The present invention relates generally to the field of nucleic acid chemistry, specifically to A^-position modified cytidine triphosphates. The present invention also relates to methods of making and using the same. The invention includes the use of the modified nucleotides for the synthesis of modified product, which can be an oligonucleotide, double- or single- stranded DNA or RNA fragment, or an aptamer. The present invention is based on the idea of using A^-modified cytidine triphosphates for the biosynthesis and application of modified nucleic acids. The present idea of biosynthesis and application of modified nucleic acids covers synthesis, modification and application of nucleic acid molecule comprising at least one cytosine nucleobase modification of the compounds described herein.

The invention is illustrated in several examples, using polymerases described herein. However, other polymerases, that are substrates for modified cytidine compounds, can be used.

BACKGROUND OF THE INVENTION

Oligonucleotides (ONs) are short DNA or RNA molecules which have found use in a wide variety of applications in the field of molecular biology, biotechnology, synthