Attorney Docket No.: 3062/193 PCT DESCRIPTION CHEMOPROTEOMIC CAPTURE OF RNA BINDING ACTIVITY IN LIVING CELLS CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Patent Application Serial No. 63/420,922, filed October 31, 2022, the disclosure of which is incorporated herein by reference in its entirety. GRANT STATEMENT This invention was made with government support under Grant Nos. GM144472, DA043571, and AI169412 awarded by National Institutes of Health. The Government has certain rights in the invention. REFERENCE TO SEQUENCE LISTING XML The Sequence Listing XML associated with the instant disclosure has been electronically submitted to the United States Patent and Trademark Office via the Patent Center as a 21,233 byte UTF- 8-encoded XML file created on October 31, 2023 and entitled “3062_193_PCT.xml”. The Sequence Listing submitted via Patent Center is hereby incorporated by reference in its entirety. TECHNICAL FIELD The presently disclosed subject matter relates to diagnostics and therapeutics. In particular, it relates to tunable chemistry for discovery of RNA-binding sites on proteins, particularly with respect to the use of competition assays involving the use of purine-based electrophilic probes for use in protein function analyses, e.g., in live cells. BACKGROUND RNA-binding proteins (RBPs) constitute a large (~7% of the human proteome) and diverse class of proteins that control RNA metabolism and function ^1-3 . RBPs bind sequence and/or structural motifs in single- or double-stranded RNA using RNA-binding domains (RBDs) as constituents of ribonucleoprotein (RNP) complexes that regulate gene expression and non-coding RNA (ncRNA) function ^4,5 . Canonical RBDs include, for example, RNA recognition motif (RRM), DEAD box helicase, and hnRNP K homology (KH) domains, which mediate binding affinity, avidity and specificity of target RNA sequences ^6-10 . RNPs regulate gene expression and non-coding RNA (ncRNA) function through dynamic RNA-protein networks to control biogenesis, transport, translation, and degradation of bound RNAs ^5,11 . Although large compendiums of RBPs have been inventoried by proteomics (i.e., the RNA interactome ^12,13 ), corresponding methods for direct quantification of RNA binding sites on proteins for activity and inhibitor profiling are lacking. Widely used proteomic methods for RNA interactome capture (RIC) deploy ultraviolet light (UV) irradiation to cross-link RBPs to polyadenylated (poly(A)) RNA in cells followed by oligo(dT)- mediated purification of RNPs and tandem liquid chromatography-mass spectrometry (LC-MS/MS) Attorney Docket No.: 3062/193 PCT identification. RBP crosslinking and immunoprecipitation in cells can be achieved through UV- excitation of native nucleoside bases (254 nm ¹⁴ ) or using a photoactivatable ribonucleoside analog 4- thiouridine (4SU ¹⁵ ) that is metabolically incorporated into nascent RNAs (365 nm) for RIC ^12,13,16 . Variations on this technique ^17-19 include metabolic labeling of RNAs with an alkynyl uridine analog combined with 4SU crosslinking for RIC of RBPs on nonpoly(A) RNAs (CARIC ²⁰ ). Methods based on differential solubility of RNPs have also been pursued for RBP investigations ^21,22 . Collectively, these methodologies have identified hundreds of RBPs as regulatory components of RNPs mediating cell differentiation, embryonic development, inflammation, and viral sensing ^1-3 . An intriguing finding from RIC experiments is the observation that a large fraction of identified proteins in yeast and human cells lack canonical RBDs or were not previously assigned a role in RNA biology ¹ . These novel RBPs (designated as ‘enigmRBPs’) are enriched for metabolic enzymes with glycolysis as a particular hotspot for enzymes with RNA-binding function ²³ . GAPDH, for example, was previously validated as an authentic RBP in post-transcriptional regulation of T cell effector function ²⁴ . One important step toward discovering non-canonical RBPs are methodologies for unbiased identification of RNA-binding regions on proteins by LC-MS/MS. Direct identification of RNA crosslinked sites on proteins has been demonstrated; however, this method (i.e., RNP ^xl ) is typically lower in sensitivity and requires specialized computational workflows to address the heterogenous character of peptide-RNA oligonucleotide conjugates detected ²⁵ . Lower resolution methods for identifying RNA-binding regions (~17 amino acids) include a variant of RIC that incorporates a protease digestion step prior to a second round of oligo(dT) capture to identify peptides flanking the crosslinked site for in silico reconstruction of the RNA-bound regions on RBPs (RBDmap ²⁶ ). The mass shift resulting from RNA crosslinked to peptides (~9 amino acids) has been used to infer the location of RNA-bound sites through depletion of tryptic peptide LC-MS/MS signals in UV- versus non- irradiated controls (RBR-ID ²⁷ ). Despite the success of the aforementioned methods, current techniques for investigating RBDs sacrifice either sensitivity or binding site resolution ¹ . Accordingly, there is an ongoing need for additional methods for identifying and/or quantifying the activity of RBPs and/or RBDs, including method for identifying RBPs in live cells and to identify RNA-sensitive sensitive residues in the RBPs. SUMMARY This Summary lists several embodiments of the presently disclosed subject matter, and in many cases lists variations and permutations of these embodiments. This Summary is merely exemplary of the numerous and varied embodiments. Mention of one or more representative features of a given embodiment is likewise exemplary. Such an embodiment can typically exist with or without the feature(s) mentioned; likewise, those features can be applied to other embodiments of the presently disclosed subject matter, whether listed in this Summary or not. To avoid excessive repetition, this Summary does not list or suggest all possible combinations of such features. Attorney Docket No.: 3062/193 PCT In some embodiments, the presently disclosed subject matter provides a method for identifying an RNA-binding site in a protein, the method comprising: (a) separating a protein sample comprising live cells into a first sample and a second sample; (b) culturing the first sample in a first cell culture medium comprising heavy isotopes and 4-thiouridine (4SU) for a first period of time, optionally wherein the first cell culture medium comprises ¹³ C- and/or ¹⁵ N-labeled amino acids, thereby providing cultured cells comprising a 4SU-RNA and exposing the first sample to ultraviolet (UV) light to covalently crosslink the 4SU-RNA to an aromatic amino acid residue in an RNA binding domain (RBD) of an RNA binding protein (RBP) if an RBP is present in the first sample; (c) culturing the second sample in a second cell culture medium comprising a naturally occurring isotope distribution for the first period of time, wherein the second culture medium further comprises 4SU or wherein the second sample is exposed to the UV light; (d) contacting the first sample and the second sample with an electrophilic probe compound for a second period of time, thereby modifying one or more probe- reactive amino acid residue in one or more proteins in the first sample and/or in the second sample to provide one or more modified amino acid residues, wherein the electrophilic probe compound has a structure of Formula (Ia) or of Formula (Ib):       Formula (Ia) Formula (Ib)  wherein: X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; and R ₁ and R ₂ are independently selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, aryloxy, arylthiol, and arylamino, subject to the proviso that at least one of R ₁ and R ₂ is halo; and (e) analyzing proteins or protein digests from the first sample and the second sample to detect the one or more modified amino acid residues; (f) determining the presence of an RNA-binding site in a protein by detecting a difference in probe-reactive amino acid residue modification between the first sample and the second sample. In some embodiments, the probe-reactive amino acid residue is a cysteine residue. In some embodiments, the modified amino acid residue has a structure of Formula (IIa-i), Formula (IIb-i), Formula (IIa-ii), or Formula (IIb-ii): Attorney Docket No.: 3062/193 PCT Formula (IIb-ii) wherein X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; R ₁ is selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthio, and arylamino; and R ₂ is selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthio, and arylamino. In some embodiments, R ₁ and R ₂ are selected from H, halo, and amino or R ₁ and R ₂ are selected from H and halo. In some embodiments, R ₁ is chloro or fluoro. In some embodiments, R ₂ is chloro or fluoro. In some embodiments, X is -CH ₂ -C ^CH. In some embodiments, the probe compound is selected from the group comprising 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine, 2,6-dichloro-9-(prop- 2-yn-1-yl)-9H-purine, 6-chloro-7-(prop-2-yn-1-yl)-7H-purine, 6-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2-chloro-7-(prop-2-yn-1-yl)-7H-purine, 2-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2,6-difluoro-7-(prop- 2-yn-1-yl)-7H-purine, 2,6,-difluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2-fluoro-7-(prop-2-yn-1- yl)-7H-purine, 6-chloro-2-fluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2-amino-7-(prop-2-yn-1-yl)- 7H-purine, and 6-chloro-2-amino-9-(prop-2-yn-1-yl)-9H-purine, optionally wherein the probe compound is 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine. In some embodiments, the first period of time is about 1 hour to about 24 hours, optionally about 16 hours. In some embodiments, the first cell culture medium comprises about 50 micromolar (µM) to about 500 µM 4SU; optionally wherein the first cell culture medium comprises about 100 µM 4SU. In some embodiments, the second cell culture medium comprises about 50 µM to about 500 µM 4SU and the second sample is not exposed to the UV light; optionally wherein the second cell culture medium comprises about 100 µM 4SU. In some embodiments, the UV light has a wavelength of about 254 nanometers (nm), about 312 nm, or about 365 nm, optionally about 312 nm, further optionally wherein the exposure is performed to provide an exposure of about 1 Joule per square centimeter (J/cm ² ). In some embodiments, the Attorney Docket No.: 3062/193 PCT contacting of step (d) is performed in a cell medium comprising a concentration of electrophilic probe compound of about 1 µM to about 50 µM, optionally a concentration of electrophilic probe compound of about 25 µM. In some embodiments, the second period of time is about 15 minutes to about 4 hours, optionally about 1 hour. In some embodiments, prior to step (e), cells in the first sample and the second sample are lysed to provide a heavy proteome and a light proteome, and the heavy proteome and the light proteome are mixed to provide a proteome mixture comprising the at least one modified reactive amino acid residue. In some embodiments, the analyzing of step (e) further comprises tagging the at least one modified reactive amino acid residue with a compound comprising a detectable labeling group, thereby forming at least one tagged reactive amino acid residue comprising said detectable labeling group. In some embodiments, the detectable labeling group comprises biotin or a biotin derivative, optionally wherein the biotin derivative is desthiobiotin. In some embodiments, the tagging comprises reacting an alkyne group in the X moiety of the at least one modified reactive amino acid residue with a compound comprising (i) an azide moiety and (ii) the detectable labeling group, optionally via a copper-catalyzed azide-alkyne cycloaddition (CuAAC) coupling reaction. In some embodiments, the analyzing further comprises digesting proteins in the proteome mixture with trypsin to provide a digested protein sample comprising a peptide comprising the at least one tagged reactive amino acid moiety comprising the detectable labeling group. In some embodiments, the analyzing further comprises enriching the digested protein sample for the detectable labeling group, optionally wherein the enriching comprises contacting the digested protein sample with a solid support comprising a binding partner of the detectable labeling group. In some embodiments, the analyzing further comprises analyzing the enriched digested protein sample via liquid chromatography-mass spectrometry (LC-MS). In some embodiments, the analyzing comprises determining a stable isotope labelling of amino acids in cell culture (SILAC) ratio of 2 or more for at least one peptide in the enriched digested protein sample, thereby identifying that the peptide is a fragment from an RBP, and identifying a probe-reactive amino acid residue in said peptide as being an RNA-sensitive amino acid residue in said RBP. In some embodiments, the RNA-sensitive amino acid residue is present at an allosteric site in the RBP. In some embodiments, the presently disclosed subject matter provides a method for identifying an RNA-binding site in a protein, the method comprising: (a) providing a protein sample comprising isolated proteins, living cells, or a cell lysate; (b) contacting a protein sample with a 4-thiouridine (4SU)- RNA; (c) exposing the protein sample to ultraviolet (UV) light to form a covalent crosslink between the 4SU-RNA and an amino acid residue in an RNA-binding site when an RNA-binding site is present in a protein in the protein sample; (d) contacting the protein sample with an electrophilic probe compound for a period of time sufficient for the electrophilic probe compound to react with at least one Attorney Docket No.: 3062/193 PCT probe reactive amino acid in the protein sample to form a probe modified amino acid residue, wherein the electrophilic probe compound has a structure of Formula (Ia) or of Formula (Ib):      ,        Formula (Ia) Formula (Ib)  wherein: X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; and R ₁ and R ₂ are independently selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, aryloxy, arylthiol, and arylamino, subject to the proviso that at least one of R ₁ and R ₂ is halo; (e) analyzing proteins in the protein sample to determine the presence and/or amount of one or more probe modified amino acid residues, thereby determining a protein sample modification profile; (f) analyzing proteins in a control sample treated as described for steps (b) and (d), but not exposed to the ultraviolet light to detect the presence and/or amount of one or more probe modified amino acid residues, thereby determining a control sample modification profile; and (g) determining the presence of at least one RNA-binding site in the protein sample when the protein modification profile in (e) has fewer probe modified amino acid residues and/or a lower amount of one or more modified amino acid residues compared to the control sample modification profile. In some embodiments, the probe-reactive amino acid residue is a cysteine residue. In some embodiments, the probe modified amino acid residue has a structure of Formula (IIa-i), Formula (IIb-i), Formula (IIa-ii), or Formula (IIb-ii): ,  Formula (IIb-ii) Attorney Docket No.: 3062/193 PCT wherein: X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; R ₁ is selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthio, and arylamino; and R ₂ is selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthio, and arylamino. IN some embodiments, R ₁ and R ₂ are selected from H, halo, and amino or R ₁ and R ₂ are selected from H and halo. In some embodiments, R ₁ is chloro or fluoro. In some embodiments, R ₂ is chloro or fluoro. In some embodiments, X is -CH ₂ -C ^CH. In some embodiments, the probe compound is selected from the group comprising 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine, 2,6-dichloro-9-(prop-2-yn-1-yl)-9H- purine, 6-chloro-7-(prop-2-yn-1-yl)-7H-purine, 6-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2-chloro-7- (prop-2-yn-1-yl)-7H-purine, 2-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2,6-difluoro-7-(prop-2-yn-1-yl)- 7H-purine, 2,6,-difluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2-fluoro-7-(prop-2-yn-1-yl)-7H- purine, 6-chloro-2-fluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2-amino-7-(prop-2-yn-1-yl)-7H- purine, and 6-chloro-2-amino-9-(prop-2-yn-1-yl)-9H-purine, optionally wherein the probe compound is 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine. In some embodiments, the UV light has a wavelength of about 254 nanometers (nm), about 312 nm, or about 365 nm, optionally about 312 nm. In some embodiments, the method further comprises labeling the protein sample and/or the control sample, optionally wherein the labeling comprises an isotopic labeling technique, further optionally wherein the isotopic labeling technique comprises stable isotope labelling of amino acids in cell culture (SILAC) or tandem mass tags (TMT). Accordingly, it is an object of the presently disclosed subject matter to provide methods for identifying an RNA-binding site in a protein. This and other objects are achieved in whole or in part by the presently disclosed subject matter. An object of the presently disclosed subject matter having been stated above, other objects and advantages of the presently disclosed subject matter will become apparent to those of ordinary skill in the art after a study of the following description of the presently disclosed subject matter and non- limiting Figures and Examples. BRIEF DESCRIPTION OF THE FIGURES Figures 1A-1D: Development of clickable electrophilic purines (CEPs) for chemical proteomic profiling. Figure 1A is a schematic drawing showing the purine base-based structure of exemplary covalent probes for activity-based profiling. The electronic features of the purine heterocycle were used to install electrophilic and reporter tag groups for developing CEPs. Figure 1B is a schematic drawing showing how CEPs facilitate the enrichment and identification of purine-binding proteins from complex samples (lysates, cells) using standard chemoproteomic workflows for protein and binding site identifications. Figure 1C is a graph showing that Gene Ontology (GO) analyses identified RNA binding as an enriched function from chemoproteomic analyses of cells treated with an exemplary CEP, i.e., AHL-Pu-1. GO analysis of desthiobiotin-tagged iodoacetamide (DBIA) datasets were included as Attorney Docket No.: 3062/193 PCT a comparison. The top 5 domains based on false discovery rate (FDR) are displayed based on the CEP data and based on the DBIA data. Figure 1D is a graph showing that the aggregate datasets for an exemplary CEP probe (i.e., AHL-Pu-1) and DBIA binding activity are enriched for RNA-binding domains (RBDs) detected in cell proteomes. Figure 2: Methodology for capturing RNA-binding activity of proteins in situ using photoaffinity competition. Figure 2 is a schematic diagram showing workflow for a methodology referred to herein as Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE). UV crosslinking stabilizes cellular RNA binding protein (RBP)-RNA interactions and blocks clickable electrophilic purine (CEP) probe binding to RNA-sensitive cysteine (RS-Cys) sites (shown in inset with dashed line outline on the upper right) found in protein-RNA interfaces proteome-wide. Homogeneous probe adducts produced from covalent binding of CEP to cysteine sites on proteins via nucleophilic aromatic substitution (S _N Ar) are compared with UV-mediated crosslinking of heterogenous cellular 4SU-RNA to various residues on protein sites in the insets at the top shown in solid line outlines. Figures 3A-3C. Quantifying RNA-sensitive cysteine sites in situ using Photo-Activatable- Competition and Chemoproteomic Enrichment (PACCE). Figure 3A is a graph showing a comparison of in vitro (IV) vs in situ (IS) clickable electrophilic purine (CEP) probe labeling for PACCE. Iodoacetamide (IA)-alkyne is included as a general, cysteine-reactive probe counterpart for benchmarking PACCE. Data shown are derived from human embryonic kidney (HEK293T) cells. Figure 3B is a Venn diagram showing proteins containing RNA-sensitive cysteine (RS-Cys) sites (aggregate from PACCE in situ analysis of melanoma (DM93) and HEK293T cells) compared with proteins detected by RNA-based proteomic capture methods and the human RNA binding protein (RBP) database ²⁰ . Significant overlap between RS-Cys-containing proteins and human RBPs was determined by a hypergeometric test (p = 2.97 x 10 ^-5 ). RS-Cys-containing proteins (top inset) were enriched for RNA and nucleic acid binding as determined by Gene Ontology (GO). RS-Cys-containing proteins that did not overlap with annotated human RBPs (putative RBPs, bottom inset) were enriched for general binding functions. Figure 3C is a graph showing domain enrichment analysis of aggregate RS-Cys sites (DM93 and HEK293T) compared with the inferred binding regions from RNA-based proteomic detection methods revealed prevalent binding at RNA-binding domains (RBDs). Castello 2016 refers to reference 26, He 2016 refers to reference 27, and Mullari 2017 refers to reference 45. Enriched domain assignments are those with a Q < 0.01 after Benjamini–Hochberg correction of a two- sided binomial test. Figures 4A-4D: Location of RNA-sensitive cysteine sites in protein-RNA interfaces. Figure 4A is a schematic drawing showing the location of an RNA-sensitive cysteine (RS-Cys) detected on DEAD-box helicase 19B (DDX19B, PDB ID: 3G0H), cysteine 393 (C393) in proximity to bound RNA (8 angstroms). The inset shows representative primary mass spectrum (MS1) extracted ion chromatograms (XICs) demonstrating significant blockade of clickable electrophilic purine (CEP) Attorney Docket No.: 3062/193 PCT labeling at DDX19B C393 (SILAC ratio (SR) >2) from RNA crosslinking (312 nanometer (nm)) vs. the 1:1 mixing control for equivalent CEP labeling and mixing of SILAC light (red) and heavy proteomes (blue) using a one-tailed Mann-Whitney U test (p = 0.0238). DDX19B probe-modified peptide is shared with DDX19A. Figure 4B is a schematic diagram showing that the DEAD-box 3X (DDX3X) cysteine 298 (C298) is not sensitive to RNA crosslinking (red XICs = SILAC light, blue XICs = SILAC heavy), which agrees with its larger calculated distance (29 angstroms) to the bound RNA (PDB ID: 6O5F). Statistics were calculated using a one-tailed Mann-Whitney U test (p = 0.4524). Figure 4C is a plot showing aggregate RS-Cys SR values (DM93 and HEK293T) as a function of the distance between the respective site and interacting RNA across RNA-binding protein (RBP)-RNA structures analyzed (289 structures in total). Figure 4D is a graph showing that the distance between the quantified site and bound RNA of RBP-RNA structures is generally reduced for RS-Cys (SR >2). Data shown are sites from HEK293T membrane proteomes subjected to RNA crosslinking competition (4-thiouridine (4SU)- and native-RNA). RNA insensitive sites include non-probe modified Cys-sites. Data shown are mean + SEM for n=3 biologically independent replicates for 1:1 mixing sample and n=6 independent replicates for Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE). *p < 0.05. Figures 5A-5C: Discovery of moonlighting RNA-binding activity in proteomes. Figure 5A shows representative primary mass spectrum (MS1) extracted ion chromatograms (XICs) for cysteine 957 (C957) of exocyst complex component 4 (EXOC4 C957: mean SILAC ratio (SR) = 2.7 vs. 0.7, p = 0.0119) and cysteines 121 (C121) and 691 (C691) of ubiquitin interaction motif containing 1 (UIMC1 C121: mean SR = 7.2 vs 1.1, p = 0.0119; C691: mean SR = 7.9 vs 1.1, p = 0.0119 for Photo-Activatable- Competition and Chemoproteomic Enrichment (PACCE) compared with the SILAC light/heavy 1:1 mixing control, respectively) RS-Cys sites. Statistics were calculated using a one-tailed Mann-Whitney U test. Red XICs represent SILAC light, while blue XICs represent SILAC heavy proteomes. Figure 5B is a fluorescent image showing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS- PAGE) separating fluorescent-adapter ligated, crosslinked FLAG-polypyrimidine tract binding protein 1 (PTBP1), FLAG-EXOC4, and FLAG-UIMC1 ribonucleoproteins (RNPs). Bands boxed in red correspond to the molecular weight of the FLAG-tagged protein plus the fluorescently labeled adapter. Counterpart western blots (a-FLAG, black box) confirmed expression of recombinant protein. Figure 5C is a graph showing western blot analyses comparing RNP formation of wild-type (WT) and corresponding RNA-binding protein (RBP) mutants. Integrated band intensities from the WT and mutant RNP bands were used to quantify the impact of deleting RNA-binding domains or regions on RNP formation. Data shown are mean + SEM; n=3 biological replicates for 1:1 mixing condition liquid chromatography-mass spectroscopy (LC-MS) studies, n=6 biologically independent replicates for PACCE LC-MS studies, n=6 biologically independent replicates for western blots. *p < 0.05; **p < 0.01. EXOC4 was analyzed using a ratio paired t-test (paired, parametric, one-tailed, p = 0.0035), while Attorney Docket No.: 3062/193 PCT PTBP1 was analyzed using a one-way ANOVA (p = 0.0118) followed by a Tukey’s post hoc test for multiple comparisons (p = 0.9007, 0.0309, 0.0151, respectively). Figures 6A-6C: Reaction mechanism of purine-based probe molecules. Figure 6A shows a schematic drawing of the proposed nucleophilic aromatic substitution (S _N Ar) reaction of clickable electrophilic purines (CEPs) with nucleophilic groups. Figure 6B is a schematic drawing of chemical reactions used in a high-performance liquid chromatography (HPLC) assay for measuring solution reactivity of CEPs. Time-dependent reactions were performed between nucleophiles (10.8 millimolar (mM)) with CEPs (AHL-Pu-1 or AHL-Pu-2; 9.8 mM). Tetramethylguanidine (TMG) was included as a base to facilitate covalent reaction. The following nucleophiles were chosen to mimic amino acid side chain groups: butanethiol (cysteine), n-butylamine (lysine), p-cresol (tyrosine), butyric acid (aspartate/glutamate), and propionamide (asparagine/glutamine). Figure 6C shows HPLC chromatographs of a representative example of HPLC analysis of AHL-Pu-1 reaction to form the butanethiol-CEP adduct. Covalent reaction at C6 to form the AHL-Pu-1-butanethiol adduct was confirmed by retention times that matched those of the synthetic standard (Pa-1). The elapsed reaction times for each HPLC trace is shown on the right axis (0 to 360 minutes). Data shown are representative of three independent experiments (n=3). Figures 7A-7C: Comparison of the reactivity of exemplary clickable electrophilic purine (CEP) probes (AHL-Pu-1 and AHL-Pu-2) against nucleophiles in solution. Figure 7A is a pair of graphs comparing individual CEP reactivity (percent (%) starting material consumed versus time (in minutes)) against nucleophiles that mimic side chain functional groups of the indicated amino acid: aspartic acid/glutamic acid (Asp/Glu, squares); cysteine (Cys, circles), glutamine/asparagine (Gln/Asn, circles), lysine (Lys, “x”s), and tyrosine (Tyr, vertical lines). Figure 7B is a series of graphs comparing AHL-Pu-1 (vertical lines) vs AHL-Pu-2 (“x”s) reactivity (percent (%) starting material consumed versus time (in minutes)) against nucleophiles in solution. Data shown are representative of three independent experiments (n=3). Figure 7C is a graph showing the stability (% degradation of the CEP versus time in hours) of AHL-Pu-1 and AHL-Pu-2 in phosphate-buffered saline (PBS) buffer. Solutions of CEPs (9.8 millimolar (mM)) were prepared and HPLC analysis of these probes measured at the indicated time points. Negligible degradation, as determined by reduction of CEP signal, was observed after 48 hours (2 days). Data shown are representative of three independent experiments (n=3). Figures 8A-8D: Live cell labeling using CEPs. Figure 8A shows an image of the gel-based activity-based purine profiling (ABPP) analysis of melanoma (DM93) cells treated with clickable electrophilic purine (CEP) probes using optimized treatment conditions for liquid chromatography tandem mass spectroscopy (LC-MS/MS) quantitative chemical proteomics. DM93 cells were treated with 25 micromolar (µM) AHL-Pu-1 or AHL-Pu-2 for 4 hours (hr) at 37 degrees Celsius (°C). After treatment, cells were lysed, probe-modified soluble (left panel; 2 milligrams per milliliter (mg/mL)) and membrane proteomes (right panel; 2 mg/mL) subjected to copper (I)-catalyzed azide-alkyne Attorney Docket No.: 3062/193 PCT cycloaddition (CuAAC) with rhodamine-azide followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis and in-gel fluorescence scanning. Figure 8B shows an image of gel-based ABPP analyses of concentration-dependent labeling of DM93 cells treated with CEP probes. DM93 cells were treated with indicated concentrations of AHL-Pu-1 or AHL-Pu-2 for 4 hour at 37°C followed by gel-based ABPP analyses. Figure 8C is an image showing the time-dependent labeling of DM93 cells treated with CEP probes (25 µM of AHL-Pu-1 or AHL-Pu-2 for the indicated times at 37°C) and subjected to gel-based ABPP analyses. Figure 8D is a graph showing cell viability (as a %) of DM93 cells treated with AHL-Pu-1 (25 µM, 4 hr, 37 ^C) as determined by the WST-1 assay for cell proliferation and viability. Cell viability was not statistically significantly different between dimethyl sulfoxide (DMSO) vehicle and AHL-Pu-1 treated cells (p = 0.4). Statistical significance was determined using a Mann-Whitney test. Data shown are representative of n=3 biologically independent experiments. Figures 9A-9C: Experimental conditions used for Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE) in human embryonic kidney (HEK293T) cells do not generally alter the transcriptome or proteome. Figure 9A shows normalized expression values (fragments per kilobase of transcript per million mapped reads [FPKM]) as determined by paired-end RNA-sequencing (RNA-Seq). Respective samples were treated with either 4-thiouridine (4SU, 100 micromolar ( ^M), 16 hours (hr)), clickable electrophilic purine (CEP, 25 ^M, 1 hr), or both at 37 degrees Celsius (°C). Figures 9B and 9C are graphs showing principal component (PC) analysis results derived from RNA-Seq samples. The X and Y axes indicate PC1 and PC2, which explain 14% and 19% of the total variation, respectively. The correlation scatter plot of SILAC ratio (SR) values (log ₂ ) of (Figure 9B) different cell treatments (CEP, CEP+4SU, or 4SU) compared to dimethyl sulfoxide (DMSO) or (Figure 9C) PACCE condition (CEP+4SU) to assess proteomic alterations. Peaks defined by log ₂ (L/H ratios) >5 were set to 5. Data shown are representative of n=3 biologically independent experiments. Figures 10A-10D: Experimental workflow for clickable electrophilic purine (CEP)- mediated chemical proteomics. Figure 10A is a schematic diagram showing a workflow for gel based profiling studies (in situ). Cells are treated with either CEP or dimethyl sulfoxide (DMSO). Cells are lysed and conjugated to a fluorescent tag by copper (I)-catalyzed azide-alkyne cycloaddition (CuAAC) and analyzed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Figure 10B is a schematic diagram showing a workflow for gel-based competition studies. Proteome derived from lysed cells is pretreated with either CEP + DMSO or CEP + inhibitor. Next, samples are conjugated to a fluorescent dye via CuAAC and analyzed with SDS-PAGE. Loss of fluorescent signal indicates competition. Figure 10C is a schematic diagram showing a non-competitive workflow to identify proteins and corresponding binding sites that are enriched from CEP labeling of proteins in cells. For stable isotope labeling by amino acids in cell culture (SILAC) workflows, proteomes are Attorney Docket No.: 3062/193 PCT derived from cells cultured in SILAC media supplemented with either “light” ¹² C, ¹⁴ N- (denoted in red) or “heavy” ¹³ C, ¹⁵ N-labeled lysine and arginine (denoted in blue). To identify CEP-enriched proteins, light and heavy cells are treated with CEP probe (25 micromolar (µM), 4 hours (hr), 37 degrees Celsius (°C)) or DMSO vehicle, respectively. Afterwards, cells are lysed followed by CuAAC conjugation of desthiobiotin-azide, avidin affinity chromatography, and liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. The resulting SILAC ratios (SR) are quantified using the area under the curve of primary mass spectrum (MS1) extracted ion chromatograms (EICs). CEP-enriched proteins are identified using probe-modified peptides that meet quality control criteria and show a substantial increase in peptide abundance in CEP probe-treated compared with vehicle control samples (SR >>1). Figure 10D is a schematic diagram showing a competitive workflow to evaluate inhibitor activity in proteomes. Cell proteomes derived from either light- or heavy-labeled cells were co-treated with DMSO vehicle or nitrogenous base (0.025-25 millimolar (mM), 30 minutes (min), 37°C), respectively, and CEP probe (25 µM). Non-competed sites are expected to show equivalent probe labeling intensity in vehicle (L)- and fragment (H)-treated conditions (SR~1). Nitrogenous base- competed sites are identified by probe-modified peptides showing a substantial reduction in peptide abundance (due to competition of CEP labeling) in nitrogenous base- compared with vehicle-treated control samples (SR >>1). Figures 11A-11B: Amino acid preference of clickable electrophilic purines (CEPs) in live cell environment. Distribution of CEP modifications on nucleophilic amino acid residues from probe- modified peptides of detected proteins (soluble proteomes) from quantitative chemoproteomic analyses of CEP-treated melanoma (DM93) cells. Data shown are high confidence sites (Byonic score > 600) for AHL-Pu-1 (Figure 11A) and AHL-Pu-2 (Figure 11B) and representative of n=3 biologically independent experiments. Figures 12A-12D: Clickable electrophilic purine (CEP) competition against purine and pyrimidine bases. The protein-binding profiles of AHL-Pu-1-labeled human embryonic kidney (HEK293T) proteomes are competed by pretreatment of purines but not pyrimidines in vitro as determined by gel (Figure 12A) and quantitative chemical proteomics (Figure 12B). These studies provide evidence that AHL-Pu-1 covalent binding activity is dependent on purine recognition. Quantitative chemical proteomics showed that CEP-modified proteins (Figure 12C) and sites (Figure 12D) are largely competed with purine (25 millimolar (mM)) and adenine (2.5 mM) but not uracil (2.5 mM) or cytosine (2.5 mM). Proteomes were co-treated with nitrogenous bases at the indicated concentrations for 30 minutes (min) at 37 degrees Celsius ( ^C) and AHL-Pu-1 (25 µM, 30 min, 37 ^C). Data shown are representative of n=3 biologically independent experiments. Figures 13A-13D: Structure-activity relationship (SAR) of clickable electrophilic purine (CEP) analogs. Melanoma (DM93) cells were treated with AHL-Pu-1 (N ₇ alkyne handle) or AHL-Pu- 2 (N ₉ alkyne handle) at 25 micromolar (µM) for 4 hours (hr) at 37 degrees Celsius (°C). Figure 13A Attorney Docket No.: 3062/193 PCT shows a Venn diagram of overlapping sites between CEP analogs. Functional protein classes enriched in CEP-treated DM93 cells as determined by Gene Ontology ^1,2 , highlighting AHL-Pu-1 and AHL-Pu- 2 modify an equivalent array of protein classes using panther classification (Figure 13B) and GO functions (Figure 13C). Sites with specific enrichment (SR > 5) were quantified. Data shown are representative of n=3 biologically independent experiments. Figure 13D shows a Venn diagram showing overlap of CEP and desthiobiotin-iodoacetamide (DBIA) datasets with human annotated RNA binding proteins (RBPs) ³ . Overlap between CEP-enriched proteins and human annotated RBPs was statistically significant as determined by hypergeometric distribution (p = 4.09 X 10 ^-5 ). Figures 14A-14D: Identifying optimal conditions for 4-thiouridine (4SU) metabolic labeling of cellular RNA in human embryonic kidney (HEK293T) cells. Optimizing non-toxic conditions for metabolic incorporation of 4SU into cellular RNA using RNA dot blots (Figures 14A and 14B) and agarose gel analyses following published methods ⁴ . Figure 14C shows an image of agarose gel-shift assays confirming that ultraviolet (UV) irradiation at 312 nanometers (nm) crosslinks 4SU-labeled cellular RNA to proteins. Photocrosslinking of native RNA to proteins was also observed at this wavelength. Figure 14D is a graph showing that photocrosslinking using optimized 4SU conditions (100 micromolar (µM), 16 hours (hr)) did not result in overt toxicity to cells (as indicated by lack of cell viability). Data shown are representative of n=3 biologically independent experiments. Figures 15A-15D: Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE) conditions for quantifying RNA-sensitive cysteine (RS-Cys) sites. Figure 15A is a schematic diagram showing that ultraviolet (UV) crosslinking of RNA to proteins protects cysteines located in or proximal to RNA-binding sites from clickable electrophilic purine (CEP) labeling. PACCE captures RS-Cys sites competed (SILAC ratio (SR) >2) by crosslinking (i) 4-thiouridine (4SU)-RNA and (ii) 4SU- and native-RNA. Figures 15B and 15C show Venn diagrams of RS-Cys sites (5,530, Figure 15B) and proteins (3,018, Figure 15C) determined by the aggregate of all sites that show sensitivity to RNA crosslinking competition from PACCE in situ (live cell labeling using CEP probe). Figure 15D is a graph showing the average SR value for peptides found in respective groups. The number of reported sites per group is highlighted. Data shown are representative of n=3 independent experiments from PACCE studies in human embryonic kidney (HEK) and melanoma (DM93) cells. Figures 16A and 16B: Comparison of Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE) using clickable electrophilic purine (CEP) (in vitro and in situ) and iodoacetamide (IA)-alkyne (in vitro) probe in human embryonic kidney (HEK293T) cells and proteomes. Figure 16A is a Venn diagram showing overlapping sites between IA-alkyne and CEP. Figure 16B is a graph showing domain enrichment analysis of CEP- and IA-alkyne modified sites identified from HEK soluble and membrane fractions. Data shown are representative of n=3 biologically independent experiments. Attorney Docket No.: 3062/193 PCT Figures 17A and 17B: RNase treatment reduces RNA-sensitive cysteine (RS-Cys) detection by Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE) in human embryonic kidney (HEK293T) cell proteomes. Figure 17A is a schematic diagram showing a workflow for evaluating effects of RNase treatment on RS-Cys detection by PACCE. Stable isotope labeling by amino acids in cell culture (SILAC) light and heavy cells are cultured in the absence or presence of 4-thiouridine (4SU)-RNA, respectively. Cells were lysed and proteomes exposed to ultraviolet (UV) irradiation followed by clickable electrophilic purine (CEP) probe labeling and quantitative chemical proteomics to identify RS-Cys sites (SILAC ratio (SR) >2). The role of crosslinked 4SU-RNA in protecting RNA-binding sites from CEP labeling was verified by addition of RNase to proteomes prior to UV irradiation. Figure 17B is a graph showing the quantitation of RNase treatments in PACCE studies. RNase treatment of lysates resulted in a 60% reduction in the number of RS-Cys sites detected [a total of 934 and 317 RS-Cys sites in (-)RNase and (+)RNase sample groups, respectively]. Data shown are mean + SEM for n = 2-3 biologically independent experiments. Figure 18 is a schematic diagram comparing different RNA-binding protein identification methods, i.e., the presently disclosed Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE) methodology versus other mass spectrometry based proteomic methods for RNA binding protein (RBP) analysis, i.e., RBR-ID and RNP ^XL . The methods are compared with respect to an exemplary amino acid sequence CAVFYIWK (SEQ ID NO: 1) that could be present in an RNA-binding domain (RBD). The RNP ^XL method involves tandem mass spectrometry analysis of product ions of peptides from cross-linked proteins, as illustrated by the MS2 spectrum shown for an exemplary peptide GNEFEDYCLKR (SEQ ID NO: 2). Figure 19A-19C: Identification of known RBPs using PACCE. Figure 19A is a pair of schematic diagrams showing the location and distance of RS-Cys sites to RNA in RBP-RNA structures: DDX17, C298 (6UV2, X-ray); SF3B6, C83 (7Q4O, X-ray). Cys residues reside in canonical RBDs, including RRM and Helicase-ATP binding domains. DDX17 probe-modified peptide is shared with DDX5. Distances were calculated using Pymol. Data shown are mean + SEM for n=3 biologically independent replicates for each treatment condition. *p ≤ 0.05. Figure 19B is a graph showing the MS2 (fragmentation) spectra and mass spectrometry analysis of an RNA sensitive, AHL-Pu-1-modified tryptic peptide fragment (GVEIC*IATPGR, SEQ ID NO: 3) on DDX17/DDX5 (C298/C221). Covalent addition of AHL-Pu-1 onto the Cys residues results in a modified Cys (C*) with a mass addition of +604.2631 Da. In addition to standard b- and y-fragment ions, internal fragment ions due to fragmentation of AHL-Pu-1 probe are highlighted. Internal fragmentation of y- and b-fragment ions that contain a portion of the probe are denoted by iX and fX annotations, respectively. Yellow peaks denote desthiobiotin fragments. Data shown are representative of 3 biologically independent replicates. Figure 19C is a graph showing the MS2 (fragmentation) spectra and mass spectrometry analysis of a Attorney Docket No.: 3062/193 PCT modified peptide fragment (NACDHLSGFNVC*NR; SEQ ID NO: 4) at the SF3B6 C83 site. The modified Cys residue is indicated by the star (C*). Figure 20 is a graph showing the MS2 (fragmentation) spectra and mass spectrometry analysis of the RS-Cys site on DDX19B (C393). The modified sequence, VLVTTNVC*AR (SEQ ID NO: 5; where the modified Cys residue is indicated as C*) is also found in DDX19A (C392). Figure 21 is a graph showing the MS2 (fragmentation) mass spectrometry analysis of the RS- Cys site on EXOC4 (C957). The modified sequence is LKEIIC*EQAAIK (SEQ ID NO: 6; wherein the modified Cys residue is indicated as C*). The inset contains a schematic of fragments from probe and internal fragmentation. Figures 22A-22F: Validation of novel RBP activity in PACCE-identified proteins. Figures 22A-22C show western blot analyses showing RNP formation upon UV crosslinking in HEK293T cells expressing recombinant proteins of PTBP1 (Figure 22A), EXOC4 (Figure 22B), and UIMC1 (Figure 22C). RNA crosslinking in cells forms higher molecular weight RNA-protein complexes that are detected by reduced migration using SDS-PAGE gels and exhibit sensitivity to RNase treatment (highlighted in the dotted boxes). Quantification showed significant reductions in UIMC1 band signals from RNase treatment. Data shown are mean + SEM for n=3 biologically independent replicates. **p < 0.01. Figures 22D and 22E show the Full-length fluorescent gel (Figure 22D) and western blot (Figure 22E) of fPAR-CLIP evaluation of FLAG-PTBP1, -EXOC4, and -UIMC1. Figure 22F shows the cellular lysates recombinantly expressing proteins of interest from UV-treated or control cells were subjected to concentration-dependent RNaseA treatments. Proteins were immunoprecipitated followed by P32 radioactive labeling (top) and western blotting (bottom). Proteins were immunoprecipitated followed by radioactive labeling (32P) of RNA 5′ ends with T4 polynucleotide kinase and imaging of autoradiographic film. Western blotting was performed to confirm recombinant protein expression. Figures 23A-23C: Mass Spectrometry analysis of the RS-Cys sites on UIMC1. Figure 23A shows the UIMC1 domains and sequence alignments showing conservation of RS-Cys sites. One RS- Cys site, centered on cysteine 121 (C121), is represented by the sequences ESLNSCRPSDA (SEQ ID NO: 7), ESLNSCWSSAA (SEQ ID NO: 8), and ESLNSRWSSDA (SEQ ID NO: 9). A second RS-Cys site, centered on cysteine 691 (C691), is represented by the sequences SEATDCLVDFK (SEQ ID NO: 10), SEATNCLVDFK (SEQ ID NO: 11), and SEAADCLVDFK (SEQ ID NO: 12). Figure 23B shows the modified sequence and MS2 fragment ion annotation of AIAESLNSC*RPSDASATR (SEQ ID NO: 13) where C* represents the modified cysteine residue at UIMC1 C121. Figure 23C shows the modified sequence and MS2 fragment ion annotation of SFVSISEATDC*LVDFKK (SEQ ID NO: 14) wherein C* represents the modified cysteine residue at UIMC1 C691. Figures 24A-24C: Analysis of RNA-binding protein (RBP) binding capacity using deletion mutants. Figures 24A and 24B are schematic diagrams of PTBP1 (Figure 24A) and EXOC4 (Figure 24B) wild-type (WT) protein and domains/regions deleted in corresponding mutants. Vertical Attorney Docket No.: 3062/193 PCT lines for EXOC4 represent a 20 amino acid deletion surrounding the RS-Cys C957 site. Figure 24C shows a representative western blot comparing RNP formation of wild-type (WT) and corresponding RBP mutants in HEK293T cells used for quantitation. Data shown are representative of n=6 independent experiments. DETAILED DESCRIPTION Described herein is a method for global quantification of protein-RNA interactions that can be performed in living cells. This method, referred to as Photo-Activatable-Competition and Chemoproteomic Enrichment, or PACCE, is differentiated from currently available RIC methods by deploying chemical probes, i.e., clickable electrophilic purine (CEP) probes, to covalently bind, enrich, and identify the human RNA-binding proteome. Of particular note, PACCE can detect protein-RNA interfaces with amino acid resolution by quantifying the sensitivity of probe-modified sites to competition with photoactivatable cellular RNA. As disclosed herein, using PACCE, >5,500 RNA- sensitive cysteine sites were assessed that mapped to a large fraction of proteins mediating recognition of coding and noncoding RNA. PACCE provided functional profiling of RNA-binding regions on known RBPs as well as discovery of moonlighting RNA binding activity in situ. Of further note, the presently disclosed method can detect allosteric sites involved in the modulation of RNA binding of proteins, as shown from the detection of several cysteine sites that were sensitive to RNA crosslinking but that were located relatively far from a bound RNA (e.g., more than 50 angstroms from or more than 100 angstroms from a bound RNA) based on co-crystal structures. Accordingly, the presently disclosed method appears uniquely positioned to find “druggable” sites on RNA-binding proteins (including druggable allosteric sites), particularly as the method can be performed in live cells, a capability that is difficult with other cysteine-reactive probes used for global profiling in lysates, such as the iodoacetamide-alkyne (or desthiobiotin) probe. The presently disclosed subject matter will now be described more fully hereinafter with reference to the accompanying Figures and EXAMPLES, in which representative embodiments are shown. The presently disclosed subject matter can, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art. Certain components in the Figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the presently disclosed subject matter (in some cases schematically). Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the presently described subject matter belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. Attorney Docket No.: 3062/193 PCT Throughout the specification and claims, a given chemical formula or name shall encompass all active optical and stereoisomers, as well as racemic mixtures where such isomers and mixtures exist. I. Definitions The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the presently disclosed subject matter. While the following terms are believed to be well understood by one of ordinary skill in the art, the following definitions are set forth to facilitate explanation of the presently disclosed subject matter. References to techniques employed herein are intended to refer to the techniques as commonly understood in the art, including variations on those techniques or substitutions of equivalent techniques that would be apparent to one of skill in the art. In describing the presently disclosed subject matter, it will be understood that a number of techniques and steps are disclosed. Each of these has individual benefit and each can also be used in conjunction with one or more, or in some cases all, of the other disclosed techniques. Accordingly, for the sake of clarity, this description will refrain from repeating every possible combination of the individual steps in an unnecessary fashion. Nevertheless, the specification and claims should be read with the understanding that such combinations are entirely within the scope of the presently disclosed and claimed subject matter. Following long-standing patent law convention, the terms “a”, “an”, and “the” refer to “one or more” when used in this application, including in the claims. For example, the phrase “a protein” refers to one or more proteins, including a plurality of the same protein. Similarly, the phrase “at least one”, when employed herein to refer to an entity, refers to, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 75, 100, or more of that entity, including but not limited to whole number values between 1 and 100 and greater than 100. Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. The term “about”, as used herein when referring to a measurable value such as an amount of mass, weight, time, volume, concentration, or percentage, is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1 % from the specified amount, as such variations are appropriate to perform the disclosed methods and/or employ the disclosed compositions. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter. A disease or disorder is “alleviated” if the severity of a symptom of the disease, condition, or disorder, or the frequency at which such a symptom is experienced by a subject, or both, are reduced. Attorney Docket No.: 3062/193 PCT The term “allosteric” as used herein refers to a site (e.g., a single residue or a multi-residue domain) in an RNA-binding protein that it not part of an RNA-binding domain. As used herein, the term “and/or” when used in the context of a list of entities, refers to the entities being present singly or in combination. Thus, for example, the phrase “A, B, C, and/or D” includes A, B, C, and D individually, but also includes any and all combinations and subcombinations of A, B, C, and D. The terms “additional therapeutically active compound” and “additional therapeutic agent”, as used in the context of the presently disclosed subject matter, refers to the use or administration of a compound for an additional therapeutic use for a particular injury, disease, or disorder being treated. Such a compound, for example, could include one being used to treat an unrelated disease or disorder, or a disease or disorder which may not be responsive to the primary treatment for the injury, disease, or disorder being treated. As used herein, the term “adjuvant” refers to a substance that elicits an enhanced immune response when used in combination with a specific antigen. As use herein, the terms “administration of” and/or “administering” a compound should be understood to refer to providing a compound of the presently disclosed subject matter to a subject in need of treatment. The term “comprising”, which is synonymous with “including” “containing”, or “characterized by”, is inclusive or open-ended and does not exclude additional, unrecited elements and/or method steps. “Comprising” is a term of art that means that the named elements and/or steps are present, but that other elements and/or steps can be added and still fall within the scope of the relevant subject matter. As used herein, the phrase “consisting essentially of” limits the scope of the related disclosure or claim to the specified materials and/or steps, plus those that do not materially affect the basic and novel characteristic(s) of the disclosed and/or claimed subject matter. For example, a pharmaceutical composition can “consist essentially of” a pharmaceutically active agent or a plurality of pharmaceutically active agents, which means that the recited pharmaceutically active agent(s) is/are the only pharmaceutically active agent(s) present in the pharmaceutical composition. It is noted, however, that carriers, excipients, and/or other inactive agents can and likely would be present in such a pharmaceutical composition and are encompassed within the nature of the phrase “consisting essentially of”. As used herein, the phrase “consisting of” excludes any element, step, or ingredient not specifically recited. It is noted that, when the phrase “consists of” appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. Attorney Docket No.: 3062/193 PCT With respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms. For example, a composition that in some embodiments comprises a given active agent also in some embodiments can consist essentially of that same active agent, and indeed can in some embodiments consist of that same active agent. The term “aqueous solution” as used herein can include other ingredients commonly used, such as sodium bicarbonate described herein, and further includes any acid or base solution used to adjust the pH of the aqueous solution while solubilizing a peptide. The term “binding” refers to the adherence of molecules to one another, such as, but not limited to, enzymes to substrates, ligands to receptors, antibodies to antigens, DNA binding domains of proteins to DNA, and DNA or RNA strands to complementary strands. “Binding partner”, as used herein, refers to a molecule capable of binding to another molecule. The term “biocompatible”, as used herein, refers to a material that does not elicit a substantial detrimental response in the host. As used herein, the terms “biologically active fragment” and “bioactive fragment” of a peptide encompass natural and synthetic portions of a longer peptide or protein that are capable of specific binding to their natural ligand and/or of performing a desired function of a protein, for example, a fragment of a protein of larger peptide which still contains the epitope of interest and is immunogenic. The term “biological sample”, as used herein, refers to samples obtained from a subject, including but not limited to skin, hair, tissue, blood, plasma, cells, sweat, and urine. A “coding region” of a gene comprises the nucleotide residues of the coding strand of the gene and the nucleotides of the non-coding strand of the gene which are homologous with or complementary to, respectively, the coding region of an mRNA molecule which is produced by transcription of the gene. “Complementary” as used herein refers to the broad concept of subunit sequence complementarity between two nucleic acids (e.g., two DNA molecules). When a nucleotide position in both of the molecules is occupied by nucleotides normally capable of base pairing with each other at a given position, the nucleic acids are considered to be complementary to each other at this position. Thus, two nucleic acids are complementary to each other when a substantial number (in some embodiments at least 50%) of corresponding positions in each of the molecules are occupied by nucleotides that can base pair with each other (e.g., A:T and G:C nucleotide pairs). Thus, it is known that an adenine residue of a first nucleic acid region is capable of forming specific hydrogen bonds (“base pairing”) with a residue of a second nucleic acid region which is antiparallel to the first region if the residue is thymine or uracil. Similarly, it is known that a cytosine residue of a first nucleic acid strand is capable of base pairing with a residue of a second nucleic acid strand which is antiparallel to the first strand if the residue is guanine. A first region of a nucleic acid is complementary to a second Attorney Docket No.: 3062/193 PCT region of the same or a different nucleic acid if, when the two regions are arranged in an antiparallel fashion, at least one nucleotide residue of the first region is capable of base pairing with a residue of the second region. By way of example and not limitation, the first region comprises a first portion and the second region comprises a second portion, whereby, when the first and second portions are arranged in an antiparallel fashion, in some embodiments at least about 50%, in some embodiments at least about 75%, in some embodiments at least about 90%, and in some embodiments at least about 95% of the nucleotide residues of the first portion are capable of base pairing with nucleotide residues in the second portion. In some embodiments, all nucleotide residues of the first portion are capable of base pairing with nucleotide residues in the second portion. A “control” cell, tissue, sample, or subject is a cell, tissue, sample, or subject of the same type as a test cell, tissue, sample, or subject. The control may, for example, be examined at precisely or nearly the same time the test cell, tissue, sample, or subject is examined. The control may also, for example, be examined at a time distant from the time at which the test cell, tissue, sample, or subject is examined, and the results of the examination of the control may be recorded so that the recorded results may be compared with results obtained by examination of a test cell, tissue, sample, or subject. The control may also be obtained from another source or similar source other than the test group or a test subject, where the test sample is obtained from a subject suspected of having a condition, disease, or disorder for which the test is being performed. A “test” cell is a cell being examined. A “pathogenic” cell is a cell that, when present in a tissue, causes or contributes to a condition, disease, or disorder in the animal in which the tissue is located (or from which the tissue was obtained). A tissue “normally comprises” a cell if one or more of the cell are present in the tissue in an animal not afflicted with a condition, disease, or disorder. As used herein, the terms “condition”, “disease condition”, “disease”, “disease state”, and “disorder” refer to physiological states in which diseased cells or cells of interest can be targeted with the compositions of the presently disclosed subject matter. In some embodiments, a disease is leukemia, which in some embodiments is Acute Myeloid Leukemia (AML). As used herein, the term “diagnosis” refers to detecting a risk or propensity to a condition, disease, or disorder. In any method of diagnosis exist false positives and false negatives. Any one method of diagnosis does not provide 100% accuracy. A “disease” is a state of health of an animal wherein the animal cannot maintain homeostasis, and wherein if the disease is not ameliorated then the animal’s health continues to deteriorate. In contrast, a “disorder” in an animal is a state of health in which the animal is able to maintain homeostasis, but in which the animal’s state of health is less favorable than it would be in the absence of the disorder. Left untreated, a disorder does not necessarily cause a further decrease in the animal’s state of health. Attorney Docket No.: 3062/193 PCT As used herein, an “effective amount” or “therapeutically effective amount” refers to an amount of a compound or composition sufficient to produce a selected effect, such as but not limited to alleviating symptoms of a condition, disease, or disorder. In the context of administering compounds in the form of a combination, such as multiple compounds, the amount of each compound, when administered in combination with one or more other compounds, may be different from when that compound is administered alone. Thus, an effective amount of a combination of compounds refers collectively to the combination as a whole, although the actual amounts of each compound may vary. The term “more effective” means that the selected effect occurs to a greater extent by one treatment relative to the second treatment to which it is being compared. “Encoding” refers to the inherent property of specific sequences of nucleotides in a polynucleotide, such as a gene, a cDNA, or an mRNA, to serve as templates for synthesis of other polymers and macromolecules in biological processes having either a defined sequence of nucleotides (e.g., rRNA, tRNA, and mRNA) or a defined sequence of amino acids and the biological properties resulting therefrom. Thus, a gene encodes a protein if transcription and translation of an mRNA corresponding to or derived from that gene produces the protein in a cell or other biological system and/or an in vitro or ex vivo system. Both the coding strand, the nucleotide sequence of which is identical to the mRNA sequence (with the exception of uracil bases presented in the latter) and is usually provided in Sequence Listing, and the non-coding strand, used as the template for transcription of a gene or cDNA, can be referred to as encoding the protein or other product of that gene or cDNA. As used herein, an “essentially pure” preparation of a particular protein or peptide is a preparation wherein in some embodiments at least about 95% and in some embodiments at least about 99%, by weight, of the protein or peptide in the preparation is the particular protein or peptide. In some embodiments, the terms “fragment”, “segment”, or “subsequence” as used herein refers to a portion of an amino acid sequence, comprising at least one amino acid, or a portion of a nucleic acid sequence comprising at least one nucleotide. Thus, in some embodiments, the terms “fragment”, “segment”, and “subsequence” are used interchangeably herein. In some embodiments, the term “fragment” refers to a compound (e.g., a small molecule compound, such as a small molecule comprising a purine scaffold) that can react with a reactive amino acid residue (e.g., a reactive cysteine) to form an adduct comprising a modified amino acid residue. Thus, in some embodiments, the terms “fragment” and “ligand” are used interchangeably. In some embodiments, the term “fragment” refers to that portion of a ligand that remains covalently attached to the reactive amino acid residue. As used herein, a “ligand” is a compound (e.g., a purine-based compound) that specifically binds to a target compound or molecule, such as a reactive nucleophilic amino acid residue in a protein. In some embodiments, the ligand can bind to the target covalently. A ligand “specifically binds to” or “is specifically reactive with” a compound (e.g., a reactive amino acid residue) when the ligand Attorney Docket No.: 3062/193 PCT functions in a binding reaction which is determinative of the presence of the compound in a sample of heterogeneous compounds. As used herein, a “functional” biological molecule is a biological molecule in a form in which it exhibits a property by which it can be characterized. A functional enzyme, for example, is one that exhibits the characteristic catalytic activity by which the enzyme can be characterized. As used herein “injecting”, “applying”, and administering” include administration of a compound of the presently disclosed subject matter by any number of routes and modes including, but not limited to, topical, oral, buccal, intravenous, intramuscular, intra-arterial, intramedullary, intrathecal, intraventricular, transdermal, subcutaneous, intraperitoneal, intranasal, enteral, topical, sublingual, vaginal, ophthalmic, pulmonary, vaginal, and rectal approaches. As used herein, the term “linkage” refers to a connection between two groups. The connection can be either covalent or non-covalent, including but not limited to ionic bonds, hydrogen bonding, and hydrophobic/hydrophilic interactions. As used herein, the term “linker” refers to a molecule that joins two other molecules either covalently or noncovalently, such as but not limited to through ionic or hydrogen bonds or van der Waals interactions. The terms “measuring the level of expression” and “determining the level of expression” as used herein refer to any measure or assay which can be used to correlate the results of the assay with the level of expression of a gene or protein of interest. Such assays include measuring the level of mRNA, protein levels, etc. and can be performed by assays such as northern and western blot analyses, binding assays, immunoblots, etc. The level of expression can include rates of expression and can be measured in terms of the actual amount of an mRNA or protein present. Such assays are coupled with processes or systems to store and process information and to help quantify levels, signals, etc. and to digitize the information for use in comparing levels. The term “otherwise identical sample”, as used herein, refers to a sample similar to a first sample, that is, it is obtained in the same manner from the same subject from the same tissue or fluid, or it refers a similar sample obtained from a different subject. The term “otherwise identical sample from an unaffected subject” refers to a sample obtained from a subject not known to have the disease or disorder being examined. The sample may of course be a standard sample. By analogy, the term “otherwise identical” can also be used regarding regions or tissues in a subject or in an unaffected subject. As used herein, “parenteral administration” of a pharmaceutical composition includes any route of administration characterized by physical breaching of a tissue of a subject and administration of the pharmaceutical composition through the breach in the tissue. Parenteral administration thus includes, but is not limited to, administration of a pharmaceutical composition by injection of the composition, by application of the composition through a surgical incision, by application of the composition through Attorney Docket No.: 3062/193 PCT a tissue-penetrating non-surgical wound, and the like. In particular, parenteral administration is contemplated to include, but is not limited to, subcutaneous, intraperitoneal, intramuscular, intrasternal injection, and kidney dialytic infusion techniques. The term “pharmaceutical composition” refers to a composition comprising at least one active ingredient, whereby the composition is amenable to investigation for a specified, efficacious outcome in a mammal (for example, without limitation, a human). Those of ordinary skill in the art will understand and appreciate the techniques appropriate for determining whether an active ingredient has a desired efficacious outcome based upon the needs of the artisan. “Pharmaceutically acceptable” means physiologically tolerable, for either human or veterinary application. Similarly, “pharmaceutical compositions” include formulations for human and veterinary use. As used herein, the term “pharmaceutically acceptable carrier” means a chemical composition with which an appropriate compound or derivative can be combined and which, following the combination, can be used to administer the appropriate compound to a subject. As used herein, the term “physiologically acceptable” ester or salt means an ester or salt form of the active ingredient which is compatible with any other ingredients of the pharmaceutical composition, which is not deleterious to the subject to which the composition is to be administered. “Plurality” means at least two. “Polypeptide” refers to a polymer composed of amino acid residues, related naturally occurring structural variants, and synthetic non-naturally occurring analogs thereof linked via peptide bonds, related naturally occurring structural variants, and synthetic non-naturally occurring analogs thereof. “Synthetic peptides or polypeptides” refers to non-naturally occurring peptides or polypeptides. Synthetic peptides or polypeptides can be synthesized, for example, using an automated polypeptide synthesizer. Various solid phase peptide synthesis methods are known to those of skill in the art. As used herein, the term "mass spectrometry" (MS) refers to a technique for the identification and/or quantitation of molecules in a sample. MS includes ionizing the molecules in a sample, forming charged molecules; separating the charged molecules according to their mass-to-charge ratio; and detecting the charged molecules. MS allows for both the qualitative and quantitative detection of molecules in a sample. The molecules can be ionized and detected by any suitable means known to one of skill in the art. Some examples of mass spectrometry are "tandem mass spectrometry" or "MS/MS," which are the techniques wherein multiple rounds of mass spectrometry occur, either simultaneously using more than one mass analyzer or sequentially using a single mass analyzer. The term "mass spectrometry" can refer to the application of mass spectrometry to protein analysis. In some embodiments, electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) can be used in this context. In some embodiments, intact protein molecules can be ionized by the above techniques, and then introduced to a mass analyzer. Alternatively, protein molecules can be broken Attorney Docket No.: 3062/193 PCT down into smaller peptides, for example, by enzymatic digestion by a protease, such as trypsin. Subsequently, the peptides are introduced into the mass spectrometer and identified by peptide mass fingerprinting or tandem mass spectrometry. As used herein, the term "mass spectrometer" is used to refer an apparatus for performing mass spectrometry that includes a component for ionizing molecules and detecting charged molecules. Various types of mass spectrometers can be employed in the methods of the presently disclosed subject matter. For example, whole protein mass spectroscopy analysis can be conducted using time-of-flight (TOF) or Fourier transform ion cyclotron resonance (FT-ICR) instruments. For peptide mass analysis, MALDI time-of-flight instruments can be employed, as they permit the acquisition of peptide mass fingerprints (PMFs) at high pace. Multiple stage quadrupole-time-of-flight and the quadrupole ion trap instruments can also be used. The terms "high throughput protein identification," "proteomics" and other related terms are used herein to refer to the processes of identification of a large number or (in some cases, all) proteins in a certain protein complement. Post-translational protein modifications and quantitative information can also be assessed by such methods. One example of "high throughput protein identification" is a gel- based process that includes the pre-fractionation and purification of proteins by one-dimensional protein gel electrophoresis. The gel can then be fractionated into several molecular weight fractions to reduce sample complexity, and proteins can be in-gel digested with trypsin. The tryptic peptides are extracted from the gel, further fractionated by liquid chromatography, and analyzed by mass spectrometry. In another approach, a sample can be fractionated without using the gels, for example, by protein extraction followed by liquid chromatography. The proteins can then be digested in-solution, and the proteolytic fragments further fractionated by liquid chromatography and analyzed by mass spectrometry. As used herein, the term "Western blot," which can be also referred to as "immunoblot", and related terms refer to an analytical technique used to detect specific proteins in a sample. The technique uses gel electrophoresis to separate the proteins, which are then transferred from the gel to a membrane (typically nitrocellulose or PVDF) and stained, in membrane, with antibodies specific to the target protein. The expression "stable isotope labeling by amino acids in cell culture" (SILAC) is used herein to refer to an approach for incorporation of a label into proteins for mass spectrometry (MS)-based quantitative proteomics. SILAC comprises metabolic incorporation of a given "light" or "heavy" form of the amino acid into the proteins. For example, SILAC comprises the incorporation of amino acids with substituted stable isotopic nuclei (e.g. deuterium, ¹³ C, ¹⁵ N). In an illustrative SILAC experiment, two cell populations are grown in culture media that are identical, except that one of them contains a "light" and the other a "heavy" form of a particular amino acid (for example, ¹² C and ¹³ C labeled L- lysine, respectively). When the labeled analog of an amino acid is supplied to cells in culture instead of Attorney Docket No.: 3062/193 PCT the natural amino acid, it is incorporated into all newly synthesized proteins. After a number of cell divisions, each instance of the amino acid is replaced by its isotope-labeled analog. Since there is little chemical difference between the labeled amino acid and the natural amino acid isotopes, the cells behave substantially similar to the control cell population grown in the presence of a normal amino acid. The term “prevent”, as used herein, means to stop something from happening, or taking advance measures against something possible or probable from happening. In the context of medicine, “prevention” generally refers to action taken to decrease the chance of getting a disease or condition. It is noted that “prevention” need not be absolute, and thus can occur as a matter of degree. A “preventive” or “prophylactic” treatment is a treatment administered to a subject who does not exhibit signs, or exhibits only early signs, of a condition, disease, or disorder. A prophylactic or preventative treatment is administered for the purpose of decreasing the risk of developing pathology associated with developing the condition, disease, or disorder. The term “protein” typically refers to large polypeptides. Conventional notation is used herein to portray polypeptide sequences: the left-hand end of a polypeptide sequence is the amino-terminus; the right-hand end of a polypeptide sequence is the carboxyl-terminus. As used herein, the term “purified” and like terms relate to an enrichment of a molecule or compound relative to other components normally associated with the molecule or compound in a native environment. The term “purified” does not necessarily indicate that complete purity of the particular molecule has been achieved during the process. A “highly purified” compound as used herein refers to a compound that is in some embodiments greater than 90% pure, that is in some embodiments greater than 95% pure, and that is in some embodiments greater than 98% pure. As used herein, the term “mammal” refers to any member of the class Mammalia, including, without limitation, humans, and nonhuman primates such as chimpanzees and other apes and monkey species; farm animals such as cattle, sheep, pigs, goats, and horses; domestic mammals such as dogs and cats; laboratory animals including rodents such as mice, rats and guinea pigs, and the like. The term does not denote a particular age or sex. Thus, adult and newborn subjects, as well as fetuses, whether male or female, are intended to be included within the scope of this term. The term “subject” as used herein refers to a member of species for which treatment and/or prevention of a disease or disorder using the compositions and methods of the presently disclosed subject matter might be desirable. Accordingly, the term “subject” is intended to encompass in some embodiments any member of the Kingdom Animalia including, but not limited to the phylum Chordata (e.g., members of Classes Osteichythyes (bony fish), Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals), and all Orders and Families encompassed therein. Attorney Docket No.: 3062/193 PCT The compositions and methods of the presently disclosed subject matter are particularly useful for warm-blooded vertebrates. Thus, in some embodiments the presently disclosed subject matter concerns mammals and birds. More particularly provided are compositions and methods derived from and/or for use in mammals such as humans and other primates, as well as those mammals of importance due to being endangered (such as Siberian tigers), of economic importance (animals raised on farms for consumption by humans) and/or social importance (animals kept as pets or in zoos) to humans, for instance, carnivores other than humans (such as cats and dogs), swine (pigs, hogs, and wild boars), ruminants (such as cattle, oxen, sheep, giraffes, deer, goats, bison, and camels), rodents (such as mice, rats, and rabbits), marsupials, and horses. Also provided is the use of the disclosed methods and compositions on birds, including those kinds of birds that are endangered, kept in zoos, as well as fowl, and more particularly domesticated fowl, e.g., poultry, such as turkeys, chickens, ducks, geese, guinea fowl, and the like, as they are also of economic importance to humans. Thus, also provided is the use of the disclosed methods and compositions on livestock, including but not limited to domesticated swine (pigs and hogs), ruminants, horses, poultry, and the like. A “sample”, as used herein, refers in some embodiments to a biological sample from a subject, including, but not limited to, normal tissue samples, diseased tissue samples, biopsies, blood, saliva, feces, semen, tears, and urine. A sample can also be any other source of material obtained from a subject which contains proteins, cells, tissues, or fluid of interest. A sample can also be obtained from cell or tissue culture. The term “standard”, as used herein, refers to something used for comparison. For example, it can be a known standard agent or compound which is administered and used for comparing results when administering a test compound, or it can be a standard parameter or function which is measured to obtain a control value when measuring an effect of an agent or compound on a parameter or function. Standard can also refer to an “internal standard”, such as an agent or compound which is added at known amounts to a sample and is useful in determining such things as purification or recovery rates when a sample is processed or subjected to purification or extraction procedures before a marker of interest is measured. Internal standards are often a purified marker of interest which has been labeled, such as with a radioactive isotope, allowing it to be distinguished from an endogenous marker. A “subject” of analysis, diagnosis, or treatment is an animal. Such animals include mammals, in some embodiments, humans. As used herein, a “subject in need thereof” is a patient, animal, mammal, or human, who will benefit from the method of this presently disclosed subject matter. The term “substantially pure” describes a compound, e.g., a protein or polypeptide, which has been separated from components which naturally accompany it. Typically, a compound is substantially pure when in some embodiments at least 10%, in some embodiments at least 20%, in some embodiments at least 50%, in some embodiments at least 60%, in some embodiments at least 75%, in Attorney Docket No.: 3062/193 PCT some embodiments at least 90%, and in some embodiments at least 99% of the total material (by volume, by wet or dry weight, or by mole percent or mole fraction) in a sample is the compound of interest. Purity can be measured by any appropriate method, e.g., in the case of polypeptides by column chromatography, gel electrophoresis, or HPLC analysis. A compound, e.g., a protein, is also substantially purified when it is essentially free of naturally associated components or when it is separated from the native contaminants which accompany it in its natural state. The term “symptom”, as used herein, refers to any morbid phenomenon or departure from the normal in structure, function, or sensation, experienced by the patient and indicative of disease. In contrast, a “sign” is objective evidence of disease. For example, a bloody nose is a sign. It is evident to the patient, doctor, nurse, and other observers. A “therapeutic” treatment is a treatment administered to a subject who exhibits signs of pathology for the purpose of diminishing or eliminating those signs. A “therapeutically effective amount” of a compound is that amount of compound which is sufficient to provide a beneficial effect to the subject to which the compound is administered. As used herein, the phrase “therapeutic agent” refers to an agent that is used to, for example, treat, inhibit, prevent, mitigate the effects of, reduce the severity of, reduce the likelihood of developing, slow the progression of, and/or cure, a disease or disorder. The terms “treatment” and “treating” as used herein refer to both therapeutic treatment and prophylactic or preventative measures, wherein the object is to prevent or slow down (lessen) the targeted pathologic condition, prevent the pathologic condition, pursue or obtain beneficial results, and/or lower the chances of the individual developing a condition, disease, or disorder, even if the treatment is ultimately unsuccessful. Those in need of treatment include those already with the condition as well as those prone to have or predisposed to having a condition, disease, or disorder, or those in whom the condition is to be prevented. As used herein, the terms “vector”, “cloning vector”, and “expression vector” refer to a vehicle by which a polynucleotide sequence (e.g., a foreign gene) can be introduced into a host cell, so as to transduce and/or transform the host cell in order to promote expression (e.g., transcription and translation) of the introduced sequence. Vectors include plasmids, phages, viruses, etc. All genes, gene names, and gene products disclosed herein are intended to correspond to homologs and/or orthologs from any species for which the compositions and methods disclosed herein are applicable. Thus, the terms include, but are not limited to genes and gene products from humans and mice. It is understood that when a gene or gene product from a particular species is disclosed, this disclosure is intended to be exemplary only, and is not to be interpreted as a limitation unless the context in which it appears clearly indicates. As used herein the term “alkyl” refers to C _1-20 inclusive, linear (i.e., "straight-chain"), branched, or cyclic, saturated or at least partially and in some cases fully unsaturated (i.e., alkenyl and alkynyl) Attorney Docket No.: 3062/193 PCT hydrocarbon chains, including for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, octyl, ethenyl, propenyl, butenyl, pentenyl, hexenyl, octenyl, butadienyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, and allenyl groups. "Branched" refers to an alkyl group in which a lower alkyl group, such as methyl, ethyl or propyl, is attached to a linear alkyl chain. In some embodiments, the alkyl group is “lower alkyl.” "Lower alkyl" refers to an alkyl group having 1 to about 8 carbon atoms (i.e., a C _1-8 alkyl), e.g., 1, 2, 3, 4, 5, 6, 7, or 8 carbon atoms. In some embodiments, the alkyl is “higher alkyl.” "Higher alkyl" refers to an alkyl group having about 10 to about 20 carbon atoms, e.g., 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In certain embodiments, "alkyl" refers, in particular, to C _1-8 straight-chain alkyls. In other embodiments, “alkyl” refers, in particular, to C _1-8 branched-chain alkyls. Alkyl groups can optionally be substituted (a “substituted alkyl”) with one or more alkyl group substituents, which can be the same or different. The term "alkyl group substituent" includes but is not limited to alkyl, substituted alkyl, halo, arylamino, acyl, hydroxyl, aryloxyl, alkoxyl, alkylthio, arylthio, aralkyloxyl, aralkylthio, carboxyl, alkoxycarbonyl, oxo, and cycloalkyl. There can be optionally inserted along the alkyl chain one or more oxygen, sulfur or substituted or unsubstituted nitrogen atoms, wherein the nitrogen substituent is hydrogen, lower alkyl (also referred to herein as “alkylaminoalkyl”), or aryl. Thus, as used herein, the term "substituted alkyl" includes alkyl groups, as defined herein, in which one or more atoms or functional groups of the alkyl group are replaced with another atom or functional group, including for example, alkyl, substituted alkyl, halogen, aryl, substituted aryl, alkoxyl, hydroxyl, nitro, amino, alkylamino, dialkylamino, sulfate, and mercapto. The term "aryl" is used herein to refer to an aromatic moiety that can be a single aromatic ring, or multiple aromatic rings that are fused together, linked covalently, or linked to a common group, such as, but not limited to, a methylene or ethylene moiety. The common linking group also can be a carbonyl, as in benzophenone, or oxygen, as in diphenylether, or nitrogen, as in diphenylamine. The term "aryl" specifically encompasses heterocyclic aromatic compounds. The aromatic ring(s) can comprise phenyl, naphthyl, biphenyl, diphenylether, diphenylamine and benzophenone, among others. In particular embodiments, the term “aryl” means a cyclic aromatic comprising about 5 to about 10 carbon atoms, e.g., 5, 6, 7, 8, 9, or 10 carbon atoms, and including 5- and 6-membered hydrocarbon and heterocyclic aromatic rings. The aryl group can be optionally substituted (a “substituted aryl”) with one or more aryl group substituents, which can be the same or different, wherein “aryl group substituent” includes alkyl, substituted alkyl, aryl, substituted aryl, aralkyl, hydroxyl, alkoxyl, aryloxyl, aralkyloxyl, carboxyl, carbonyl, acyl, halo, nitro, alkoxycarbonyl, aryloxycarbonyl, aralkoxycarbonyl, acyloxyl, acylamino, aroylamino, carbamoyl, alkylcarbamoyl, dialkylcarbamoyl, arylthio, alkylthio, alkylene, and -NR'R'', Attorney Docket No.: 3062/193 PCT wherein R' and R'' can each be independently hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, and aralkyl. Thus, as used herein, the term "substituted aryl" includes aryl groups, as defined herein, in which one or more atoms or functional groups of the aryl group are replaced with another atom or functional group, including for example, alkyl, substituted alkyl, halogen, aryl, substituted aryl, alkoxyl, hydroxyl, nitro, amino, alkylamino, dialkylamino, sulfate, and mercapto. Specific examples of aryl groups include, but are not limited to, cyclopentadienyl, phenyl, furan, thiophene, pyrrole, pyran, pyridine, imidazole, benzimidazole, isothiazole, isoxazole, pyrazole, pyrazine, triazine, pyrimidine, quinoline, isoquinoline, indole, carbazole, and the like. The term “heteroaryl” refers to aryl groups wherein at least one atom of the backbone of the aromatic ring or rings is an atom other than carbon. Thus, heteroaryl groups have one or more non- carbon atoms selected from the group including, but not limited to, nitrogen, oxygen, and sulfur. As used herein, the term "acyl" refers to an organic carboxylic acid group wherein the -OH of the carboxyl group has been replaced with another substituent (i.e., as represented by RC(=O)—, wherein R is an alkyl, substituted alkyl, aralkyl, substituted aralkyl, aryl or substituted aryl group as defined herein). As such, the term "acyl" specifically includes arylacyl groups, such as an acetylfuran and a phenacyl group. Specific examples of acyl groups include acetyl and benzoyl. “Cyclic” and "cycloalkyl" refer to a non-aromatic mono- or multicyclic ring system of about 3 to about 10 carbon atoms, e.g., 3, 4, 5, 6, 7, 8, 9, or 10 carbon atoms. The cycloalkyl group can be optionally partially unsaturated. The cycloalkyl group also can be optionally substituted with an alkyl group substituent as defined herein, oxo, and/or alkylene. There can be optionally inserted along the cyclic alkyl chain one or more oxygen, sulfur or substituted or unsubstituted nitrogen atoms, wherein the nitrogen substituent is hydrogen, alkyl, substituted alkyl, aryl, or substituted aryl, thus providing a heterocyclic group. Representative monocyclic cycloalkyl rings include cyclopentyl, cyclohexyl, and cycloheptyl. Multicyclic cycloalkyl rings include adamantyl, octahydronaphthyl, decalin, camphor, camphane, and noradamantyl. The terms “heterocycle”, “heterocyclyl” “heterocycloalkyl” or “heterocyclic” refer to cycloalkyl groups (i.e., non-aromatic, cyclic groups as described hereinabove) wherein one or more of the backbone carbon atoms of a cyclic ring is replaced by a heteroatom (e.g., nitrogen, sulfur, or oxygen). Examples of heterocycles include, but are not limited to, tetrahydrofuran, tetrahydropyran, morpholine, dioxane, piperidine, piperazine, and pyrrolidine. Additional examples of heterocycles include, for example, the cyclic forms of sugars, such as ribose, glucose, galactose, and the like. “Alkylene" refers to a straight or branched bivalent aliphatic hydrocarbon group having from 1 to about 20 carbon atoms, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. The alkylene group can be straight, branched or cyclic. The alkylene group also can be optionally unsaturated and/or substituted with one or more "alkyl group substituents." There can be optionally Attorney Docket No.: 3062/193 PCT inserted along the alkylene group one or more oxygen, sulfur or substituted or unsubstituted nitrogen atoms (also referred to herein as “alkylaminoalkyl”), wherein the nitrogen substituent is alkyl as previously described. Exemplary alkylene groups include methylene (-CH ₂ -); ethylene (-CH ₂ -CH ₂ -); propylene (-(CH ₂ ) ₃ -); cyclohexylene (-C ₆ H ₁₀ -); -CH=CH—CH=CH-; -CH=CH-CH ₂ -; -(CH ₂ ) _q -N(R)- (CH ₂ ) _r -, wherein each of q and r is independently an integer from 0 to about 20, e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, and R is hydrogen or lower alkyl; methylenedioxyl (-O-CH ₂ -O-); and ethylenedioxyl (-O-(CH ₂ ) ₂ -O-). An alkylene group can have about 2 to about 3 carbon atoms and can further have 6-20 carbons. "Alkoxyl" or “alkoxy” refers to an alkyl-O- group wherein alkyl is as previously described. The term "alkoxyl" as used herein can refer to, for example, methoxyl, ethoxyl, propoxyl, isopropoxyl, butoxyl, t-butoxyl, and pentoxyl. The term “oxyalkyl” can be used interchangably with “alkoxyl”. The terms “aryloxy” and “aryloxyl” refer to an aryl-O-group, wherein aryl is as previously described. The term “aryloxy as used herein can refer to, for example, phenoxy, p-chlorophenoxy, p- fluorophenoxy, p-methylphenoxy, p-methoxyphenoxy, and the like. "Aralkyl" refers to an aryl-alkyl- group wherein aryl and alkyl are as previously described and include substituted aryl and substituted alkyl. Exemplary aralkyl groups include benzyl, phenylethyl, and naphthylmethyl. In some embodiments, the aromatic portion of the aralkyl group can be substituted by one or more aryl group substituents and/or the alkyl portion of the aralkyl group can be substituted by one or more alkyl group substituents and the aralkyl group can be a “substituted aralkyl” group. The term “amino” refers to the -NR’R” group, wherein R’ and R” are each independently selected from the group including H and substituted and unsubstituted alkyl, cycloalkyl, heterocycle, aralkyl, aryl, and heteroaryl. In some embodiments, the amino group is -NH ₂ . The terms “alkylamino” and “aminoalkyl” refer to a -NHR group where R is alkyl or substituted alkyl. The term “arylamino” refers to a -NHR group where R is aryl or substituted aryl. The term “carbonyl” refers to the -(C=O)- or a double bonded oxygen substituent attached to a carbon atom of a previously named parent group. The terms “carboxylate” and “carboxylic acid” can refer to the groups -C(=O)-O- and -C(=O)- OH, respectively. In some embodiments, “carboxylate” can refer to either the -C(=O)-O- or -C(=O)-OH group. In some embodiments, the term “carboxyl” can also be used to refer to a carboxylate or carboxylic acid group. The terms “sulfonyl”, “sulfone”, and “sulphone” as used herein refer to the -S(=O) ₂ - or - S(=O) ₂ R group, wherein R is alkyl, substituted alkyl, cycloalkyl, heterocycloalkyl, aralkyl, substituted aralkyl, aryl, substituted aryl, heteroaryl, or substituted heteroaryl. The term “sulfonamide” refers to the -S(=O) ₂ -N(R) ₂ group, wherein each R is independently selected from H, alkyl, substituted alkyl, aralkyl, substituted aralkyl, aryl, and substituted aryl, or Attorney Docket No.: 3062/193 PCT wherein the two R together can for a ring with the nitrogen atom (e.g., wherein the two R are together an alkylene group, such as a butylene or pentylene group). The term “sulfonate” as used herein refers to a -S(=O) ₂ -O-R group, wherein R is selected from alkyl, substituted alkyl, aralkyl, substituted aralkyl, aryl, and substituted aryl. The terms "halo", "halide", or "halogen" as used herein refer to fluoro, chloro, bromo, and iodo groups. The term “perhaloalkyl” refers to an alkyl group wherein all of the hydrogen atoms are replaced by halo. Thus, for example, perhaloalkyl can refer to a “perfluroalkyl” group wherein all of the hydrogen atoms of the alkyl group are replaced by fluoro. Perhaloalkyl groups include, but are not limited to, - CF ₃ . The terms "hydroxyl" and “hydroxy” refer to the -OH group. The term “oxo” refers to a compound described previously herein wherein a carbon atom is replaced by an oxygen atom. The term “thio” refers to the -S- or -SH group. The terms “alkylthio” and “thioalkyl” refer to a -SR group where R is alkyl or substituted alkyl. The term “arylthiol” refers to a -SR group where R is aryl or substituted aryl. The term “cyano” refers to the -CN group. The term “nitro” refers to the -NO ₂ group. A line crossed by a wavy line, e.g., in the structure: indicates the site where the indicated substituent can bond to another group. II. General Considerations Covalent probes serve as invaluable tools for the global investigation of protein function and ligand binding capacity. While several probes have been deployed for the interrogation of nucleophilic residues such as cysteine (e.g., iodoacetamide-alkyne (IA-Alkyne)), lysine (NaTFBS-Alkyne), and methionine, a large fraction of the human proteome still remains inaccessible to pharmacological modulation. Activity-based protein profiling (ABPP) utilizes active-site directed chemical probes to measure the functional state of large numbers of enzymes in native biological systems (e.g., cells or tissues). Activity-based probes consist of a reactive group for targeting a specific enzyme class and a reporter tag for detection by in-gel fluorescence scanning or by avidin-enrichment coupled with liquid chromatography mass spectrometry (LCMS), respectively. For ligand (e.g., inhibitor) discovery, the Attorney Docket No.: 3062/193 PCT potency and selectivity of small molecules can be profiled against many enzymes in parallel by performing competitive ABPP in complex proteomes, where ligands compete for probe labeling of enzyme targets. Exemplary approaches to ABPP are disclosed in U.S. Patent Application Publication No.2022/0251085, which is incorporated herein by reference in its entirety. Purines are essential components of DNA and RNA and have been fine-tuned by nature for biological activity. Purines have historically been explored as a scaffold for the development of inhibitors but their application in chemical biology as probes to discover new target proteins and druggable sites has been limited. In one aspect, the presently disclosed subject matter relates to purine- derived chemical probes and their use as chemoproteomic tools for activity-based profiling of the proteome (e.g., the human proteome). The chemical structure of purine with the atoms numbered, is shown at the top of Scheme 1, below. Scheme 1 further shows structures of the four purine tautomers, the main two tautomers, i.e., 9H-purine and 7H-purine, and the two minor tautomers, i.e., 3H-purine and 1H-purine. Chemically, the reactions of purine reflect the interplay between the constituent pyrimidine and imidazole rings of the purine scaffold. Electron localization within the nitrogen atoms of the pyrimidine ring can render the C2 and particularly the C6 site amenable to nucleophilic attack by protein residues. Thus, one aspect of the presently disclosed purine-based probes (and ligands) is the addition of an electrophilic group (e.g., a chloro or other halo group) on the pyrimidine ring that can serve as an effective leaving group during nucleophilic attack by a nucleophilic group on a side chain of a protein residue. See Figures 1A and 6A. Meanwhile, the more electron rich imidazole ring can provide sites for facile derivatization, e.g., to attach detectable tags or taggable groups. Alternatively, the electron rich imidazole ring can serve for derivatization of the purine scaffold to provide a wide variety of covalent protein modulators (e.g., inhibitors or activators), also referred to herein as “ligands”. Figure 6A shows the mechanism whereby a covalent protein/probe adduct is formed when the probe is contacted with a protein having a reactive nucleophilic amino acid residue. After adduct formation (e.g., in a cell lysate, a live cell, a tissue, a living organism, or another sample comprising one or more proteins), the covalent protein/probe adduct can be analyzed in-gel and/or by LC-MS/MS. More particularly, as disclosed herein, purine-based clickable electrophilic purines (CEPs) can be used as probes in a photo-activatable-competition and chemoproteomic enrichment (PACCE) method to target reactive cysteines (e.g., RNA-sensitive purine- reactive cysteines) in RBPs and quantify protein-RNA binding activity. Attorney Docket No.: 3062/193 PCT Scheme 1. Chemical Structure of Purine and Purine Tautomers. III. Methods of Detecting RNA-Binding in Proteins As described above, proteomic methods for RNA interactome capture (RIC) rely principally on crosslinking native or labeled cellular RNA to enrich and investigate RBP composition and function in cells. The ability to measure RBP activity at individual binding sites by RIC, however, has been challenging due to the heterogenous nature of peptide adducts derived from the RNA-protein crosslinked site. According to one aspect of the presently disclosed subject matter, an orthogonal strategy is disclosed that utilizes CEPs to directly quantify protein-RNA interactions on proteins through photoaffinity competition with 4-thiouridine (4SU)-labeled RNA in cells. As described in the examples below, this strategy, i.e., PACCE, facilitated the detection of >5,500 cysteine sites across ~3,000 proteins displaying RNA-sensitive alterations in probe binding. Importantly, PACCE provided for the functional profiling of canonical RNA-binding domains as well as the discovery of “moonlighting” RNA binding activity in the human proteome. As shown in Figure 4A, the presently disclosed method can detect allosteric sites involved in the modulation of RNA-protein binding, as several RNA-sensitive cysteine sites were identified via the PACCE method that were located relatively far from the RNA (e.g., more than 50 angstroms or more than 100 angstroms or about 150 angstroms from the RNA). Collectively, the present method provides a unique chemoproteomic platform for global quantification of protein-RNA binding activity in living cells. Thus, in some embodiments the presently disclosed subject matter relates to methods for identifying RNA-binding sites in proteins. The term “identifying an RNA-binding site” as used herein can refer to detecting the presence of RNA-binding activity in a protein sample (e.g., detecting the presence of an RNA-binding residue, sequence, or domain in a protein present in a sample). In some embodiments, “identifying an RNA-binding site” refers to determining the identity of the RBP and/or determining a particular amino acid residue or sequence (or domain) in the RBP associated with the Attorney Docket No.: 3062/193 PCT RNA binding. In some embodiments, “identifying an RNA-binding site” refers to identifying an RBP and quantitating the protein-RNA binding activity. In some embodiments, “identifying an RNA-binding site” refers to identifying an RNA- sensitive (RS)-amino acid residue (e.g., an RS-cysteine residue (RS-Cys)) in an RBP. The RS-amino acid residue (e.g., RS-Cys) identified according to the presently disclosed methods represent potential “druggable” sites, the modification of which can be used to modulate a biological activity of the RBP. Thus, in some embodiments, the presently disclosed subject matter provides a method of identifying a protein or peptide that contains a druggable cysteine residue and/or of identifying the druggable cysteine residue itself (i.e., identifying a particular druggable cysteine residue in a particular protein). In some embodiments, the presently disclosed subject matter provides a method of identifying a protein or peptide that contains a druggable amino acid residue other than cysteine and/or of identifying the druggable amino acid residue other than cysteine itself (i.e., identifying a particular druggable amino acid residue in a particular protein). III.A. PACCE in Living Cells In some embodiments, the presently disclosed method comprises, consists essentially of, or consists of: (a) separating a protein sample comprising live cells into a first sample and a second sample; (b) culturing the first sample (which can also be referred to as the “heavy” sample) in a first cell culture medium comprising heavy isotopes and 4-thiouridine (4SU) for a first period of time (e.g., wherein the first cell culture medium comprises ¹³ C- and/or ¹⁵ N-labeled amino acids), thereby providing cultured cells comprising a 4SU-RNA and exposing the first sample (i.e., after the first period of time) to ultraviolet (UV) light to covalently crosslink the 4SU-RNA to an aromatic amino acid residue in an RBD of an RBP if an RBP is present in the first sample; (c) culturing the second sample (which can also be referred to as the “light” sample) in a second cell culture medium comprising a naturally occurring isotope distribution for the first period of time, wherein the second culture medium further comprises 4SU or wherein the second sample is exposed to the UV light; (d) contacting the first sample and the second sample with an electrophilic probe compound for a second period of time, thereby modifying one or more probe-reactive amino acid residue in one or more proteins in the first sample and/or in the second sample to provide one or more modified amino acid residues, wherein the electrophilic probe compound has a structure of Formula (I): wherein: represents a single or a double bond, subject to the proviso that one of Formula (I) is a single bond and the other of Formula (I) is a double bond; X covalently attached to the Attorney Docket No.: 3062/193 PCT nitrogen atom at position 7 (N7) or the nitrogen atom at position 9 (N) and is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; and R ₁ and R ₂ are independently selected from the group comprising H, halo, amino (e.g., - NH ₂ ), alkyl (e.g., C1-C5 alkyl), alkoxy (e.g., C1-C5 alkoxy), alkylthio, aryloxy, arylthiol, and arylamino, subject to the proviso that at least one of R ₁ and R ₂ is halo; (e) analyzing proteins or protein digests from the first sample and the second sample to detect the one or more modified amino acid residues; and (f) determining the presence of an RNA-binding site in a protein by detecting a difference in probe-reactive amino acid residue modification between the first sample and the second sample. In some embodiments, at least one of R ₁ and R ₂ is halo. As indicated above, X (which comprises a detectable or taggable group) is attached at one of the nitrogen atoms of the imidazole ring (i.e., N9 or N7). Thus, the electrophilic probe compound of Formula (I) has a structure of Formula (Ia) or a structure of Formula (Ib):       Formula (Ia) Formula (Ib)  wherein X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; and R ₁ and R ₂ are independently selected from the group comprising H, halo (e.g., Cl, Br, F, or I), amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthiol, and arylamino, subject to the proviso that at least one of R ₁ and R ₂ is halo. In some embodiments, R ₁ and R ₂ are independently selected from the group consisting of H, halo, and amino, subject to the proviso that at least one of R ₁ and R ₂ is halo (e.g., Cl or F). In some embodiments, the reactive amino acid residue is selected from the group comprising cysteine, lysine, and tyrosine. In some embodiments, the reactive amino acid residue is a cysteine residue. When the reactive amino acid reside is a cysteine residue, the sulfur atom of the cystine residue can replace R ₁ or R ₂ in the electrophilic probe compound to form a covalent adduct. Thus, in some embodiments, the modified amino acid residue has a structure of Formula (IIa-i), Formula (IIb-i), Formula (IIa-ii), or Formula (IIb-ii). Attorney Docket No.: 3062/193 PCT Formula (IIa-i), Formula (IIb-i), Formula (IIa-ii), or Formula (IIb-ii), wherein: X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; R ₁ is selected from the group consisting of H, halo, amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthio, and arylamino; and R ₂ is selected from the group consisting of H, halo, amino, alkyl, alkoxy, alkylthio, alkylamino, aryloxy, arylthio, and arylamino. In some embodiments, X comprises a fluorophore or a detectable labeling group. The fluorophore of X can be any suitable fluorophore. In some embodiments, the fluorophore is selected from the group including, but not limited to, rhodamine, rhodol, fluorescein, thiofluorescein, aminofluorescein, carboxyfluorescein, chlorofluorescein, methylfluorescein, sulfofluorescein, aminorhodol, carboxyrhodol, chlororhodol, methylrhodol, sulforhodol; aminorhodamine, carboxyrhodamine, chlororhodamine, methylrhodamine, sulforhodamine, thiorhodamine, cyanine, indocarbocyanine, oxacarbocyanine, thiacarbocyanine, merocyanine, cyanine 2, cyanine 3, cyanine 3.5, cyanine 5, cyanine 5.5, cyanine 7, oxadiazole derivatives, pyridyloxazole, nitrobenzoxadiazole, benzoxadiazole, pyren derivatives, cascade blue, oxazine derivatives, Nile red, Nile blue, cresyl violet, oxazine 170, acridine derivatives, proflavin, acridine orange, acridine yellow, arylmethine derivatives, auramine, crystal violet, malachite green, tetrapyrrole derivatives, porphin, phtalocyanine, bilirubin 1- dimethylaminonaphthyl-5-sulfonate, 1-anilino-8-naphthalene sulfonate, 2-p-touidinyl-6-naphthalene sulfonate, 3-phenyl-7-isocyanatocoumarin, N-(p-(2-benzoxazolyl)phenyl)maleimide, stilbenes, pyrenes, 6-FAM (Fluorescein), 6-FAM (NHS Ester), 5(6)-FAM, 5-FAM, Fluorescein dT, 5-TAMRA- cadavarine, 2-aminoacridone, HEX, JOE (NHS Ester), MAX, TET, ROX, and TAMRA. In some embodiments, X comprises a fluorophore moiety. In some cases, the fluorophore of X is obtained from a compound library. In some cases, the compound library comprises ChemBridge fragment library, Pyramid Platform Fragment-Based Drug Discovery, Maybridge fragment library, FRGx from AnalytiCon, TCI-Frag from AnCoreX, Bio Building Blocks from ASINEX, BioFocus 3D Attorney Docket No.: 3062/193 PCT from Charles River, Fragments of Life (FOL) from Emerald Bio, Enamine Fragment Library, IOTA Diverse 1500, BIONET fragments library, Life Chemicals Fragments Collection, OTAVA fragment library, Prestwick fragment library, Selcia fragment library, TimTec fragment-based library, Allium from Vitas-M Laboratory, or Zenobia fragment library. In some embodiments, X is a monovalent moiety comprising an alkyne group (i.e., a carbon- carbon triple bond). For example, in some embodiments, X comprises or consists of -C ^CH, -alkylene- C ^CH, -C(=O)-alkylene-C ^CH, or -C(=O)-NH-alkylene-C ^CH (e.g., C(=O)-NH-CH ₂ -C ^CH). In some embodiments, the alkylene group is a C ₁ -C ₅ alkylene group. In some embodiments, the alkylene group is methylene. In some embodiments, X is a propargyl group, i.e., -CH ₂ -C ^CH. In some embodiments, e.g., in the electrophilic probe compound (i.e., the compound of Formula (I), (Ia), or (Ib)), R ₁ and R ₂ are selected from H, halo, and amino. In some embodiments, R ₁ and R ₂ are selected from H and halo. In some embodiments, R ₁ is chloro or fluoro, R ₂ is chloro or fluoro, or both R ₁ and R ₂ are chloro or fluoro. In some embodiments, R ₁ and R ₂ are the same. In some embodiments, R ₁ and R ₂ are different. Examples of purine-based probe compounds that can be used in the presently disclosed methods are described, for example, in U.S. Patent Application Publication No.2022/0251085, the disclosure of which is incorporated herein by reference in its entirety. In some embodiments, the electrophilic probe compound is selected from the group comprising 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine, 2,6- dichloro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-7-(prop-2-yn-1-yl)-7H-purine, 6-chloro-9-(prop-2- yn-1-yl)-9H-purine, 2-chloro-7-(prop-2-yn-1-yl)-7H-purine, 2-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2,6-difluoro-7-(prop-2-yn-1-yl)-7H-purine, 2,6,-difluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2- fluoro-7-(prop-2-yn-1-yl)-7H-purine, 6-chloro-2-fluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2- amino-7-(prop-2-yn-1-yl)-7H-purine, and 6-chloro-2-amino-9-(prop-2-yn-1-yl)-9H-purine. In some embodiments, the electrophilic probe compound is selected from 2,6-dichloro-7-(prop-2-yn-1-yl)-7H- purine (also referred to herein as “AHL-Pu-1”) and 2,6-dichloro-9-(prop-2-yn-1-yl)-9H-purine (also referred to herein as “AHL-Pu-2”). Culturing conditions can be selected to provide sufficient conditions for incorporation of the heavy isotopes into proteins in the cells and/or for incorporating 4SU into cellular RNA, as well as for minimizing damage to the cells. Suitable culturing conditions can be selected based on cell type, temperature, heavy isotope concentration, and/or 4SU concentration. In some embodiments, the first period of time is about 1 hour to about 24 hours (e.g., about 1, about 2, about 3, about 4, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, about 11 hours, about 12 hours, about 13 hours, about 14 hours, about 15 hours, about 16 hours, about 17 hours, about 18 hours, about 19 hours, about 20 hours, about 21 hours, about 22 hours, about 23 hours or about 24 hours). In some embodiments, the first period of time is about 16 hours. Culturing can be performed at about 37°C. In some embodiments, the first cell culture medium comprises about 50 micromolar (µM) to Attorney Docket No.: 3062/193 PCT about 500 µM 4SU (e.g., about 50 µM, about 75 µM, about 100 µM, about 125 µM, about 150 µM, about 175 µM, about 200 µM, about 225 µM, about 250 µM, about 275 µM, about 300 µM, about 325 µm, about 350 µM, about 375 µM, about 400 µM, about 425 µM, about 450 µM, about 475 µM, or about 500 µM 4SU). In some embodiments, the first cell culture medium comprises about 100 µM 4SU. The UV light can be provided at a wavelength sufficient for crosslinking 4SU-RNA to a protein residue present in an RNA-binding domain. In some embodiments, the protein residue being crosslinked to the 4SU-RNA is a tyrosine (Tyr or Y) residue, a phenylalanine (Phe or F) residue, or a tryptophan (Trp or W) residue. In some embodiments, the UV light has a wavelength of about 200 nanometers to about 400 nm. In some embodiments, the UV light has a wavelength of about 254 nm to about 365 nm. Higher wavelengths (lower energy) can improve specificity, but provide lower coverage of RNA-sensitive sites, while lower wavelengths (higher energy) can provide higher coverage of RNA-sensitive sites, but lower sensitivity. In some embodiments, the UV light has a wavelength of about 254 nm. In some embodiments, the UV light has a wavelength of about 365 nm. In some embodiments, the UV light has a wavelength of about 312 nm. In some embodiments, the UV exposure is performed to provide an exposure of about 1 Joule per square centimeter (J/cm ² ). In some embodiments, the second cell culture medium comprises 4SU and the second sample is not exposed to UV light. Thus, in some embodiments, step (c) comprises culturing the second sample in a second cell culture medium comprising a naturally occurring isotope distribution and further comprising 4SU for the first period of time to provide cultured cells comprising 4SU-RNA, but not exposing the second sample to UV light. Under these conditions, the method is configured to study competition between probe binding and binding of cellular native RNA and cellular 4SU-RNA. The second cell culture medium can contain about the same concentration of 4SU as the first cell culture medium. In some embodiments, the second cell culture medium comprises about 50 µM to about 500 µM 4SU (e.g., about 50 µM, about 75 µM, about 100 µM, about 125 µM, about 150 µM, about 175 µM, about 200 µM, about 225 µM, about 250 µM, about 275 µM, about 300 µM, about 325 µm, about 350 µM, about 375 µM, about 400 µM, about 425 µM, about 450 µM, about 475 µM, or about 500 µM 4SU). In some embodiments, the second cell culture medium comprises about 100 µM 4SU. Alternatively, in some embodiments, step (c) comprises culturing the second sample in a second cell culture medium comprising a naturally occurring isotope distribution for the first period of time and in the absence of 4SU and (after the first period of time) exposing the second sample to the UV light. Under these conditions, the method is configured to study competition between probe binding and cellular 4SU-RNA binding. The UV light exposure conditions can be the same as for the first sample. Thus, in some embodiments, the UV light has a wavelength of about 312 nm. In some embodiments, the exposure is performed to provide an exposure of about 1 J/cm ² . Attorney Docket No.: 3062/193 PCT Following the culturing of step (b) and step (c), the first sample and the second sample are each exposed to an electrophilic probe compound for the second period of time. For instance, the first sample can be exposed to media comprising the electrophilic probe compound and the second sample can be separately exposed to media comprising the same electrophilic probe compound. The contacting conditions (i.e. temperature, time, probe compound concentration, cell concentration) for the two samples can be the same. In some embodiments, the contacting of step (d) can be performed under conditions where the concentration of the electrophilic probe compound is about 1 µM to about 50 µM (e.g., about 1 µM, about 5 µM, about 10 µM, about 15 µM, about 20 µM, about 25 µM, about 30 µM, about 35 µM, about 40 µM, about 45 µM, or about 50 µM). In some embodiments, the concentration of the electrophilic probe compound is about 25 µM. In some embodiments, the second period of time is about 15 minutes to about 4 hours (e.g., about 15 minutes, about 30 minutes, about 1 hour, about 1.5 hours, about 2 hours, about 2.5 hours, about 3 hours, about 3.5 hours, or about 4 hours). In some embodiments, the second period of time is about 1 hour. In some embodiments, prior to step (e), cells in the first sample and cells in the second sample are lysed to provide a heavy proteome and a light proteome. In some embodiments, the heavy proteome and the light proteome are mixed together to provide a proteome mixture comprising the at least one modified reactive amino acid residue. For example, the cells can be lysed after the second period of time and lysate from the first sample and lysate from the second sample can be mixed together. In some embodiments, analyzing step (e) further comprises tagging the at least one modified reactive amino acid residue with a compound comprising a detectable labeling group, thereby forming at least one tagged reactive amino acid residue comprising said detectable labeling group. In some embodiments, the detectable labeling group comprises biotin or a biotin derivative. In some embodiments, the biotin derivative is desthiobiotin. In some embodiments, the tagging comprises reacting an alkyne group in the X moiety of the at least one modified reactive amino acid residue with a compound comprising: (i) an azide moiety; and (ii) the detectable labeling group. In some embodiments, the tagging comprises, consists essentially of, or consists of performing a copper- catalyzed azide-alkyne cycloaddition (CuAAC) coupling reaction. Conditions for conducting CuAAC reactions and other “click” chemistry reactions that can be used to tag a modified reactive amino acid residue are known in the art. In some embodiments analyzing step (e) further comprises digesting proteins to provide a digested protein sample comprising a protein fragment comprising the at least one tagged reactive amino acid moiety comprising the detectable labeling group. In some embodiments, the digesting is performed with trypsin. In some embodiments, analyzing step (e) further comprises enriching the digested protein sample for the detectable labeling group. In some embodiments, the enriching comprises contacting the digested protein sample with a solid support comprising a binding partner of the detectable labeling group. For example, if the detectable labeling group comprises biotin or a Attorney Docket No.: 3062/193 PCT derivative thereof, the solid support can comprise streptavidin. In some embodiments, the binding partner comprises an antibody that binds the detectable labeling group. In some embodiments of the presently disclosed methods, analyzing step (e) further comprises analyzing the digested protein sample or the enriched digested protein sample via liquid chromatography-mass spectrometry (LC-MS). In some embodiments, the LC-MS comprises liquid chromatography tandem mass spectrometry (LC-MS/MS). RNA-sensitivity can be detected using the PACCE method by comparing protein modification between the first sample and the heavy sample. In some embodiments, the comparing comprises determining one or more SILAC ratio from MS analysis of the enriched digested protein sample. Methods of determining SILAC ratios are known in the art. For example, a SILAC ratio can be determined by comparing the MS intensities (e.g., the relative MS1 intensities) of matched “light” and “heavy” peptide signals. Stated another way, a SILAC ratio can be a ratio of intensities between a peak or peaks of a peptide fragment corresponding to a light peptide and a peak or peaks of a corresponding heavy peptide. Generally, the more sensitive a site is to RNA binding, the higher the SILAC ratio associated with the corresponding peptide fragment (i.e., RNA-binding in the heavy sample blocks electrophilic probe compound binding, resulting in a lower intensity of a signal in a heavy peptide fragment compared to the signal in the corresponding light peptide fragment). See Figure 15A. In some embodiments, an RNA-sensitive site is identified when a peptide fragment is detected to have a SILAC ratio that is about 2 or greater (e.g., >2). Particular amino acid sequences of peptide fragments can be detected by analysis of the MS fragmentation spectrum. Accordingly, in some embodiments, the analyzing comprises determining a SILAC ratio of 2 or more for at least one peptide in the enriched digested protein sample, thereby identifying that the peptide is a fragment from an RBP and/or identifying a probe-reactive amino acid residue in said peptide as being an RNA-sensitive amino acid residue in said RBP. In some embodiments, the RNA-sensitive amino acid residue is located close to an RNA-binding residue (e.g., inside an RNA-binding domain or pocket). In some embodiments, the RNA-sensitive amino acid is farther away from the site of RNA binding (e.g., at least about 50, at least about 100, or at least about 150 angstroms from a site of RNA binding.). Accordingly, in some embodiments, the analyzing comprises determining a SILAC ratio of 2 or more for at least one peptide in the enriched digested protein sample, thereby identifying that the peptide is a fragment from an RBP. In some embodiments, the method further comprises identifying a probe-reactive amino acid residue in said peptide as being an RNA-sensitive amino acid residue in said RBP (e.g., by analysis of fragmentation patterns of the peptide). In some embodiments, the RNA- sensitive amino acid residue is present at an allosteric site in the RBP (i.e., the RNA-sensitive amino acid residue is an allosteric RS-residue). Attorney Docket No.: 3062/193 PCT III.B. PACCE for Protein-Containing Samples While there are many benefits for preforming the presently disclosed method in live cells, in some embodiments, the presently disclosed method can be performed on other samples that comprises proteins, including in samples comprising isolated proteins or cell lysates. Thus, in some embodiments, the presently disclosed method does not include incorporating 4SU into cellular RNA, but rather supplying a 4SU-RNA to a protein sample from another source. In some embodiments, the presently disclosed subject matter provides a method for identifying an RNA-binding site in a protein by a method comprising, consisting essentially of, or consisting of the following steps: (a) providing a protein sample comprising isolated proteins, living cells, or a cell lysate; (b) contacting a protein sample with a 4SU-RNA (e.g., a synthetic 4SU-RNA); (c) exposing the protein sample to UV light to form a covalent crosslink between the 4SU-RNA and an RNA-binding site when an RNA-binding site is present in a protein in the protein sample; (d) contacting the protein sample with an electrophilic probe compound for a period of time sufficient for the electrophilic probe compound to react with at least one probe reactive amino acid in the protein sample, wherein the electrophilic probe compound has a structure of Fomula (I): wherein: represents a single or a double bond, subject to the proviso that one of Formula (I) is a single bond and the other of Formula (I) is a double bond; X covalently attached to the nitrogen atom at position 7 (N7) or the nitrogen atom at position 9 (N) and is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; and R ₁ and R ₂ are independently selected from the group comprising H, halo, amino (e.g., - NH ₂ ), alkyl (e.g., C1-C5 alkyl), alkoxy (e.g., C1-C5 alkoxy), alkylthio, aryloxy, arylthiol, and arylamino, subject to the proviso that at least one of R ₁ and R ₂ is halo; (e) analyzing proteins in the protein sample to determine the presence and/or amount of one or more probe modified amino acid residues, thereby determining a protein sample modification profile; (f) analyzing proteins in a control sample treated as described for steps (b) and (d), but not exposed to the ultraviolet light to detect the presence and/or amount of one or more probe modified amino acid residues, thereby determining a control sample modification profile; and (g) determining the presence of at least one RNA-binding site in the protein sample when the protein modification profile in (e) has fewer probe modified amino acid residues and/or a lower amount of one or more modified amino acid residues compared to the control sample modification profile. As described above, the electrophilic probe compound of Formula (I) has a structure of Formula (Ia) or a structure of Formula (Ib): Attorney Docket No.: 3062/193 PCT         ,        Formula (Ia) Formula (Ib)  wherein: X is a monovalent moiety comprising an alkyne moiety, a fluorophore moiety, a detectable labeling group, or a combination thereof; and R ₁ and R ₂ are independently selected from the group comprising H, halo, amino, alkyl, alkoxy, alkylthio, aryloxy, arylthiol, and arylamino, subject to the proviso that at least one of R ₁ and R ₂ is halo. In some embodiments, the probe-reactive amino acid residue is a cysteine residue, a tyrosine residue or a lysine residue. In some embodiments, the probe-reactive amino acid is a cysteine residue. Thus, in some embodiments, the probe-modified amino acid residue has a structure of one of Formulas (IIa-i), (IIb-i), (IIa-ii) or (IIb-ii) as described above. In some embodiments, X comprises a fluorophore or a detectable labeling group. The fluorophore of X can be any suitable fluorophore. In some embodiments, the fluorophore is selected from the group including, but not limited to, rhodamine, rhodol, fluorescein, thiofluorescein, aminofluorescein, carboxyfluorescein, chlorofluorescein, methylfluorescein, sulfofluorescein, aminorhodol, carboxyrhodol, chlororhodol, methylrhodol, sulforhodol; aminorhodamine, carboxyrhodamine, chlororhodamine, methylrhodamine, sulforhodamine, thiorhodamine, cyanine, indocarbocyanine, oxacarbocyanine, thiacarbocyanine, merocyanine, cyanine 2, cyanine 3, cyanine 3.5, cyanine 5, cyanine 5.5, cyanine 7, oxadiazole derivatives, pyridyloxazole, nitrobenzoxadiazole, benzoxadiazole, pyren derivatives, cascade blue, oxazine derivatives, Nile red, Nile blue, cresyl violet, oxazine 170, acridine derivatives, proflavin, acridine orange, acridine yellow, arylmethine derivatives, auramine, crystal violet, malachite green, tetrapyrrole derivatives, porphin, phtalocyanine, bilirubin 1- dimethylaminonaphthyl-5-sulfonate, 1-anilino-8-naphthalene sulfonate, 2-p-touidinyl-6-naphthalene sulfonate, 3-phenyl-7-isocyanatocoumarin, N-(p-(2-benzoxazolyl)phenyl)maleimide, stilbenes, pyrenes, 6-FAM (Fluorescein), 6-FAM (NHS Ester), 5(6)-FAM, 5-FAM, Fluorescein dT, 5-TAMRA- cadavarine, 2-aminoacridone, HEX, JOE (NHS Ester), MAX, TET, ROX, and TAMRA. In some embodiments, X comprises a fluorophore moiety. In some cases, the fluorophore of X is obtained from a compound library. In some cases, the compound library comprises ChemBridge fragment library, Pyramid Platform Fragment-Based Drug Discovery, Maybridge fragment library, FRGx from AnalytiCon, TCI-Frag from AnCoreX, Bio Building Blocks from ASINEX, BioFocus 3D from Charles River, Fragments of Life (FOL) from Emerald Bio, Enamine Fragment Library, IOTA Diverse 1500, BIONET fragments library, Life Chemicals Fragments Collection, OTAVA fragment Attorney Docket No.: 3062/193 PCT library, Prestwick fragment library, Selcia fragment library, TimTec fragment-based library, Allium from Vitas-M Laboratory, or Zenobia fragment library. In some embodiments, X is a monovalent moiety comprising an alkyne group (i.e., a carbon- carbon triple bond). For example, in some embodiments, X comprises or consists of -C ^CH, -alkylene- C ^CH, -C(=O)-alkylene-C ^CH, or -C(=O)-NH-alkylene-C ^CH (e.g., C(=O)-NH-CH ₂ -C ^CH). In some embodiments, the alkylene group is a C ₁ -C ₅ alkylene group. In some embodiments, the alkylene group is methylene. In some embodiments, X is a propargyl group, i.e., -CH ₂ -C ^CH. In some embodiments, e.g., in the electrophilic probe compound (i.e., the compound of Formula (I), (Ia), or (Ib)), R ₁ and R ₂ are selected from H, halo, and amino. In some embodiments, R ₁ and R ₂ are selected from H and halo. In some embodiments, R ₁ is chloro or fluoro, R ₂ is chloro or fluoro, or both R ₁ and R ₂ are chloro or fluoro. In some embodiments, R ₁ and R ₂ are the same. In some embodiments, R ₁ and R ₂ are different. In some embodiments, the electrophilic probe compound is selected from the group comprising 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine, 2,6-dichloro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-7- (prop-2-yn-1-yl)-7H-purine, 6-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2-chloro-7-(prop-2-yn-1-yl)-7H- purine, 2-chloro-9-(prop-2-yn-1-yl)-9H-purine, 2,6-difluoro-7-(prop-2-yn-1-yl)-7H-purine, 2,6,- difluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2-fluoro-7-(prop-2-yn-1-yl)-7H-purine, 6-chloro-2- fluoro-9-(prop-2-yn-1-yl)-9H-purine, 6-chloro-2-amino-7-(prop-2-yn-1-yl)-7H-purine, and 6-chloro-2- amino-9-(prop-2-yn-1-yl)-9H-purine. In some embodiments, the electrophilic probe compound is selected from 2,6-dichloro-7-(prop-2-yn-1-yl)-7H-purine (also referred to herein as “AHL-Pu-1”) and 2,6-dichloro-9-(prop-2-yn-1-yl)-9H-purine (also referred to herein as “AHL-Pu-2”). In some embodiments, the UV light used in step (c) can have a wavelength of about 200 nm to about 400 nm or of about 254 nm to about 365 nm. In some embodiments, the UV light has a wavelength of about 254 nm. In some embodiments, the UV light has a wavelength of about 365 nm. In some embodiments, the UV light has a wavelength of about 312 nm. In some embodiments, the protein sample is a biological organism and the presently disclosed methods can be used to detect reactive amino acid residues of proteins in vivo. When the protein sample is a biological organism (e.g., a living biological organism), such as an animal, contacting the protein sample with the probe compound of Formula (I) comprises administering the probe compound of Formula (I) to the biological organism via a suitable route of administration. The administration can be systemic or localized (e.g., to a site of disease, such as a tumor). In some embodiments, the administration is oral administration or injection, e.g., i.v. or i.p. injection. In some embodiments, prior to analyzing the proteins, a tissue sample is removed from the biological organism and homogenized. Alternatively, a biological fluid sample (e.g., blood or saliva) can be collected and the proteins therein can be analyzed for detection of a modified amino acid residue. In some embodiments, a tissue sample Attorney Docket No.: 3062/193 PCT can be isolated from a subject, processed, and the tissue sample and/or cells extracted therefrom can be exposed to a UV light for analysis. In some embodiments, providing the protein sample further comprises separating the protein sample (e.g., a cell or cell lysate sample) into a first protein sample and a second protein sample. Then, in the contacting step, the first protein sample can be contacted with a first probe compound of Formula (I) at a first probe concentration for a first period of time and the second protein sample can be contacted with a second probe compound of Formula (I) (i.e., a probe compound of Formula (I) having a different structure than that of the first probe compound of Formula (I)) or with a purine ligand compound at the same concentration (i.e., at the first probe concentration) for the same time period (i.e., for the first period of time). Alternatively, the second protein sample can be contacted with the same probe compound as the first protein sample, but at a different probe concentration (i.e., a second probe concentration) or for a different period of time. In some embodiments, analyzing proteins comprises analyzing the first and second protein samples to determine the presence and/or identity of a modified reactive amino acid residue (e.g., a modified reactive cysteine residue) in the first sample and the presence and/or identity of a modified reactive amino acid residue (e.g., a modified reactive cysteine residue) in the second sample. In some embodiments, the identities and/or amounts of identified modified reactive amino acid residues (e.g., the modified reactive cysteine residues) from the first and second protein samples are compared. In some embodiments, a purine ligand (e.g., an electrophilic purine compound that is free of an alkyne group or other reporter tag) can be screened for ability to modulate the activity of a RBP (e.g., by covalently modifying a RS-sensitive amino acid residue identified by a method as described herein). For example, in some embodiments, a purine ligand or ligands can be screened using a competitive study that includes treating a light sample with DMSO vehicle and a heavy sample with a purine ligand at a predetermined concentration and time followed by labeling both the light and heavy samples with a purine probe (purine with alkyne or other reporter tag). The sample can be a purified protein, complete lysate, live cell, or live organism. In some embodiments, e.g., to identify a small molecule that can bind a RBP-RNA complex (i.e., a ribonucleoprotein), an RNA cross-linking step can be performed in conjunction with the ligand treatment (e.g., by exposing a heavy sample comprising a ligand with UV light). As described in Section III.A., in some embodiments, the protein sample comprises living cells. In some embodiments, providing the protein sample further comprises separating the protein sample into a first protein sample and a second protein sample and culturing the first protein sample in a first cell culture medium comprising heavy isotopes prior to the contacting step and culturing the second protein sample in a second cell culture medium, wherein the second culture medium comprises a naturally occurring isotope distribution prior to the contacting step. In some embodiments, the first cell culture medium comprises ¹³ C- and/or ¹⁵ N-labeled amino acids. In some embodiments, the first cell Attorney Docket No.: 3062/193 PCT culture medium comprises ¹³ C-, ¹⁵ N-labeled lysine and arginine. Thus, in some embodiments, the method comprises performing SILAC. In some embodiments, the first culture medium further comprises 4SU, thereby resulting in the first culture medium containing cellular RNA that includes incorporated 4SU and this cellular 4SU-RNA is the 4SU-RNA contacted with the protein sample in step (b). In some embodiments, the sample for use in the methods is from any tissue or fluid from an individual. Samples include, but are not limited to, tissue (e.g. connective tissue, muscle tissue, nervous tissue, or epithelial tissue), whole blood, dissociated bone marrow, bone marrow aspirate, pleural fluid, peritoneal fluid, central spinal fluid, abdominal fluid, pancreatic fluid, cerebrospinal fluid, brain fluid, ascites, pericardial fluid, urine, saliva, bronchial lavage, sweat, tears, ear flow, sputum, hydrocele fluid, semen, vaginal flow, milk, amniotic fluid, and secretions of respiratory, intestinal or genitourinary tract. In some embodiments, the sample is a tissue sample, such as a sample obtained from a biopsy or a tumor tissue sample. In some embodiments, the sample is a blood serum sample. In some embodiments, the sample is a blood cell sample containing one or more peripheral blood mononuclear cells (PBMCs). In some embodiments, the sample contains one or more circulating tumor cells (CTCs). In some embodiments, the sample contains one or more disseminated tumor cells (DTC, e.g., in a bone marrow aspirate sample). In some embodiments, the samples are obtained from the individual by any suitable means of obtaining the sample using well-known and routine clinical methods. Procedures for obtaining tissue samples from an individual are well known. For example, procedures for drawing and processing tissue sample such as from a needle aspiration biopsy is well-known and is employed to obtain a sample for use in the methods provided. Typically, for collection of such a tissue sample, a thin hollow needle is inserted into a mass such as a tumor mass for sampling of cells that, after being stained, will be examined under a microscope. In some embodiments, the sample is a biological organism. In some embodiments, the biological organism is a rodent, e.g., a mouse or a rat. In some embodiments, the biological organism is a primate, e.g., a monkey. In some embodiments, the biological organism is a bacterium or a fungus. In some embodiments, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is a sample solution. In some instances, the sample solution comprises a solution such as a buffer (e.g. phosphate buffered saline) or a media. In some embodiments, the media is an isotopically labeled media. In some instances, the sample solution is a cell solution. In some embodiments, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is incubated with one or more compound probes for analysis of protein-probe interactions. In some instances, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is further incubated in the presence of an additional compound (e.g., potential RBP modulators) prior to addition of the one or more probes. Attorney Docket No.: 3062/193 PCT The presently disclosed methods can further comprise labeling one or more sample according to one or more labeling techniques known in the art for proteomics studies. For example, in some embodiments, the method further comprises labeling one or more sample (e.g., the heavy and/or light sample) using an isotopic labeling technique can be a metabolic technique (e.g., SILAC) or a chemical labeling technique, e.g., using an isobaric tag, such as a tandem mass tag (TMT) or an isobaric tag for relative and absolute quantitation (iTRAQ). In some embodiments, a method comprises incubating a sample (e.g. cell sample, cell lysate sample, or comprising isolated proteins) with a labeling group (e.g., an isotopically labeled labeling group) to tag one or more proteins of interest for further analysis. In such cases, the detectable labeling group comprises a biotin, a streptavidin, bead, resin, a solid support, or a combination thereof, and further comprises a linker that is optionally isotopically labeled. As described above, the linker can be about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more residues in length and might further comprise a cleavage site, such as a protease cleavage site (e.g., TEV cleavage site). In some cases, the labeling group is a biotin-linker moiety, which is optionally isotopically labeled with ¹³ C and ¹⁵ N atoms at one or more amino acid residue positions within the linker. IV. Samples In some embodiments, one or more of the methods disclosed herein comprise a sample (e.g., a cell sample, cell lysate sample or a biological organism). In some embodiments, the sample for use with the methods described herein is obtained from cells of an animal. In some instances, the animal cell includes a cell from a marine invertebrate, fish, insects, amphibian, reptile, or mammal. In some instances, the mammalian cell is a primate, ape, equine, bovine, porcine, canine, feline, or rodent. In some instances, the mammal is a primate, ape, dog, cat, rabbit, ferret, or the like. In some cases, the rodent is a mouse, rat, hamster, gerbil, hamster, chinchilla, or guinea pig. In some embodiments, the bird cell is from a canary, parakeet or parrots. In some embodiments, the reptile cell is from a turtles, lizard or snake. In some cases, the fish cell is from a tropical fish. In some cases, the fish cell is from a zebrafish (e.g. Danino rerio). In some cases, the worm cell is from a nematode (e.g. C. elegans). In some cases, the amphibian cell is from a frog. In some embodiments, the arthropod cell is from a tarantula or hermit crab. In some embodiments, the sample for use with the methods described herein is obtained from a mammalian cell. In some instances, the mammalian cell is an epithelial cell, connective tissue cell, hormone secreting cell, a nerve cell, a skeletal muscle cell, a blood cell, or an immune system cell. Exemplary mammalian cell lines include, but are not limited to, 293A cells, 293FT cells, 293F cells, 293H cells, HEK 293 cells, CHO DG44 cells, CHO-S cells, CHO-K1 cells, and PC12 cells. In some embodiments, the sample for use with the methods described herein is obtained from cells of a tumor cell line. In some instances, the sample is obtained from cells of a solid tumor cell line. In some instances, the solid tumor cell line is a sarcoma cell line. In some instances, the solid tumor cell Attorney Docket No.: 3062/193 PCT line is a carcinoma cell line. In some embodiments, the sarcoma cell line is obtained from a cell line of alveolar rhabdomyosarcoma, alveolar soft part sarcoma, ameloblastoma, angiosarcoma, chondrosarcoma, chordoma, clear cell sarcoma of soft tissue, dedifferentiated liposarcoma, desmoid, desmoplastic small round cell tumor, embryonal rhabdomyosarcoma, epithelioid fibrosarcoma, epithelioid hemangioendothelioma, epithelioid sarcoma, esthesioneuroblastoma, Ewing sarcoma, extrarenal rhabdoid tumor, extraskeletal myxoid chondrosarcoma, extraskeletal osteosarcoma, fibrosarcoma, giant cell tumor, hemangiopericytoma, infantile fibrosarcoma, inflammatory myofibroblastic tumor, Kaposi sarcoma, leiomyosarcoma of bone, liposarcoma, liposarcoma of bone, malignant fibrous histiocytoma (MFH), malignant fibrous histiocytoma (MFH) of bone, malignant mesenchymoma, malignant peripheral nerve sheath tumor, mesenchymal chondrosarcoma, myxofibrosarcoma, myxoid liposarcoma, myxoinflammatory fibroblastic sarcoma, neoplasms with perivascular epitheioid cell differentiation, osteosarcoma, parosteal osteosarcoma, neoplasm with perivascular epitheioid cell differentiation, periosteal osteosarcoma, pleomorphic liposarcoma, pleomorphic rhabdomyosarcoma, PNET/extraskeletal Ewing tumor, rhabdomyosarcoma, round cell liposarcoma, small cell osteosarcoma, solitary fibrous tumor, synovial sarcoma, and telangiectatic osteosarcoma. In some embodiments, the carcinoma cell line is obtained from a cell line of adenocarcinoma, squamous cell carcinoma, adenosquamous carcinoma, anaplastic carcinoma, large cell carcinoma, small cell carcinoma, anal cancer, appendix cancer, bile duct cancer (i.e., cholangiocarcinoma), bladder cancer, brain tumor, breast cancer, cervical cancer, colon cancer, cancer of Unknown Primary (CUP), esophageal cancer, eye cancer, fallopian tube cancer, gastroenterological cancer, kidney cancer, liver cancer, lung cancer, medulloblastoma, melanoma, oral cancer, ovarian cancer, pancreatic cancer, parathyroid disease, penile cancer, pituitary tumor, prostate cancer, rectal cancer, skin cancer, stomach cancer, testicular cancer, throat cancer, thyroid cancer, uterine cancer, vaginal cancer, or vulvar cancer. In some instances, the sample is obtained from cells of a hematologic malignant cell line. In some instances, the hematologic malignant cell line is a T-cell cell line. In some instances, B-cell cell line. In some instances, the hematologic malignant cell line is obtained from a T-cell cell line of: peripheral T-cell lymphoma not otherwise specified (PTCL-NOS), anaplastic large cell lymphoma, angioimmunoblastic lymphoma, cutaneous T-cell lymphoma, adult T-cell leukemia/lymphoma (ATLL), blastic NK-cell lymphoma, enteropathy-type T-cell lymphoma, hematosplenic gamma-delta T-cell lymphoma, lymphoblastic lymphoma, nasal NK/T-cell lymphomas, or treatment-related T-cell lymphomas. In some instances, the hematologic malignant cell line is obtained from a B-cell cell line of: acute lymphoblastic leukemia (ALL), acute myelogenous leukemia (AML), chronic myelogenous leukemia (CML), acute monocytic leukemia (AMoL), chronic lymphocytic leukemia (CLL), high-risk chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), high-risk small Attorney Docket No.: 3062/193 PCT lymphocytic lymphoma (SLL), follicular lymphoma (FL), mantle cell lymphoma (MCL), Waldenstrom's macroglobulinemia, multiple myeloma, extranodal marginal zone B cell lymphoma, nodal marginal zone B cell lymphoma, Burkitt's lymphoma, non-Burkitt high grade B cell lymphoma, primary mediastinal B-cell lymphoma (PMBL), immunoblastic large cell lymphoma, precursor B- lymphoblastic lymphoma, B cell prolymphocytic leukemia, lymphoplasmacytic lymphoma, splenic marginal zone lymphoma, plasma cell myeloma, plasmacytoma, mediastinal (thymic) large B cell lymphoma, intravascular large B cell lymphoma, primary effusion lymphoma, or lymphomatoid granulomatosis. In some embodiments, the sample for use with the methods described herein is obtained from a tumor cell line. Exemplary tumor cell lines include, but are not limited to, 600MPE, AU565, BT-20, BT-474, BT-483, BT-549, Evsa-T, Hs578T, MCF-7, MDA-MB-231, SkBr3, T-47D, HeLa, DU145, PC3, LNCaP, A549, H1299, NCI-H460, A2780, SKOV-3/Luc, Neuro2a, RKO, RKO-AS45-1, HT-29, SW1417, SW948, DLD-1, SW480, Capan-1, MC/9, B72.3, B25.2, B6.2, B38.1, DMS 153, SU.86.86, SNU-182, SNU-423, SNU-449, SNU-475, SNU-387, Hs 817.T, LMH, LMH/2A, SNU-398, PLHC-1, HepG2/SF, OCI-Ly1, OCI-Ly2, OCI-Ly3, OCI-Ly4, OCI-Ly6, OCI-Ly7, OCI-Ly10, OCI-Ly18, OCI- Ly19, U2932, DB, HBL-1, RIVA, SUDHL2, TMD8, MEC1, MEC2, 8E5, CCRF-CEM, MOLT-3, TALL-104, AML-193, THP-1, BDCM, HL-60, Jurkat, RPMI 8226, MOLT-4, RS4, K-562, KASUMI- 1, Daudi, GA-10, Raji, JeKo-1, NK-92, and Mino. V. Sample Preparation and Analysis In some embodiments, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is a sample solution. In some instances, the sample solution comprises a solution such as a buffer (e.g. phosphate buffered saline) or a media. In some embodiments, the media is an isotopically labeled media. In some instances, the sample solution is a cell solution. In some embodiments, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is incubated with one or more compound probes for analysis of protein-probe interactions. In some instances, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is further incubated in the presence of an additional compound probe prior to addition of the one or more probes. In other instances, the sample (e.g., cell sample, cell lysate sample, or comprising isolated proteins) is further incubated with a non-probe small molecule ligand, e.g., in which the non-probe small molecule ligand does not contain a photoreactive moiety and/or an alkyne group. In such instances, the sample is incubated with a probe and non-probe small molecule ligand for competitive protein profiling analysis. In some embodiments, one or more methods are utilized for labeling a sample (e.g. cell sample, cell lysate sample, or comprising isolated proteins) for analysis of probe protein interactions. In some instances, a method comprises labeling the sample (e.g. cell sample, cell lysate sample, or comprising isolated proteins) with an enriched media. In some cases, the sample (e.g. cell sample, cell lysate Attorney Docket No.: 3062/193 PCT sample, or comprising isolated proteins) is labeled with isotope-labeled amino acids, such as ¹³ C or ¹⁵ N- labeled amino acids. In some cases, the labeled sample is further compared with a non-labeled sample to detect differences in probe protein interactions between the two samples. In some instances, this difference is a difference of a target protein and its interaction with a small molecule ligand in the labeled sample versus the non-labeled sample. In some instances, the difference is an increase, decrease or a lack of protein-probe interaction in the two samples. In some instances, the isotope-labeled method is termed SILAC, stable isotope labeling using amino acids in cell culture. In some embodiments, a method comprises incubating a sample (e.g. cell sample, cell lysate sample, or comprising isolated proteins) with a labeling group (e.g., an isotopically labeled labeling group) to tag one or more proteins of interest for further analysis. In such cases, the detectable labeling group comprises a biotin, a streptavidin, bead, resin, a solid support, or a combination thereof, and further comprises a linker that is optionally isotopically labeled. As described above, the linker can be about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more residues in length and might further comprise a cleavage site, such as a protease cleavage site (e.g., TEV cleavage site). In some cases, the labeling group is a biotin-linker moiety, which is optionally isotopically labeled with ¹³ C and ¹⁵ N atoms at one or more amino acid residue positions within the linker. In some cases, the biotin-linker moiety is a isotopically- labeled TEV-tag as previously described. ¹⁰ In some embodiments, an isotopic reductive dimethylation (ReDi) method is utilized for processing a sample. In some cases, the ReDi labeling method involves reacting peptides with formaldehyde to form a Schiff base, which is then reduced by cyanoborohydride. This reaction dimethylates free amino groups on N-termini and lysine side chains and monomethylates N-terminal prolines. In some cases, the ReDi labeling method comprises methylating peptides from a first processed sample with a "light" label using reagents with hydrogen atoms in their natural isotopic distribution and peptides from a second processed sample with a "heavy" label using deuterated formaldehyde and cyanoborohydride. Subsequent proteomic analysis (e.g., mass spectrometry analysis) based on a relative peptide abundance between the heavy and light peptide version might be used for analysis of probe-protein interactions. In some embodiments, isobaric tags for relative and absolute quantitation (iTRAQ) method is utilized for processing a sample. In some cases, the iTRAQ method is based on the covalent labeling of the N-terminus and side chain amines of peptides from a processed sample. In some cases, reagent such as 4-plex or 8-plex is used for labeling the peptides. In some embodiments, the probe-protein complex is further conjugated to a chromophore, such as a fluorophore. In some instances, the probe-protein complex is separated and visualized utilizing an electrophoresis system, such as through a gel electrophoresis, or a capillary electrophoresis. Exemplary gel electrophoresis includes agarose based gels, polyacrylamide based gels, or starch based gels. In Attorney Docket No.: 3062/193 PCT some instances, the probe-protein is subjected to a native electrophoresis condition. In some instances, the probe-protein is subjected to a denaturing electrophoresis condition. In some instances, the probe-protein after harvesting is further fragmentized to generate protein fragments. In some instances, fragmentation is generated through mechanical stress, pressure, or chemical means. In some instances, the protein from the probe-protein complexes is fragmented by a chemical means. In some embodiments, the chemical means is a protease. Exemplary proteases include, but are not limited to, serine proteases such as chymotrypsin A, penicillin G acylase precursor, dipeptidase E, DmpA aminopeptidase, subtilisin, prolyl oligopeptidase, D-Ala-D-Ala peptidase C, signal peptidase I, cytomegalovirus assemblin, Lon-A peptidase, peptidase Clp, Escherichia coli phage KIF endosialidase CIMCD self-cleaving protein, nucleoporin 145, lactoferrin, murein tetrapeptidase LD-carboxypeptidase, or rhomboid-1; threonine proteases such as ornithine acetyltransferase; cysteine proteases such as TEV protease, amidophosphoribosyltransferase precursor, gamma-glutamyl hydrolase (Rattus norvegicus), hedgehog protein, DmpA aminopeptidase, papain, bromelain, cathepsin K, calpain, caspase-1, separase, adenain, pyroglutamyl-peptidase I, sortase A, hepatitis C virus peptidase 2, sindbis virus-type nsP2 peptidase, dipeptidyl-peptidase VI, or DeSI-1 peptidase; aspartate proteases such as beta-secretase 1 (BACE1), beta-secretase 2 (BACE2), cathepsin D, cathepsin E, chymosin, napsin-A, nepenthesin, pepsin, plasmepsin, presenilin, or renin; glutamic acid proteases such as AfuGprA; and metalloproteases such as peptidase_M48. In some instances, the fragmentation is a random fragmentation. In some instances, the fragmentation generates specific lengths of protein fragments, or the shearing occurs at particular sequence of amino acid regions. In some instances, the protein fragments are further analyzed by a proteomic method such as by liquid chromatography (LC) (e.g. high performance liquid chromatography), liquid chromatography- mass spectrometry (LC-MS), matrix-assisted laser desorption/ionization (MALDI-TOF), gas chromatography-mass spectrometry (GC-MS), capillary electrophoresis-mass spectrometry (CE-MS), or nuclear magnetic resonance imaging (NMR). In some embodiments, the LC method is any suitable LC methods well known in the art, for separation of a sample into its individual parts. This separation occurs based on the interaction of the sample with the mobile and stationary phases. Since there are many stationary/mobile phase combinations that are employed when separating a mixture, there are several different types of chromatography that are classified based on the physical states of those phases. In some embodiments, the LC is further classified as normal-phase chromatography, reverse-phase chromatography, size- exclusion chromatography, ion-exchange chromatography, affinity chromatography, displacement chromatography, partition chromatography, flash chromatography, chiral chromatography, and aqueous normal-phase chromatography. Attorney Docket No.: 3062/193 PCT In some embodiments, the LC method is a high performance liquid chromatography (HPLC) method. In some embodiments, the HPLC method is further categorized as normal-phase chromatography, reverse-phase chromatography, size-exclusion chromatography, ion-exchange chromatography, affinity chromatography, displacement chromatography, partition chromatography, chiral chromatography, and aqueous normal-phase chromatography. In some embodiments, the HPLC method of the present disclosure is performed by any standard techniques well known in the art. Exemplary HPLC methods include hydrophilic interaction liquid chromatography (HILIC), electrostatic repulsion-hydrophilic interaction liquid chromatography (ERLIC) and reverse phase liquid chromatography (RPLC). In some embodiments, the LC is coupled to a mass spectroscopy as a LC-MS method. In some embodiments, the LC-MS method includes ultra-performance liquid chromatography-electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC-ESI-QTOF-MS), ultra-performance liquid chromatography-electro spray ionization tandem mass spectrometry (UPLC-ESI-MS/MS), reverse phase liquid chromatography-mass spectrometry (RPLC-MS), hydrophilic interaction liquid chromatography-mass spectrometry (HILIC-MS), hydrophilic interaction liquid chromatography-triple quadrupole tandem mass spectrometry (HILIC-QQQ), electrostatic repulsion-hydrophilic interaction liquid chromatography-mass spectrometry (ERLIC-MS), liquid chromatography time-of-flight mass spectrometry (LC-QTOF-MS), liquid chromatography-tandem mass spectrometry (LC-MS/MS), multidimensional liquid chromatography coupled with tandem mass spectrometry (LC/LC-MS/MS). In some instances, the LC-MS method is LC/LC-MS/MS. In some embodiments, the LC-MS methods of the present disclosure are performed by standard techniques well known in the art. In some embodiments, the GC is coupled to a mass spectroscopy as a GC-MS method. In some embodiments, the GC-MS method includes two-dimensional gas chromatography time-of-flight mass spectrometry (GC*GC-TOFMS), gas chromatography time-of-flight mass spectrometry (GC-QTOF- MS) and gas chromatography-tandem mass spectrometry (GC-MS/MS). In some embodiments, CE is coupled to a mass spectroscopy as a CE-MS method. In some embodiments, the CE-MS method includes capillary electrophoresis-negative electrospray ionization- mass spectrometry (CE-ESI-MS), capillary electrophoresis-negative electrospray ionization- quadrupole time of flight-mass spectrometry (CE-ESI-QTOF-MS) and capillary electrophoresis- quadrupole time of flight-mass spectrometry (CE-QTOF-MS). In some embodiments, the nuclear magnetic resonance (NMR) method is any suitable method well known in the art for the detection of one or more cysteine binding proteins or protein fragments disclosed herein. In some embodiments, the NMR method includes one dimensional (1D) NMR methods, two dimensional (2D) NMR methods, solid state NMR methods and NMR chromatography. Exemplary 1D NMR methods include ¹ Hydrogen, ¹³ Carbon, ¹⁵ Nitrogen, ¹⁷ Oxygen, ¹⁹ Fluorine, ³¹ Phosphorus, ³⁹ Potassium, ²³ Sodium, ³³ Sulfur, ⁸⁷ Strontium, ²⁷ Aluminium, ⁴³ Calcium, ³⁵ Chlorine, Attorney Docket No.: 3062/193 PCT ³⁷ Chlorine, ⁶³ Copper, ⁶⁵ Copper, ⁵⁷ Iron, ²⁵ Magnesium, ¹⁹⁹ Mercury or ⁶⁷ Zinc NMR method, distortionless enhancement by polarization transfer (DEPT) method, attached proton test (APT) method and 1D- incredible natural abundance double quantum transition experiment (INADEQUATE) method. Exemplary 2D NMR methods include correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), 2D-INADEQUATE, 2D-adequate double quantum transfer experiment (ADEQUATE), nuclear overhauser effect spectroscopy (NOSEY), rotating-frame NOE spectroscopy (ROESY), heteronuclear multiple-quantum correlation spectroscopy (HMQC), heteronuclear single quantum coherence spectroscopy (HSQC), short range coupling and long range coupling methods. Exemplary solid state NMR method include solid state .sup.13Carbon NMR, high resolution magic angle spinning (HR-MAS) and cross polarization magic angle spinning (CP-MAS) NMR methods. Exemplary NMR techniques include diffusion ordered spectroscopy (DOSY), DOSY-TOCSY and DOSY-HSQC. In some embodiments, the results from the mass spectroscopy method are analyzed by an algorithm for protein identification. In some embodiments, the algorithm combines the results from the mass spectroscopy method with a protein sequence database for protein identification. In some embodiments, the algorithm comprises ProLuCID algorithm, Probity, Scaffold, SEQUEST, or Mascot. In accordance with the presently disclosed subject matter, as described above or as discussed in the EXAMPLES below, there can be employed conventional chemical, cellular, histochemical, biochemical, molecular biology, microbiology, recombinant DNA, and clinical techniques which are known to those of skill in the art. Such techniques are explained fully in the literature. See for example, Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Publications, Cold Spring Harbor, New York, United States of America; Glover (1985) DNA Cloning: A Practical Approach. Oxford Press, Oxford; Gait (1984) Oligonucleotide Synthesis: A Practical Approach, IRL Press, Oxford, England; Harlow & Lane, 1988, Antibodies, A Laboratory Manual, Cold Spring Harbor Publications, New York; Roe et al. (1996) DNA Isolation and Sequencing: Essential Techniques, John Wiley, New York, New York, United States of America; and Ausubel et al. (1995) Current Protocols in Molecular Biology, Greene Publishing. XI. Kits/Articles of Manufacture Disclosed herein, in certain embodiments, are kits and articles of manufacture for use with one or more methods described herein. In some embodiments, described herein is a kit for generating a protein comprising a detectable group and/or a fragment of a ligand compound described herein. In some embodiments, such kit includes a probe or ligand as described herein, small molecule fragments or libraries, and/or controls, and reagents suitable for carrying out one or more of the methods described herein. In some instances, the kit further comprises samples, such as a cell sample, and suitable solutions such as buffers or media. In some embodiments, the kit further comprises recombinant proteins for use in one or more of the methods described herein. In some embodiments, additional components of the kit comprises a carrier, package, or container that is compartmentalized to receive one or more Attorney Docket No.: 3062/193 PCT containers such as vials, tubes, and the like, each of the container(s) comprising one of the separate elements to be used in a method described herein. Suitable containers include, for example, bottles, vials, plates, syringes, and test tubes. In one embodiment, the containers are formed from a variety of materials such as glass or plastic. The articles of manufacture provided herein contain packaging materials. Examples of pharmaceutical packaging materials include, but are not limited to, bottles, tubes, bags, containers, and any packaging material suitable for a selected formulation and intended mode of use. For example, the container(s) include probes, 4SU and/or 4SU-RNA, ligands, control compounds, and one or more reagents for use in a method disclosed herein. The presently disclosed kits and articles of manufacture optionally include an identifying description or label or instructions relating to its use in the methods described herein. For example, a kit typically includes labels listing contents and/or instructions for use, and package inserts with instructions for use. A set of instructions will also typically be included. In some embodiments, a label is on or associated with the container. In some embodiments, a label is on a container when letters, numbers or other characters forming the label are attached, molded or etched into the container itself; a label is associated with a container when it is present within a receptacle or carrier that also holds the container, e.g., as a package insert. In some embodiments, a label is used to indicate that the contents are to be used for a specific therapeutic application. The label also indicates directions for use of the contents, such as in the methods described herein. EXAMPLES The following EXAMPLES provide illustrative embodiments. In light of the present disclosure and the general level of skill in the art, those of skill will appreciate that the following EXAMPLES are intended to be exemplary only and that numerous changes, modifications, and alterations can be employed without departing from the scope of the presently disclosed subject matter. Without further description, it is believed that one of ordinary skill in the art can, using the preceding description and the following illustrative EXAMPLES, make and utilize the compounds of the presently disclosed subject matter and practice the methods of the presently disclosed subject matter. The following EXAMPLES therefore particularly point out embodiments of the presently disclosed subject matter and are not to be construed as limiting in any way the remainder of the disclosure. Materials and Methods for the EXAMPLES General Information. Reagent grade chemicals were used without further purification. N,N- Dimethylformamide (DMF), ethyl acetate, chloroform and n-heptane were used without further purification (Fisher; Hampton, New Hampshire, United States of America). Merck silica gel 60 F254 plates (0.25 mm; Merck KGaA, Darmstadt, Germany) were used for analytical thin layer chromatography (TLC). Flash column chromatography was carried out using the indicated solvents on an automated flash chromatography purification system with a UV detector (sold under the tradename Attorney Docket No.: 3062/193 PCT ISOLERA™ One (Biotage, Uppsala, Sweden) using Teledyne ISCO columns (Teledyne ISCO, Lincoln, Nebraska, United States of America). Compounds were visualized by UV-irradiation and iodine chamber. A Shimadzu 1100 Series spectrometer (Shimadzu Scientific Instruments, Kyoto, Japan) was used for analytical HPLC chromatograms. Proton ( ¹ H) and carbon ( ¹³ C) NMR spectra were recorded on a Varian Inova 500 (500 MHz) or 600 (600MHz) spectrometer in CDCl ₃ or DMSO-d ₆ with chemical shifts referenced to internal standards (CDCl ₃ : 7.26 ppm ¹ H, 77.16 ppm ¹³ C; (CD ₃ ) ₂ SO: 2.50 ppm ¹ H, 39.52 ppm ¹³ C). Splitting patterns are indicated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad singlet for 1H-NMR data. NMR chemical shifts (δ) are reported in ppm. Coupling constants (J) are reported in Hz. NMR studies were performed once per compound stock following standard synthetic protocols. An Agilent 6545B LC/Q-TOF (Agilent Technologies, Santa Clara, California, United States of America) was used for high resolution mass spectral (HRMS) analysis. Structural analysis by crystallography was performed as previously described ³⁶ . Chemical Suppliers. The chemicals listed below were purchased commercially and reported as >95% purity. Fisher Scientific (Hampton, New Hampshire, United States of America): N,N- Diisopropylethylamine, Acetic acid (optima LC/MS grade), Water (HPLC grade grade), and Acetonitrile (ACN, optima LC/MS grade) Combi-Blocks, Inc. (San Diego, California, United States of America): p-Cresol, n-Butylamine Acros Organics (Antwerp, Belgium): 1,1,3,3-Tetramethylguanidine, 99% (TMG), Butyric acid, Propargyl bromide (80 wt% in toluene), Propanamide Alfa Aesar (Haverhill, Massachusetts, United States of America): Caffeine Oakwood Chemical (Oakwood Products, Estill, South Carolina, United States of America): 1- Butanethiol General Protocol for synthesis of purine compounds. The purine base (21.7 mmol 1.0 eq), dimethylformamide (DMF, 100 mL), potassium carbonate (K ₂ CO ₃ , 21.7 mmol, 1.0 eq) and propargyl bromide (80 wt% in toluene, 21.7 mmol, 1.0 eq) were mixed in a round bottom flask. The reaction was stirred under nitrogen at room temperature for 12 hrs. The reaction was treated with water (400 mL) and extracted with ethyl acetate (3 x 100 mL). The combined organic layer was dried over sodium sulfate and concentrated to a tan solid. This solid was dissolved in chloroform (100 mL). The solution was concentrated and heated to reflux to dissolve all the solids. Upon cooling a white crystalline solid formed which was isolated by filtration. The solid was rinsed with fresh chloroform (20 mL) and heptane (20 mL) to give the N-9 substituted product after air drying. The filtrate contained a mixture of the N-7 and N-9 products. These were separated using the flash chromatography system (5% acetone to 20% acetone/chloroform) to afford respective products. Attorney Docket No.: 3062/193 PCT 2,6-Dichloro-7-(prop-2-yn-1-yl)-7H-purine (AHL-Pu-1). Yield: 9%. ¹ H NMR (600 MHz, CDCl ₃ ) δ 8.48 (s, 1H), 5.27 (d, J = 2.6 Hz, 2H), 2.70 (t, J = 2.6 Hz, 1H). ¹³ C NMR (151 MHz, DMSO-d6) δ 163.24, 151.84, 151.30, 143.40, 121.61, 78.08, 77.55, 36.62. ESI-TOF (HRMS) m/z [M+H] ⁺ calculated for C ₈ H ₅ Cl ₂ N ₄ ⁺ 226.9891, found 226.9885. 2,6-Dichloro-9-(prop-2-yn-1-yl)-9H-purine (AHL-Pu-2). Yield: 45%. ¹ H NMR (600 MHz, CDCl ₃ ) δ 8.33 (s, 1H), 5.04 (d, J = 2.6 Hz, 2H), 2.61 (t, J = 2.6 Hz, 1H). ¹³ C NMR (151 MHz, DMSO-d ₆ ) δ 152.91, 151.21, 149.89, 147.71, 130.44, 77.07, 76.92, 33.52. ESI-TOF (HRMS) m/z [M+H] ⁺ calculated for C ₈ H ₅ Cl ₂ N ₄ ⁺ 226.9891, found 226.9887. General procedure for the preparation of 1-alkylthiol adducts. Dichloropurine compound (831 mg, 3.66 mmol), DMF (10 mL), potassium carbonate (powdered, 556 mg, 4.03 mmol) and n-butanethiol (373 mg, 4.14 mmol) were placed in a 50 mL round bottom flask. The reaction was stirred under nitrogen at ambient temperature for 16 hrs. The reaction was partitioned between ethyl acetate and water (25 mL/40 mL). The layers were separated and the aqueous layer was extracted with ethyl acetate (2 x 25 mL). The combined organic layer was washed with water (25 mL) and brine (25 mL). It was then dried over magnesium sulfate and concentrated to give crude product that was purified on the flash chromatography system (20% ethyl acetate to 80% hexanes) to give 650 mg of product as an off-white solid. 6-(Butylthio)-2-chloro-7-(prop-2-yn-1-yl)-7H-purine (Pa-1). Yield: 63%. ¹ H NMR (600 MHz, dmso) δ 8.68 (s, 1H), 5.32 (d, J = 2.5 Hz, 2H), 3.67 (t, J = 2.5 Hz, 1H), 3.39 - 3.36 (m, 2H), 1.74 - 1.68 (m, 2H), 1.44 (dq, J = 14.7, 7.4 Hz, 2H), 0.92 (t, J = 7.4 Hz, 3H). ¹³ C NMR (151 MHz, dmso) δ 159.97, 155.92, 152.44, 149.41, 121.69, 78.26, 77.88, 36.97, 30.57, 29.11, 21.24, 13.43. ESI-TOF (HRMS): m/z [M+H]+ calculated for C ₁₂ H ₁₄ ClN ₄ S ⁺ 281.0622, found 281.0621. HPLC Analysis of Compound Purity and Reactivity. CEP probes were prepared in ACN (10 mM final). The compound stock (50 μL) was then mixed with 10 μL of ACN. This sample mixture was Attorney Docket No.: 3062/193 PCT injected (1 μL) and analyzed by reverse-phase HPLC on a Shimadzu 1100 Series spectrometer with UV detection at 254 nm (Shimadzu Scientific Instruments, Kyoto, Japan). Chromatographic separation was performed using a Phenomenex Kinetex C18 column (2.6 μm, 50 x 4.6 mm; Phenomenex, Torrance, California, United States of America). For HPLC Method A, mobile phase A was composed of H ₂ O + 0.1% HOAc, while mobile phase B was composed of ACN + 0.1% HOAc. Samples were analyzed using the following analytical conditions: 0-0.5 min, 15% B; 0.5-6.5 min 85% B; 6.5-7 min 100% B; 7-8.5 min 100% B; 8.5-9 min 15% B; 9-9.8 min 15% B using a flow rate of 0.8 mL min ^-1 . The purities of AHL-Pu-1, AHL-Pu-2, and Pa-1 were each determined to be ≥ 95% using HPLC method A. HPLC Method B: Probes were dissolved in 500 µL DMF-ACN solution and stirred on ice with TMG and the amino acid mimetics. At the indicated time point, a 50 µL aliquot was removed and quenched in a solution of acetic acid and caffeine. Solutions were analyzed by HPLC and consumption of CEP probe was quantified as described in the HPLC solution reactivity assay below. The HPLC gradient from Method A was used in these assays. HPLC solution reactivity assay. The following reagents were prepared and stored on ice prior to use. 0.1 molar (M) solution of caffeine in acetonitrile, 1.0 M solution of nucleophile, tetramethylguanidine (TMG), 1 M acetic acid (HOAc) in acetonitrile (ACN) and 10 mM solution of CEP probe in ACN. The following nucleophiles that mimic amino acid side chain groups were used: butanethiol (Cys mimetic), n-butylamine (Lys mimetic), p-cresol (Tyr mimetic), propionamide (Asn/Gln mimetic); butyric acid (Asp/Glu mimetic). The CEP solution (500 μL) was transferred to a dram vial on ice. TMG (5.5 μL) and the respective nucleophile (5.5 μL) were added and the solutions stirred on ice for 6 hr. Aliquots (50 μL) were removed at the indicated time points and quenched with 10 μL of a 1:1 mixture of caffeine and HOAc to monitor probe reactivity. Reaction progress was evaluated by monitoring consumption of starting material (CEP) normalized to the caffeine standard. CEP consumption was calculated using the area under the curve (AUC) for the CEP peak at time (t) = experimental / t = 0. All CEP peak AUCs used for calculations were normalized to caffeine standard AUCs at respective time points to account for run-to-run variations by HPLC. The amount of CEP consumed (% starting material) was plotted as a function of time. Cell Culture. Cell lines were cultured with 5% CO ₂ at 37 ˚C with manufacturer recommended media supplemented with 10% fetal bovine serum (FBS, U.S. Source, Omega Scientific, Inc., Tarzana, California, United States of America) and 1% L-glutamine (Fisher Scientific, Hampton, New Hampshire, United States of America): HEK293T, HeLa: DMEM; DM93, A549, Jurkat: RPMI. Cells were harvested for experimental use when they reached ∼90% confluency. Plates were rinsed with cold PBS. Cells were scraped and washed (2X) with cold PBS with pelleting (400 x g, 3 min, 4°C) and aspiration between washes. PBS was aspirated one final time before snap-freezing. Pellets were stored at -80°C until further experimentation. HEK293T (CRL-3216), HeLa (CCL-2), A549 (CRM-CCL-185) and Jurkat (TIB-152) cells were purchased from the American Tissue Culture Collection (ATCC). Attorney Docket No.: 3062/193 PCT DM93 cells were kindly provided by Dr. Seigler (Duke University Medical Center, Durham, North Carolina, United States of America). Cell lines used for studies were not authenticated. SILAC cell culture. SILAC cells were cultured at 37˚ C with 5% CO ₂ in either ‘light’ or ‘heavy’ media supplemented with 10% dialyzed FBS (Omega Scientific, Inc., Tarzana, California, United States of America), 1% L-glutamine (Fisher Scientific, Hampton, New Hampshire, United States of America), and isotopically-labeled amino acids. HEK293T and HeLa cells were cultured in DMEM, while DM93, A549 and Jurkat cells were cultured in RPMI. Light media was supplemented with 100 μg/mL L- arginine and 100 μg/mL L-lysine. Heavy media was supplemented with 100 μg/mL [ ¹³ C ₆ ¹⁵ N ₄ ]L- arginine and 100 μg/mL [ ¹³ C ₆ ¹⁵ N ₂ ]L-lysine. The cells were cultured for 6 passages before use in proteomics experiments. SILAC cells were harvest and preserved according to methods described in the cell culture section unless otherwise noted. Reactivity of CEP Probes in situ. DM93 cells were cultured as described in the subsection Cell culture above. Cells were then treated with CEP probes (25 ^M final, 50X stock in DMSO) for 4 hours at 37 ˚C unless otherwise noted. The cells were then rinsed with cold PBS and collected via scraping and washed in cold PBS (2X). The pellet was reconstituted in PBS supplemented with cOmplete EDTA- free protease inhibitor tablets (EDTA-free) (Sigma-Aldrich (St. Louis, Missouri, United States of America), 11836170001) and sonicated (3 x 1 second pulses, 20% amplitude). Lysates were separated into soluble and membrane fractions using ultracentrifugation (100,000 x g, 45 min, 4°C). CEP-labeled probes were then prepared for gel-based click chemistry according to Gel-based chemical proteomics below. Gel-based chemical proteomics. Gel-based click chemistry was performed as previously described ³⁶ unless noted otherwise. Briefly, CEP-labeled samples were conjugated by copper-catalyzed azide-alkyne cycloaddition (CuAAC) to rhodamine-azide (1 μL of 1.25 mM stock; final concentration of 25 μM) using tris(2-carboxyethyl)phosphine (TCEP; 1 μL of fresh 50 mM stock in water; final concentration of 1 mM), tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA, 3 μL of a 1.7 mM 4:1 t-butanol/DMSO stock, final concentration of 100 μM), and copper sulfate (CuSO ₄ , 1 μL of 50 mM stock, final concentration of 1 mM). The reaction was allowed to occur for 1 hour at room temperature. Once completed, the reaction was quenched with 17 μL of 4X SDS-PAGE loading buffer and beta- mercaptoethanol (βME). Quenched samples were analyzed by SDS-PAGE gel and in-gel fluorescence scanning on a Bio-Rad ChemiDoc MP Imaging System (Bio-Rad Laboratories, Hercules, California, United States of America). Preparation of proteomes for SILAC LC-MS/MS chemical proteomics. CEP-labeled samples were prepared for LC-MS/MS analysis via CuACC as described in gel-based click chemistry methods. Desthiobiotin-azide was supplemented for rhodamine-azide for purification. After incubation (1 hr, room temperature), excess reagents were removed using chloroform/methanol extractions. The insoluble pellet was resuspended in 6 M urea, 25 mM ammonium bicarbonate (ambic). Proteins were Attorney Docket No.: 3062/193 PCT reduced (DTT, 10 mM, 65 ˚C, 15 min), cooled (4 ˚C, 5 min) and alkylated (IAA, 40 mM, room temperature, dark, 30 min). Excess reagents were removed using chloroform/methanol extractions. Pellets were resuspended in 25 mM ambic (500 µL) and proteolytically digested (trypsin, 7.5 µg, 3 hrs, 37 ˚C). Samples were enriched using avidin beads and washed with PBS (3X). Bound peptides were eluted using 150 µL of 50% ACN + 0.1% formic acid (3X). Eluates were combined and centrifuged using mini chromatography columns (sold under the tradename BIO-SPIN® (Bio-Rad Laboratories, Hercules, California, United States of America), 7326207) to remove additional avidin beads. Peptides were dried on a speed vac and reconstituted using 0.1% formic acid. Samples were stored at -80 ˚C until further analysis. Dataset comparisons. Iodoacetamide-desthiobiotin (DBIA) datasets from large-scale, cell- based screens (Kuljanin et al. ⁴⁰ ) were curated by Lai et al. ⁷⁸ and used in the current study to obtain gene names that were converted to reviewed UniProt identifications (IDs) using mapping tools on UniProt (Human, Taxon ID: 9606). A total of 24,151 cysteine sites and 6,118 proteins from DBIA datasets were used for analysis. To approximate site position of other methods, the start and end position of the peptides was averaged. The average site was then truncated, generating the site (i.e., average down) for domain enrichment analysis. Overlap between CEP and reported datasets was calculated using a hypergeometric test ²⁷ . Nitrogenous base competition studies using gel-based chemical proteomics. HEK293T cells were grown according to the cell culture methods described in the Cell culture subsection above. Cells pellets were lysed in PBS supplemented with cOmplete EDTA-free protease inhibitor tablets (EDTA- free) (Sigma-Aldrich (St. Louis, Missouri, United States of America), 11836170001). Cellular fractions were separated with ultracentrifugation (100,000 x g, 45 min, 4°C). The soluble fraction was normalized to 2mg/mL using the Bio-Rad DC protein assay (Bio-Rad Laboratories, Hercules, California, United States of America). The lysate (48 µL) was mixed with CEP probe (AHL-Pu-1, 25 µM final, 50X stock in DMSO) and nitrogenous bases (adenine, uracil and cytosine, 500X stock in DMSO; purine: 5000X stock in DMSO). Nitrogenous bases were sonicated at 37°C in a water bath sonicator to aid solubilization. Concentrations ranged from 25 µM to 2.5 mM for adenine, uracil and cytosine, while purine concentrations ranged from 25 µM to 25 mM. Lysates were incubated at 37°C for 30 min before labeling with the CEP probe. Next, click chemistry was performed and samples analyzed by gel-based chemical proteomics (n = 2 independent biological replicates). Nitrogenous base competition studies using LC-MS/MS chemical proteomics. HEK293T cells were cultured according to the SILAC cell culture methods described above. Soluble fractions were prepared as described in the nitrogenous base gel-based competition protocol. Protein concentrations were normalized to 2.3 mg/mL in 432 µL of PBS supplemented with cOmplete EDTA-free protease inhibitor tablets prior to the addition of either CEP probes or nitrogenous bases. Light SILAC cells were treated with AHL-Pu-1 (25 µM final), while heavy cells were co-treated with AHL-Pu-1 (25 µM final) Attorney Docket No.: 3062/193 PCT and nitrogenous bases (adenine, uracil, cytosine: 2.5 mM final; purine 25 mM final) for 30 min at 37°C. CEP-labeled samples were prepared as described in the preparation of proteomes for SILAC LC- MS/MS procedure. Competed sites were defined as probe-modified peptides that showed SR ≥ 5 with base competition and passed all other criteria listed in LC-MS/MS evaluation of peptides (n = 3 independent biological replicates). LC-MS/MS evaluation of peptides. Peptides were analyzed using nano-electrospray ionization- liquid chromatography-mass spectrometry (LC-MS/MS) on a HPLC system sold under the tradename EASY-NLC™ 1200 (Thermo Fisher Scientific, Waltham, Massachusetts, United States of America) coupled with a mass spectrometer sold under the tradename Q-EXACTIVE™ Plus (Thermo Fisher Scientific, Waltham, Massachusetts, United States of America) as previously described ³⁶ , utilizing a top 10 data-dependent acquisition mode (ddMS2). Reverse-phase LC was performed as follows: (A: 0.1% formic acid/H ₂ O; B: 80% ACN, 0.1% formic acid in H ₂ O): 0-1:48 min 1% B, 400 nL/min; 1:48 - 2:00 min 1% B, 300 nL/min; 2-90 min 16% B; 90-14625% B; 146-147 min 95% B; 147-153 min 95% B; 153-154 min1% B; 154.0-154.1 min 1% B, 400 nL/min; 154.1-180 min 1% B, 400 nL/min. LC-MS/MS data analysis of CEP-modified peptides. Peptides identification from LC-MS/MS was accomplished using software sold under the tradename BYONIC™ (Protein Metrics Inc., Cupertino, California, United States of America). Data were searched against the human protein database (UniProt, download date: 02/18/2016) with the following parameters: ≤ 2 missed cleavages, 10 ppm precursor mass tolerance, 20 ppm fragment mass tolerance, too high (narrow) “precursor isotope off by x”, precursor and charge assignment computed from MS1, maximum of 1 precursor per MS2, 1% protein false discovery rate. Three variable (common) modifications were included: methionine oxidation (+15.9949 Da),cysteine carbamidomethylation (+57.021464 Da), and CEP- modified Cys (+604.2631). CEP probe modifications on amino acids of interest were included as a variable modification of 604.2631. Search results were filtered in R on a per site basis as previously described ³⁶ . The median SR for all cleavage patterns (i.e., ratios are calculated on a modified site basis, combining fully tryptic, half-tryptic and missed cleavages) was reported. Peptides were manually validated as previously described ³⁶ . Peptides with a SR>2 in PACCE conditions (4SU-RNA and 4SU- and native-RNA) were analyzed with a Mann-Whitney U test for significance compared to the SILAC mixing control (light and heavy proteomes added in a 1:1 ratio). Amino acid residue selectivity of CEP probes. DM93 SILAC cells were prepped and analyzed as described in SILAC cell culture above. Searches were accomplished using variable CEP probe modification (+604.2631) on the following nucleophilic amino acids: cysteine, aspartic acid, glutamic acid, histidine, lysine, methionine, asparagine, glutamine, arginine, serine, threonine, tryptophan, and tyrosine. A stricter BYONIC™ score cutoff (≥600) was applied to minimize false positive identifications. Attorney Docket No.: 3062/193 PCT Gene Ontology (GO) analysis of proteins containing CEP probe modified sites. Combined CEP-modified protein lists were analyzed using either the Panther Classification System or GO Enrichment Analysis ^38,39 . Charts generated using Panther used protein class enrichment on default settings: Fisher’s exact test, Benjamini-Hochberg false discovery rate (FDR) < 0.05. Charts generated with GO Enrichment analysis utilized molecular function analysis on the following settings: binomial test, FDR correction. Domain enrichment analysis of CEP probe modified sites. Domain enrichment analysis of probe-modified sites was conducted as previously described ³⁶ . P-values were calculated using a binomial test, which were then corrected with a 1% false-discovery rate (Benjamini-Hochberg correction). Identifying non-toxic conditions for 4SU metabolic incorporation into cellular RNA. Protocols were adapted from reference 79. HEK293T cells were cultured with varying concentrations of 4SU (100 and 500 µM) and metabolic incorporation allowed to proceed at short and longer time intervals (1-24 hrs). The 4SU concentrations were selected for testing based on He et al., 2016.4SU incorporation in total RNA extracts from cells treated with varying conditions were determined by RNA dot blots as previously described ⁷⁹ . The integrity of RNA extracts was assessed by the sharpness and lack of degradation of the distinct 28S and 18S ribosomal RNA bands using denaturing agarose gel analyses. At high concentrations (500 µM), substantial 4SU incorporation into cellular RNA was observed within 1 hour and remained consistent across all time points tested. Higher 4SU incorporation into RNA was observed when used at lower concentrations (100 µM) for longer incubation times (16 and 24 hrs). Overt toxicity at these conditions was not observed as determined by cell viability measurements. Based on findings, optimal 4SU incorporation into cellular RNA was determined to be 100 µM for 16 hrs. Validating crosslinking of 4SU-RNA in cells. The xRNAx assay for purification of RNA- protein complexes was adapted from reference 22. HEK293T cells (2.9 million cells/mL) were grown for 40 hours (60% confluency). Cells were treated with 4SU (100 µM) for 16 hours and crosslinked (1J/cm ² ) on ice. Media was removed and cells were rinsed with PBS before adding TRIzol. Once RNP complexes were purified, samples were reconstituted in RNase-free water and normalized to 75 ng in 40 mM Tris-HCl (pH 7.5). Samples were treated with either RNase A (Thermo Fisher Scientific (Waltham, Massachusetts, United States of America), EN0531), Dnase 1 (NEB Ipswich, Massachusetts, United States of America), M0303L), or Proteinase k (Thermo Fisher Scientific (Waltham, Massachusetts, United States of America), 25530049) at recommended concentrations for 1 hour (37 ^C). RNA loading dye (95% formamide, 0.125% SDS, and 0.1% EDTA) was added and samples were denatured for 5 min (65 ^C). Samples were analyzed by 1% agarose gel electrophoresis. Ethidium bromide staining and imaging was performed on a Bio-Rad ChemiDoc MP Imaging System (Bio-Rad Laboratories, Hercules, California, United States of America). Attorney Docket No.: 3062/193 PCT Cell viability of CEP-treated cells. CEP-labeled DM93 cells were subjected to a WST-1 assay to assess cellular viability according to the manufacturer’s protocol. Cells were plated in a 96-well dish. The tetrazolium salt was added to treated cells. The conversion of tetrazolium salt to formazan was measured after 30 min incubation at 37 ˚C using a plate reader sold under the tradename CLARIOSTAR® (BMG LABTECH, GmbH, Ortenburg, Germany). HEK293T cells were treated with 4SU in complete media (16 hr, 100 μM, 100X stock in serum free media). Cells were then crosslinked (UV irradiation, 1J/cm ² ) on ice, scraped in cold PBS and centrifuged (400 x g, 3 min). The pellet was reconstituted in serum-free DMEM. Cells were then quantified using trypan blue according to the manufacturer’s recommendations (Thermo Scientific) with an automated cell counter sold under the tradename COUNTESS™ II FL (Thermo Fisher, Hampton, New Hampshire, United States of America). Statistics were calculated in Prism using a nonparametric, Kruskal-Wallis test of variance. PACCE workflow for identification of RBPs. SILAC HEK293T cells were plated at 2.9 million cells per/mL and grown for 40 hours (60% confluency). Cell media was replaced with 4SU- supplemented complete SILAC DMEM for 16 hrs. Afterwards, cells were washed with PBS and crosslinked at 312nm (1J/cm ² ) on ice. Serum-free DMEM containing CEP (AHL-Pu-1, 25 ^M final in DMSO) was added to cells after crosslinking. Probe labeling in cells was allowed to proceed for 1 hour at 37°C. RNase-free PBS was added and cells were harvested via scraping. Cells were then washed 2X times with RNase-free PBS (5 mL). The cell pellet was flash frozen and stored at -80 ^C until further use. Cells were lysed in PBS containing cOmplete EDTA-free protease inhibitor tablets (EDTA- free) (Sigma-Aldrich (St. Louis, Missouri, United States of America), 11836170001) by sonication on ice with an RNase free tip. Recombinant RNA sold under the tradename RNASIN™ (Promega Corporation (Madison, Wisconsin, United States of America), N2511) was added immediately after lysing according to the manufacturers protocol to protect crosslinked RNA. The cell lysates were then subjected to ultracentrifugation (100,000 x g, 45 min at 4°C) to isolate the cytosolic fraction in the supernatant and the membrane fraction as a pellet. The membrane pellet was resuspended in a modified RIPA buffer (50mM Tris-HCl, 1% NP-40 [Tergitol], 10% sodium deoxycholate, 10% SDS) with sonication. Protein concentrations were measured using the Bio-Rad DC protein assay (Bio-Rad Laboratories, Hercules, California, United States of America). Soluble fractions were normalized to 2.3 mg/mL prior to chloroform/methanol extraction, while membrane samples were normalized to 3.3 mg/mL. Proteomes were prepared according to methods listed above. Custom peaks were added to the CEP modification in the BYONIC™ search to account for probe fragments, including: +197.129 (d1), +240.1712 (d2), +425.2638 (d3), +453.2825 (d4). The PACCE-sensitive SR cutoff was lowered to ≥2 to account for low protein occupancy by RNA. RNase treatments. SILAC HEK293T cells were grown in 150 mm plates. Once at 80% confluency, heavy cells were treated with 100 µM 4SU for 16 hours, resulting a 95% confluent plate. Attorney Docket No.: 3062/193 PCT Cells were then washed with RNase free PBS, centrifuging at 1,400 x g for 3 min several times. Cells were then flash frozen in liquid N ₂ . Cells were lysed using a probe-tip sonicator (3 x 1 second pulse, 20% intensity) in 40 mM Tris-HCl (pH 7.5, 540 µL) with EDTA-free protease inhibitor. After sonication, 60 µL of 10x assay buffer (100 mM Tris-HCl, 25 mM MgCl ₂ , 5 mM CaCl ₂ , pH 7.6). Cell lysates were then combined, and an aliquot of each was taken to analyze concentrations. Aliquots were then split into each respective condition. RNase was added according to the manufacturer’s recommendation. All samples without RNase were treated with recombinant RNASIN™ according to the manufacturer’s protocol. Samples were incubated at 37°C for 1 hr. When complete, concentrations were normalized to 2.3 mg/mL at 500 µL and transferred to a 12-well plate. Samples were crosslinked at 312nm (1J/cm ² ) on ice. After, 432 µL were removed and treated with AHL-Pu-1 (25 µM final, 1 hr, 37°C). A desthiobiotin enrichment tag was then added using click-chemistry and the samples were processed using chloroform/methanol extractions as described above. The MS parameters discussed above were used for analysis. Unenriched proteomics of cells grown under PACCE conditions. Cells were treated, lysed and proteomes processed as described in the PACCE workflow for identification of RBPs section. Soluble and membrane concentrations were then normalized to 2mg/mL (100 µg L, 100 µg H) and subjected to a filter aided sample preparation (FASP) procedure with a 10kDa cutoff filter. Samples were washed with PBS (300 µL) to remove probe (14,000 x g, 15 min). Protein was then reduced with DTT (5 mM final, 56°C, 30 min). The mixture was then mixed with 200 µL urea/ammonium bicarbonate (UA, 8 M urea, 0.1M Tris-HCl, pH 8.5) and centrifuged (14,000 x g 15 min. An additional 200 µL was added and centrifuged again. An iodoacetamide solution (IAA, 0.05 M in UA, 100 µL) was added and incubated at room temperature for 20 min in the dark and then centrifuged at 14,000 x g for 10 min. The samples were washed 3x times with 100 µL of UA and centrifuged (14,000 x g, 15 min). The filter was washed with ammonium bicarbonate (ABC, 100 µL, 0.05 M in H ₂ O) and centrifuged (14,000 x g, 10 min). The protein was digested with trypsin (1:100 trypsin to protein) on the membrane in ABC overnight at 37°C. Once digested, an additional 40 µL of ABC was added to the unit and centrifuged (14,000 x g, 10 min) into a new tube. An additional 50 µL of 0.5 M NaCl was added to the top of the filter unit and centrifuged (14,000 x g, 10 min). The final peptide solution was acidified with acetic acid (5% final v/v). C18- Stage tips (2 discs) were conditioned with 20 µL methanol followed by 20 µL 80% ACN, 0.1% acetic acid (buffer B), and then 20 µL water, 0.1% acetic acid (buffer A) (900 x g, 1 min). Loaded samples were washed with 20 µL of buffer A three times (900 x g, 1 min) and eluted with 20 µL of buffer B three times (900 x g, 1 min). Samples were dried down and stored at -80°C. Reconstituted samples were analyzed using LC-MS/MS procedures described above. Mass spectra were analyzed using Proteome Discoverer software (PD, version 2.5; Thermo Fisher Scientific, Waltham, Massachusetts, United States of America) and searched using BYONIC™ (v. 4.1.10) with a human protein database (UniProt 02/18/2016, 20,199 entries). The following search Attorney Docket No.: 3062/193 PCT parameters were used: precursor and fragment ion mass tolerances ≤ 10 ppm and ≤ 50 ppm, respectively, signal to noise threshold ≥ 5, retention time shift ≤5 min, minimum peptide sequences per protein ≥ 1, peptide length ≥ 4, and a BYONIC™ score threshold = 300. The protein false discovery rate was 0.01. One static modification was included: cysteine carbamidomethylation (+57.021464 Da). Several dynamic modifications (3 total) were included, including: methionine oxidation (+15.9949 Da, common 1), heavy lysine (+8.0142 Da, common 2), and heavy arginine (+10.0083 Da, common 2). RNA sequencing (RNA-Seq) of cells grown under PACCE conditions. Cells were grown and treated as described in the PACCE workflow for identification of RBPs section. RNA was then purified from cells using a kit sold under the tradename PURELINK™ RNA Mini Kit (Thermo Fisher (Hampton, New Hampshire, United States of America), 12183018A) and frozen (-80°C). Samples were analyzed by Novogene (Beijing, China; PE150, 6G raw data). Briefly, messenger RNA (mRNA) was purified using poly-T magnetic beads and fragmented. Two strands were synthesized using random hexamer primers and dUTP or dTTP, respectively. The generated library was validated using Qubit and real-time PCR. Respectable libraries were pooled and sequenced using Illumina platforms. Reads containing adapters, poly-N and low quality reads were removed, while Q20, Q30 and GC content were calculated on the remaining data. featureCounts software (version 1.5.0-p3) was used to map to the Hisat2 (version 2.0.5) reference genome. Differential expression analysis was performed using DESeq2 (r package, 1.20.0). P-values were corrected using Benjamini and Hochberg’s method for controlling the FDR (P-value ≤ 0.5). Read counts were adjusted using edgeR (3.22.5). Euclidean distance calculations between Cys residues and RNA. Crystal structures were downloaded from the Research Collaboratory for Structural Bioinformatics (RCSB) protein data bank (PDB). Structures were filtered with the following criteria: protein co-crystalized with RNA, Homo sapiens, refinement resolution ≤ 3 angstroms, resulting in 270 structures. Structures with unnatural RNA (e.g., 5JS2) were manually removed. Structures were renumbered prior to processing (i.e., PDB numbers were matched to UniProt numbers) with PDBrenum ⁸⁰ . Euclidean distances (c ² = x ² + y ² + z ² ) were calculated between the sulfur atom of Cys (bio3d package, r) and any atom of the RNA, using the minimum calculated distance. Detected Cys sites were then matched to calculated distances. Structures that contained multiple chains produced slightly different distance measurements. All instances were used for analysis. Domain information for all Cys residues found in co-crystal structures was obtained from UniProt. The complete list of HEK293T membrane data obtained from PACCE studies (RNA-sensitive and -insensitive sites) were then compared to the database. Sites classified as RBDs (i.e., KH, RRM. Helicase C-terminal, Helicase ATP and Double stranded RNA-binding) were defined as domains highlighted in Figures 1D and 3C. Mean values were calculated for each corresponding group. Analysis of PACCE modified sites in intrinsically disordered regions. Intrinsically disordered regions from Homo sapiens were obtained from MobiDB predictions (v 4.1 ⁸¹ ). Attorney Docket No.: 3062/193 PCT Alignment of PACCE-modified sites. Identified PACCE sites were aligned using Clustal defaults in Jalview (available online at jalview.org). PACCE in vitro workflow. SILAC HEK293T cells were treated with 4SU, crosslinked, flash frozen and stored at -80°C as discussed above. Frozen pellets were thawed on ice. Cells were then reconstituted in PBS + protease inhibitor. The cells were lysed by passage through a 26G needle (15 times). Recombinant RNASIN™ was added according to the manufacture recommendation. The lysate was centrifuge at 100,000 x g for 45 min at 4°C. Soluble and membrane were separated. The membrane samples were reconstituted in PBS + protease inhibitor using sequential passage through various needle sizes (18G, 22G, 26G, respectively). Concentrations were normalized to 2.3 mg/mL (soluble) or 3.3 mg/mL (membrane) as described in the in situ experiments. Lysates were treated with either 500 μM iodoacetamide-alkyne or 25 μM AHL-Pu-1 for 1 hour at room temperature in the dark. The samples were then modified and processed using click chemistry and chloroform/methanol extractions as described above. Iodoacetamide-alkyne probe modifications on Cys residues were included as a variable modification of 509.2962. Custom peaks (as described above) were used for the identification of desthiobiotin fragments. Transient transfection of recombinant proteins. Recombinant protein production via transient transfection of HEK293T cells was performed according to methods described in Brulet et al., 2020, with several modifications. Briefly, HEK293T cells were plated at 2.9 million cell/mL in complete DMEM and grown for 40 hours to ~60% confluency. PTBP1 and EXOC4, were transiently transfected for 32 hours followed by a 16-hr incubation with 100 ^M 4SU prior to downstream processing. UIMC1 was transiently transfected for 47 hours prior to the addition of 500 ^M 4SU for 1 hr. 4SU was reconstituted in serum-free DMEM and then added to complete DMEM for incubation. The following plasmid constructs (human proteins) were purchased from GenScript Biotech (Piscataway, New Jersey, United States of America): pcDNA3.1-UIMC1-FLAG and pcDNA3.1-EXOC4-FLAG. Deletion mutant constructs were custom ordered from GenScript Biotech (Piscataway, New Jersey, United States of America) in a pcDNA3.1+/C-(K)-DYK plasmid backbone (PTBP1 ^1-59, PTBP1 ^1-143 and EXOC4 ^947-967). Fluorescence-based PAR-CLIP (fPAR-CLIP). HEK cells were cultured as discussed above. Recombinant proteins were transiently transfected as described above. Cell pellets were stored at -80°C until further processing. fPAR-CLIP experiments were carried out according to a protocol as previously described ⁶² . Briefly, the cell pellets were resuspended in 3 volumes of IP buffer (20 mM Tris-HCl, pH 7.5, 150 mM NaCl, 2 mM EDTA, 1% NP40, 0.5 mM DTT), incubated with 0.1 U/μl RNase I (Thermo Fischer Scientific (Waltham, Massachusetts, United States of America), AM2294) at 22°C for 10 min, then 5 μl of an inhibitor sold under the tradename SUPERASE•IN™ (Thermo Fisher Scientific (Waltham, Massachusetts, United States of America), AM2694) per mL was added to the cell extracts followed by centrifugation at 15,000 x g for 15 min at 4°C. Immunoprecipitation (IP) was done by Attorney Docket No.: 3062/193 PCT incubating the cleared extracts with anti-FLAG M2 magnetic beads (MilliporeSigma (Burlington, Massachusetts, United States of America), M8823) (25 μL of beads per mL of cell lysate) for 90 min at 4°C. Beads were washed three times with 1 mL IP buffer (without DTT) and resuspended in 2X bead volume IP buffer containing 1.5 U/μL RNase I at 22°C for 10 min. Beads were washed twice with 1 mL IP buffer, twice with 1 mL high salt wash buffer (20 mM Tris-HCl, pH 7.5, 500 mM NaCl, 2 mM EDTA, 1% NP40) and twice with 1 mL CIP-PNK-Ligation wash buffer (50 mM Tris-HCl, pH 7.5, 10 mM MgCl ₂ ). Dephosphorylation was carried out in 50 μL reaction mixture (5 μL cutsmart buffer, 2.5 μL Quick CIP, 2.5 μL SUPERASE•IN™, 40 μL nuclease free H ₂ O) (Quick CIP, New England Biolabs Inc. (Ipswich, Massachusetts, United States of America), M0525S) at 37°C for 10 min with shaking. Next beads were washed three times with 1 mL CIP-PNK-Ligation wash buffer. To fluorescently label the ribonucleoprotein complex to allow for visualization, on beads 3’ fluorescent adapter ligation was performed in 50 μ reaction mixture (0.5 μl 50 μM fluorescent 3’ adapter, 5 µL 10× T4 RNA ligase reaction buffer, 15 μl 50% aqueous PEG-8000, 2.5 μl T4 Rnl2(1–249)K227Q ligase, 2.5 μl SUPERASE•IN™, 24.5 μl nuclease free H ₂ O) (Fluorescent barcoded 3’ adapter: 5'- rAppNNTGACTGTGGAATTCTCGGGT(fl)GCCAAGG-fl* (SEQ ID NO: 15), wherein the rApp is a 5-adenylation moiety, the underlined hexamer is a barcode, and (fl) and (fl*) indicate the locations of one or two fluorescent moieties that can be attached to the 3’ adaptor, which in some embodiments can be selected from the group consisting of an Alexa dye and/or an ATTO dye, which in some embodiments can be the same or different), T4 Rnl2(1-249)K227Q ligase, New England Biolabs Inc. (NEB; Ipswich, Massachusetts, United States of America), M0351) at 4°C for overnight with gentle agitation. Next, beads were washed twice with 1 mL high salt wash buffer and resuspended in 60 μl 2x loading buffer (sold under the tradename NUPAGE™ LDS Sample Buffer +DTT, Thermo Fisher Scientific (Waltham, Massachusetts, United States of America), NP0008) and incubated for 5 min at 95°C. The supernatants were separated on a gel sold under the tradename NUPAGE™ 4-12% Bis-Tris Midi Protein gel (Thermo Fischer Scientific, Waltham, Massachusetts, United States of America, Catalog No. WG1401BOX) alongside 20 μl of 1/10 dilution of a protein size standard mixture sold under the tradename PAGERULER™ Plus Prestained Protein Ladder (Thermo Fisher Scientific, Catalog No.26620). After electrophoresis, gel was placed in clear plastic sheet protector and scanned for AF647 using a scanner sold under the tradename TYPHOON™ 9500 (GE Healthcare, Chicago, Illinois, United States of America) to visualize bands corresponding to the RBP-fluorescent 3′adapter ligation products. Western blots of RNA-sensitive proteins. Western blot analysis of recombinant protein and mutants was performed as described in Brulet et al. 2020, with several modifications. Cells overexpressing each respective plasmid were crosslinked (1J/cm ² ) on ice. Cells were then scraped in RNase-free PBS and pelleted (1,400 x g, 3 min). Cells were reconstituted in IP buffer and lysed using a 26-gauge syringe. Samples without RNase treatment were spiked with RNASIN™ and incubated on Attorney Docket No.: 3062/193 PCT ice. The RNase treated sample was incubated with RNase A and RNase I as described by the manufacturer for 10 min at room temperature. Lysates were then cleared at 15,000 x g for 15 min at 4°C. Samples were loaded onto the gel with Laemmli loading dye and heated for 1 min at 95°C. The following primary antibody was used: anti-FLAG antibody (Sigma-Aldrich, St. Louis, Missouri, United States of America), F7425-2MG, 1:1000). The following primary antibody was used as a loading control: Goat anti-GAPDH IgG (Cell Signaling Technology, Danvers, Massachusetts, United States of America), 2118S, 1:1,000). Appropriate secondary antibodies (1:10,000) were purchased from Thermo Fisher Scientific (Hampton, New Hampshire, United States of America). Polynucleotide kinase (PNK) assay. PNK addition of γ-32P rATP was conducted as previously described ⁶³ , with various modifications as highlighted below. Proteins were recombinantly overexpressed, 4SU treated, and crosslinked as described above. Cell pellets were flash frozen and stored at -80°C until use. Cells were lysed in lysis buffer (100 mM NaCl; 50 mM Tris-HCl pH 7.5; 0.1% SDS; 1 mM MgCl ₂ ; 0.1 mM CaCl ₂ ; 1% NP40; 0.5% sodium deoxycholate; protease inhibitors (Roche, Basel, Switzerland, Catalog No. 11836170001) via 10 passages through a 26g needle on ice. Lysates were then cleared at 15,000 x g for 15 min at 4°C. The supernatant was treated with various concentrations of RNase A (8 ng/ ^L, 2 ng/ ^L and 0.5 ng/ ^L; Thermo Fisher Scientific, Waltham, Massachusetts, United States of America), EN0531) and 2U/mL DNase (New England Biolabs, Ipswich, Massachusetts, United States of America), M0303L) for 15 min at 37°C. IP was conducted at 4°C for 2 hrs. The samples were then washed 3 times with lysis buffer followed by two additional washed with PNK buffer (50 mM NaCl; 50 mM Tris-HCl pH 7.5; 10 mM MgCl ₂ ; 0.5% NP-40; protease inhibitors (Roche, Basel, Switzerland, Catalog No. 11873580001)). Magnetic columns were then capped, and samples were incubated in 0.1 μCi/μl [γ-32P] rATP, 1 U/μl T4 PNK (NEB, Ipswich, Massachusetts, United States of America), 1 mM DTT and labeled for 15 min at 37°C. Samples were eluted with hot Laemmli before separation on an SDS-PAGE gel and transferred to a PVDF membrane. Membranes were then exposed to an autoradiographic film, and the developed films were scanned using an imaging system sold under the tradename IMAGEQUANT800™ (Cytiva, Uppsala, Sweden). Functional validation of RNA-sensitive Cys residues. Cell pellets were reconstituted in 40 mM Tris-HCl (pH 7.5) with protease inhibitor. The slurry was then probe tip sonicated on ice with an RNase free tip. Recombinant RNA sold under the tradename RNASIN® (Promega Corporation, Madison, Wisconsin, United States of America), N2511) was added immediately after lysing according to the manufacturers protocol to protect crosslinked RNA. Proteins were separated via SDS-PAGE. Proteins of interest were visualized using anti-FLAG antibodies. EXOC4 data was analyzed using a quantile- quantile (QQ) plot to assess normality followed by a ratio paired t-test (paired, parametric) to determine significance. PTBP1 was also tested for normality. The data were analyzed using an RM one-way ANOVA without corrections followed by a Tukey’s post hoc test for multiple comparisons. Attorney Docket No.: 3062/193 PCT EXAMPLE 1 Development of Clickable Electrophilic Purines The purine heterocycle is well suited for developing a chemoproteomic probe because the fused pyrimidine-imidazole aromatic ring system affords both electron-deficient and -rich sites for integration of electrophilic and reporter tags. See Figure 1A. The π-deficient pyrimidine ring of purine contains electron-deficient carbons at the C2 and C6 positions that can be converted into an electrophilic site for nucleophilic aromatic substitution (S _N Ar) reactions with protein nucleophiles. ²⁸ See Figure 6A. Activation of the C6 position for S _N Ar reaction using various leaving groups ^29-32 including halogens ^33,34 has been demonstrated in synthetic chemistry. A chloro group was selected as an initial leaving group, because of its tempered ability to activate purines for S _N Ar reaction ³⁵ . An alkyne reporter tag was appended at the N7 or N9 position, and the regioisomeric products were confirmed by X-ray crystallography to generate the clickable electrophilic purines (CEPs) AHL-Pu-1 and AHL-Pu-2, respectively. See Figure 6B. HPLC studies confirmed that CEPs undergo S _N Ar reaction with nucleophiles that mimic amino acid sidechain groups. ³⁶ Using HPLC Method B described herein above, consumption of the probes in the presence of butyric acid (Asp/Glu mimetic), in the presence of butanethiol (Cys mimetic), in the presence of propanamide (Gln/Asn mimetic), in the presence of n-butylamine (Lys mimetic), and in the presence of p-cresol (Tyr mimetic) was analyzed and quantified. Both probes reacted with the cysteine mimetic butanethiol in a time-dependent manner, with AHL-Pu-1 showing a modest enhancement in reactivity compared with AHL-Pu-2 (t _1/2 = 1.9 and 9.1 min, respectively). See Figures 6B, 6C, and 7A. Neither probe showed reactivity against the other nucleophiles tested except for moderate activity of AHL-Pu-1 for p-cresol. See Figures 7A and 7B. The structure of reaction product peaks observed by HPLC were confirmed by comparison with synthetic standards that were analyzed by X-ray crystallography and NMR. The stability of CEPs was tested in an aqueous buffer and negligible degradation was observed after incubation for 2 days. See Figure 7C. Summarily, compound integrity after 48 hours was determined to be >99% for AHL-Pu-1 and >92% for AHL-Pu-2. In addition, crystallography performed on AHL-Pu-2 and the unit cell determination showed an exact match to a previously published compound that is consistent with the expected product ⁸⁴ . EXAMPLE 2 CEPs as Cysteine-reactive Probes in Living Cells Concentration- and time-dependent studies were performed in live DM93 cells to identify non- toxic treatment conditions with minimal perturbation to the transcriptome and proteome. See Figures 8A-8D and 9A-9C. Next, SILAC light and heavy DM93 cells were treated with dimethyl sulfoxide (DMSO) vehicle or CEP (25 μM, 4 h) followed by cell lysis and copper-catalyzed azide-alkyne cycloaddition (CuAAC) conjugation of desthiobiotin-azide to CEP-modified proteins, avidin chromatography enrichment of CEP-modified peptides, and high-resolution LC-MS/MS and Attorney Docket No.: 3062/193 PCT bioinformatics analysis as previously described ³⁷ and as depicted in Figures 10A-10D. Consistent with the HPLC findings, both CEPs exhibited high chemoselectivity for cysteines; ~72% of probe-modified residues detected in CEP datasets were assigned to cysteines (mass adduct of 604.2637 Da). See Figures 11A and 11B. CEP-modified sites were largely competed with free purine but not pyrimidine nucleobases in competition studies in vitro, further verifying that CEP binding activity is purine dependent. See Figures 10D and 12A-12D. The position of the alkyne functional group at the N7 (AHL-Pu-1) versus N9 (AHL-Pu-2) position affected CEP binding activity. AHL-Pu-1 was selected for the remaining studies because of broader proteome reactivity with similar protein class coverage compared with AHL-Pu-2. See Figures 13A-13C. CEP-mediated chemoproteomics was applied to additional human adherent and suspension cell lines to quantify thousands of probe-modified cysteine sites on proteins in situ. Gene Ontology ^38,39 (GO) analyses of probe-modified proteins from aggregate AHL-Pu-1-treated cell datasets revealed enrichment for proteins involved in nucleic acid-, RNA- and general heterocyclic compound-binding. See Figures 1A-1C. Compared with the general cysteine-reactive probe iodoacetamide (IA) and specifically datasets using the desthiobiotin-tagged analog (DBIA ⁴⁰ ), protein function enrichments were largely comparable between probes with the exception of nucleic acid- and protein-binding that were specific for CEP and DBIA, respectively. See Figure 1C. The enrichment for RNA binding function prompted further examination of CEP coverage of this protein class given the growing interest in developing small molecule binders of RBPs ⁴¹ . Statistically significant overlap was observed with reported human RBPs ^1,12,13,20 using CEP-mediated chemoproteomics (~37% overlap, p = 4.09 x 10 ^-5 ). See Figure 13D. Both CEP and DBIA showed substantial coverage of detected RBPs (~37 and 61%, respectively) with an additional ~130 RBPs captured using the former cysteine-reactive probe. See Figure 13D. Domain enrichment ³⁶ analyses also revealed comparable coverage of RBDs, as well as other functional protein domains, for both CEP and DBIA. See Figure 1D. In summary, it was demonstrated that CEPs are non-toxic, cysteine-reactive probes that are complementary to the existing IA probes but can serve as effective chemoproteomic capture agents for global protein- and binding-site level quantification of RBPs in live cells. EXAMPLE 3 Quantifying Protein-RNA Interactions in Cells by Photoaffinity Competion A Photo-Activatable-Competition and Chemoproteomic Enrichment (PACCE) strategy was developed to identify CEP-modified sites on proteins that are competed by RNA photo-crosslinking competition (RNA-sensitive cysteine or RS-Cys site). See Figure 2. The photoactivatable nucleoside 4SU is metabolically incorporated into labeled RNAs in cell culture to facilitate UV crosslinking of photoreactive 4SU-RNA to RBPs in situ ^13,15 .4SU was chosen for UV-mediated crosslinking of cellular RNA to protein at 312 nm because of (i) higher specificity (e.g., DNA-protein crosslinks and single- Attorney Docket No.: 3062/193 PCT strand breaks are ~1000-fold less at 312 compared with 254 nm ⁴² ) without compromising proteomic sensitivity ^27,43 , (ii) less damage to cells ^1,27 , and (iii) a UV wavelength closer to the optimum extinction coefficient of 4SU ⁴⁴ . See Figures 9A-9C and 14A-14D. PACCE can globally capture RS-Cys located in or proximal to the RNA crosslinked site by identifying direct and proximal competition events (SILAC ratio or SR >2) from both 4SU-dependent and native RNA crosslinking. See Figure 15A. First, it was tested whether CEP labeling in vitro vs in situ impacted the ability to detect RS-Cys sites, because RBPs are known to function in larger RNP complexes that can be disrupted upon cell lysis ¹ . SILAC HEK293T cells were subjected to RNA crosslinking competition followed by in vitro CEP labeling (25 µM, 1 hr). An alkyne-tagged iodoacetamide probe counterpart was included for direct comparison (IA-alkyne; 500 µM, 1 hr). As expected, IA-alkyne captured ~2-fold the total number of cysteines compared with CEP, which resulted in a comparable increase in the number of detectable RS-Cys sites. See Figure 3A. In contrast and in support of using cell-active probes for maximal RS-Cys detection, CEP activity in situ (25 µM, 1 hr) produced only a modest increase in total cysteine site coverage (~35%) but a nearly 7-fold enhancement in RS-Cys sites captured. See Figures 3A, 16A, and 16B. RNA crosslinking competition was further supported by a substantial reduction in the number of RS-Cys sites detected when RNase was added prior to UV irradiation of proteomes. See Figures 17A and 17B. Using RNA crosslinking competition and in situ CEP labeling, a compendium of RS-Cys sites was produced from analyses in HEK293T and DM93 cells, which were cell lines selected based on a high number of CEP-modified sites identified. In aggregate, 11,385 CEP probe-modified peptides were detected, corresponding to 4,523 protein identifications. From this dataset, >5,000 RS-Cys sites were identified that mapped to ~3000 proteins. See Figures 15A-15C. See also Table 1 below. The mean SR for RS-Cys peptides were generally higher compared with non-RS-Cys counterparts. See Figure 15D. Approximately 37% of RS-Cys-containing proteins are known RBPs and this number of RS-Cys- containing RBPs represent ~36% coverage of the human annotated RBPs ^1,3 (~36% overlap, p = 2.97 × 10 ^-5 ). See Figure 3B. The RS-Cys-containing proteins were enriched for functional terms related to RNA binding and metabolism. See Figure 3B. Of note, analysis of RS-Cys sites revealed prominent domain enrichment of known RBDs that was comparable with published RNA-based proteomic detection methods for RBP analyses. ^26,27,45 See Figure 3C. Non-canonical RNA binding regions were also identified within known RBPs The frequency of these putative non-canonical RNA-binding regions varied between RBP class and even within members of the same RBP superfamily as exemplified by the DEAD box RNA helicase family ⁹ Analysis of the PACCE datasets also revealed substantial modification of intrinsically-disordered regions that combined with the domain enrichment analyses identified ~900 protein domains or regions with annotated RNA binding function that contain a RS-Cys site. Attorney Docket No.: 3062/193 PCT In summary, PACCE is capable of quantifying protein-RNA interactions directly in live cells by integrating established photoactivatable ribonucleosides with chemoproteomic workflows. See Figure 18. Table 2, below, provides a summary of key advantages and disadvantages of PACCE and other proteomic methods for RBP analysis. The discovery of RS-Cys sites in known and non-canonical RNA-binding regions provides additional opportunities for covalent binding to cysteines for pharmacological modulation of RBPs as demonstrated in a recent report ⁴⁶ . Table 2 Comparison of PACCE with RIC Methods EXAMPLE 4 Location of RS-Cys Sites in Protein-RNA Interfaces Closer inspection of RS-Cys site and proximity to bound RNA provided additional clues to the observed sensitivity of these sites to RNA crosslinking competition. For example, the RS-Cys detected on DDX19B (C393) and DDX17 (C298) are located close to the bound RNA and this proximity is reflected in the sensitivity of the respective sites to RNA crosslinking (SR >2 in PACCE compared to ~1 for the mixing control, p ≤ 0.05), See Figures 4A, 19A-19C, and 20. As a direct comparison, the RNA-insensitive site detected on DDX3X was not in proximity to bound RNA, which agrees with the observed lack of competition from RNA crosslinking (C298, SR ~1 for the mixing control and RS-Cys conditions). See Figure 4B. 270 protein-RNA structures (X-ray and cryo-electron microscopy) available in the protein data bank (PDB ⁴⁷ ) were searched to broaden the evaluation of RS-Cys location in proximity to bound RNA on RBPs. The distance from the thiol group of all cysteine sites to the nearest atom on the RNA molecule in structures were calculated using an in-house algorithm. From this group of cysteines, 60 RBP-RNA structures were identified that contained a RS-Cys site. These data were used to assess RNA sensitivity by PACCE (SR value) as a function of distance between the CEP-modified cysteine and interacting RNA. See Figure 4C. The mean cysteine-RNA distance across all RBP-RNA structures analyzed was ~23 angstroms and this distance was reduced to ~20 angstroms in RS-Cys sites. If RS-Cys sites found in known RBDs are considered, this mean cysteine-RNA distance is further lowered (~11 angstroms). See Figure 4D. Examples of RS-Cys located at a larger than expected distance from the RNA interaction site were also Attorney Docket No.: 3062/193 PCT found. For example, the C83 site in the RRM domain of SF3B6 had a calculated cysteine-RNA distance of 20 angstroms. See Figures 4C and 19A. This longer distance suggests dynamic RBP-RNA interactions ^48-50 that are difficult to capture in static structures but can be revealed by PACCE. See Figure 19A. A subset of RS-Cys sites with longer distances from RNA were detected on proteins found in large multi-RBP complexes that made it difficult to evaluate individual RBP-RNA distances (e.g., SPF27 C106, 98 angstroms). See Figure 4C. EXAMPLE 5 Discovery of Moonlighting RNA-binding Activity in the Human Proteome A substantial fraction of the RS-Cys sites mapped to proteins not found in the annotated RBP interactome (‘putative RBPs’ group). See Figure 3B. Analysis of this group of proteins revealed enrichment for terms related to molecular- (e.g., ligand binding) and protein-binding. The ligand binding group included E3 ligases (PPIL2), ubiquitin-binding proteins (UIMC1), and structural proteins (EXOC4). Interestingly and in further support of PACCE for RBP profiling, PPIL2 was recently demonstrated to exhibit RNA-binding activity as a component of the minor spliceosome ⁵¹ . To test whether PACCE could discover ‘moonlighting’ RBP activity ²³ , candidate proteins were selected that contained at least a single RS-Cys and that had not been previously been annotated as an RBP. The exocyst complex component 4 (EXOC4) protein was selected as a candidate for proof-of- concept studies because it contained a single RS-Cys located in an unknown region (C957). See Figures 5A and 21. EXOC4 is a one of the eight subunits of the exocyst protein complex, which functions to tether post-Golgi secretory vesicles to the plasma membrane before exocytic fusion ^52,53 . Previous studies reported protein-protein interaction function for EXOC4 ⁵⁴ but only a very limited number of reports described nucleic acid binding ^55,56 . Akin to the migration behavior of a known RBP (Polypyrimidine tract-binding protein 1 or PTBP1), a shift in molecular weight was observed upon UV irradiation of 4SU-labeled, EXOC4-expressing HEK293T cells that was muted in the absence of UV irradiation and reduced with RNase treatment. See Figures 22A and 22B. Without being bound to any one theory, the higher molecular weight signals observed without UV irradiation are likely due to crosslinking from ambient light as previously reported ⁵⁷ . A functionally orthogonal candidate, BRCA1-A complex subunit RAP80 (UIMC1), was selected to demonstrate that PACCE can discover unanticipated RBP activity across different protein classes. UIMC1 is annotated as a ubiquitin-binding protein that recognizes ubiquitinated histones found at sites of DNA damage to direct the BRCA1-BARD1 complex to repair DNA double-strand breaks (DSBs) ⁵⁸ . RS-Cys sites were identified in the ubiquitin-interacting motif (UIM, C121) and unknown region of UIMC1 (C691). See Figure 5A. See also Figure 23A-23C. The UIM domains facilitate UIMC1 recognition of Lys ⁶³ -linked polyubiquitin chains at DNA damage sites ^59-61 . The evolutionary conservation of these cysteines further support function. Photoactivated crosslinking of 4SU-RNA in Attorney Docket No.: 3062/193 PCT UIMC1-expressing HEK293T cells resulted in RNase-sensitive, gel migration behavior that supports RBP activity for UIMC1. See Figure 22C. Next, candidate RBPs, EXOC4 and UIMC1, were tested for direct binding of RNA in cultured cells by fluorescent photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation (fPAR-CLIP ⁶² ). HEK293T cells were transfected with plasmids encoding FLAG-tagged EXOC4, UIMC1, and PTBP1 (RBP control), labeled nascent RNA with 4-SU and crosslinked RNPs with 312 nm UV. Next, the FLAG-tagged RNPs were immunoprecipitated, and ligated fluorescent oligoribonucleotide adapters to interacting RNAs followed by SDS-PAGE. See Figures 5B, 22D and 22E. Fluorescently labeled FLAG-RNPs for all three proteins migrated at ~25 kDa above their predicted size, corresponding to the molecular weight of the fluorescent adapter, indicating that not only the canonical RBP, PTBP1 bound RNA in cells, but also candidate RBPs UIMC1 and EXOC4. Protein-RNA interactions were further verified by the polynucleotide kinase (PNK) assay ⁶³ . Cellular lysates from UV (312 nm) irradiated, 4SU-RNA-treated HEK293T cells recombinantly expressing PTBP1, EXOC4 or UIMC1 were subjected to RNaseA at increasing concentrations. Afterwards, proteins were immunoprecipitated followed by radioactive labeling ( ³² P) of RNA 5′ ends with T4 PNK. As shown in Figure 22F, increased higher molecular weight radiolabeled bands were observed upon UV irradiation of HEK293T cells overexpressing RBP and this expected ‘smeared’ signal corresponding to RNP complexes was reduced in an RNaseA-concentration dependent manner. Collectively, these studies demonstrated that PACCE can discover RNA binding activity for proteins without prior RBP annotation. The unbiased nature of PACCE is further highlighted by the identification of RS-Cys sites in poorly defined regions (EXOC4) or in domains lacking RBD annotation (UIM domain, UIMC1). EXAMPLE 6 Validation of EXOC4 C957 role in RNP formation A study was performed to test whether deleting protein regions containing a RS-Cys site affects RNP complex formation in 4SU-RNA crosslinked cells. Recombinant PTBP1was used for proof of concept that deletion of RNA-binding regions on a known RBP resulted in quantifiable changes in crosslinked RNP species. PTBP1 contains 4 RRM domains that are reported to function in RNA binding ^64,65 . PTBP1 mutants were recombinantly expressed that progressively deleted the N-terminal region containing RRM1 in HEK293T cells (PTBP1 ^1-59 and ^1-143 mutants) and assessed the resulting effects on PTBP1 RNP complexes formed in cells compared to WT protein. See Figure 24A. Crosslinked PTBP1 RNPs were detected in 4SU-RNA-treated cells expressing recombinant WT protein. Deletion of the N-terminus (PTBP1 ^1-59) had no effect but removal of RRM1 resulted in statistically significant loss of crosslinked PTBP1 RNPs (PTBP1 ^1-143). See Figures 5C and 24C. After benchmarking with PTBP1, a study was conducted to test whether mutagenesis of the RNA- binding interface on EXOC4 would affect RBP-RNA complex formation in cells. Since EXOC4 lacks Attorney Docket No.: 3062/193 PCT a known RBD, residues flanking the evolutionarily conserved, RS-Cys site (C957) were mutated and a statistically significant decrease in EXOC4 RNP species were detected compared with WT counterpart (EXOC4 ^947-967). See Figures 5C, 24B, and 24C. These data combined with direct identification of RNA bound to EXOC4 (see Figure 5B) further authenticates the RNA-binding activity of EXOC4. Collectively, these mutagenesis experiments with known and unannotated RBPs provide additional evidence in support of PACCE discovery of functional cysteines involved in protein-RNA interactions in cells. Discussion of the EXAMPLES RNA-protein interactions ^1-3 orchestrate complex networks of RNPs that regulate gene expression, translation, and epigenetic modulation ^5,11 . While proteomic methods exist for identifying the composition of RBPs in cells ^12,13 , corresponding assays to assess RNA-binding activity of proteins are currently lacking. The ability to measure RBP activity states with binding site resolution is an important step towards elucidating the complete inventory of RBDs in the human proteome. In particular, the use of chemical probes for RBP profiling can provide for and streamline ligand discovery efforts through competitive activity-based protein profiling (ABPP) screening ^46,66 . As described herein, PACCE is introduced as a chemoproteomic method to quantify RNA-binding activity of proteins directly in living cells. A distinct feature of the presently disclosed approach compared with existing RIC methods is the use of a small molecule probe and not RNA to covalently bind, enrich, and identify RBPs in cells. See Figure 18 and Table 2, above. CEPs act as cysteine-reactive probes that do not require UV irradiation but covalently bind proteins via S _N Ar for ABPP profiling of RBPs and other purine-binding proteins in situ. See Figures 1A-1D. CEPs are readily integrated into modern chemoproteomic workflows ⁶⁷ and do not require an RNA purification or extraction step to streamline rapid fluorescence gel-based or high-content mass spectrometry profiling of RBPs. The robust in situ activity of CEPs is believed to be one aspect resulting in maximizing RS-Cys coverage using the instant method as compared with in vitro probe labeling using CEPs and the broad-spectrum IA probe counterpart. See Figure 3A. RNA binding sites have been mapped by LC-MS/MS analysis of RNA crosslinked peptides derived from RBPs ²⁵ . While feasible, RNA-peptide adducts are heterogenous and require specialized proteomic workflows to deconvolute resulting data for increased resolution but at the cost of reduced sensitivity ¹ . See Figure 18. The presently disclosed PACCE methodology provides an alternative ‘footprinting’ strategy for localizing protein-RNA interfaces that utilizes photoactivatable cellular RNA to crosslink and protect RNA binding regions on proteins from CEP probe labeling. The protected region(s), containing RS-Cys sites on proteins, can be identified from the amino acid sequences of CEP- modified peptides displaying reduced LC-MS/MS abundance in the presence of RNA crosslinking competition. Importantly, the presently disclosed studies demonstrate (i) RNA specificity through loss Attorney Docket No.: 3062/193 PCT of 4SU-RNA-dependent probe competition upon RNase treatment (See Figures 17A and 17B), and (ii) proximity of RS-Cys sites to the bound RNA across hundreds of RBP-RNA structures analyzed. See Figures 4A-4D. The standardized chemical probe format of CEPs and the broad diversity of RBPs amenable to RNA crosslinking competition was leveraged to globally quantify RS-Cys on proteins with high resolution and sensitivity (~5,500 candidate sites). See Figures 3A-3C and Table 1, below). The site specificity afforded by PACCE enabled domain enrichment analyses to discover RS-Cys that are prominent in both known and non-canonical RBDs. See Figure 3C. Notably, the presently disclosed PACCE method provided identification of several functional domains including, for example, Q-motifs that can be further evaluated as RBDs in future studies. The unbiased nature of PACCE was further showcased by discovery of RS-Cys across diverse protein classes that lacked prior RBP annotation. While several enriched protein functions were related to nucleotide recognition (e.g., nucleoside phosphate binding), a subset of proteins belonged to protein classes not obviously related to RNA binding. See Figure 3B. This latter class was best exemplified by EXOC4 and UIMC1, which were initially discovered by PACCE followed by verification of RNA- binding activity using orthogonal crosslinking and gel-shift assays ⁵⁷ as well as direct validation of bound RNA by fPAR-CLIP ⁶² . See Figure 5B. As an additional control, it was demonstrated that deletion of RNA-binding regions on RBPs identified by PACCE impaired formation of protein-RNA complexes in cells. See Figure 5C. The discovery of RBP activity for UIMC1 is intriguing given the emerging roles of RBPs in the DNA damage response through direct repair or transcriptional and post-transcriptional control of gene expression ^68-70 . Follow-up studies can be performed to test whether the RNA-binding activity of UIMC1 is related to directing specificity of histone recognition at DNA DSBs. While the functional relevance of RNA binding activity for EXOC4 is less clear, it is interesting that RBPs are implicated in loading RNA into extracellular vesicles ^71,72 . Although the RS-Cys sites detected by PACCE are functionally enriched for RBDs, sites meeting the competition threshold can represent cysteine-containing regions with saturated binding to cellular RNA. Crosslinking at higher energy (e.g., 254 nm) to increase native RNA-protein capture can be performed to further improve coverage of RS-Cys sites detected in proteomes ²⁷ . Of note, however, it that the present selection of 312 nm for UV irradiation in PACCE has the capacity for crosslinking native RNA to proteins in cells. See Figure 14C and 15A. The versatility of 312 nm to crosslink native and 4SU-RNA was a consideration given the reported 4SU incorporation rates in cells (1-4% of uridines ⁷³ ). While RS-Cys sites are not necessarily located at the exact site of RNA crosslinking, it is believe that this feature of PACCE is not a limitation, but rather a strength of the methodology to detect cysteine sites that are potentially involved in allosteric regulation of RNA recognition by proteins. In Attorney Docket No.: 3062/193 PCT support of the latter, a recent report demonstrated the utility of using cysteine-reactive ligands to pharmacologically modulate RBP function ⁴⁶ . It is believed that PACCE can be highly complementary to RNA-focused global methods including PAR-CLIP ¹⁵ and SLAM-Seq ⁷⁵ to bridge RNA sequence specificity with RBP activity in cells. Akin to other chemoproteomic methods, PACCE is well positioned to screen for covalent ligands using competitive ABPP methods ^46,66 that can perturb RBPs with protein class- and binding site-selectivity across the human proteome ⁴⁶ . Importantly, the PACCE concept is versatile and can readily accept covalent probes that target more abundant residues found at RNA-protein interfaces ⁷⁶ including, for example, lysines ⁷⁷ and tyrosines ⁶⁷ . In summary, the presently disclosed method represents a robust and high-content discovery platform for chemoproteomic discovery and quantitation of RNA binding activity on proteins directly in living cells. The ability for covalent binding to cysteines and potentially other ligandable residues in RBDs can help advance therapeutic discovery of this important class of ‘undruggable’ targets ⁴¹ . REFERENCES All references listed in the instant disclosure (denoted with superscripted numbers which refer to the numbering below), including but not limited to all patents, patent applications and publications thereof, scientific journal articles, and database entries (including but not limited to UniProt, EMBL, and GENBANK® biosequence database entries and including all annotations available therein) are incorporated herein by reference in their entireties to the extent that they supplement, explain, provide a background for, and/or teach methodology, techniques, and/or compositions employed herein. The discussion of the references is intended merely to summarize the assertions made by their authors. No admission is made that any reference (or a portion of any reference) is relevant prior art. 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Table 1 Exemplary Identified RS-Cys Sites and Corresponding Proteins Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT Attorney Docket No.: 3062/193 PCT ^a : each entry includes a Uniprot Accession No. followed by an amino acid position after the colon. For example, the last row shows ZHX2 in the left column and Q9Y6X8: 759. This denotes that that ZHX2 is the Gene Name (in this case, the gene is Zinc fingers and homeoboxes protein 2, the Uniprot Accession Number is Q9Y6X8, and the amino acid that is modified is amino acid 759 of Uniprot Accession Number Q9Y6X8). Furthermore, unless otherwise noted, the amino acid at the noted position is a cysteine that in some embodiments has been covalently modified by a clickable electrophilic purine (CEP) compound (e.g., modification from AHL-Pu-1) and in Attorney Docket No.: 3062/193 PCT some embodiments has alternatively been modified by alkylation with iodoacetamide treatment (carbamidomethylation) of samples as part of the LC-MS sample preparation. It is further noted that in some embodiments, additional cysteines, in some embodiments within about 20 amino acids of the noted position, can also be modified by CEP modification and/or alkylation. b: In some embodiments, at least one methionine in the vicinity (e.g., within about 20 amino acids) of certain of the noted cysteine amino acid positions can be oxidized to methionine sulfoxide. It will be understood that various details of the presently disclosed subject matter can be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.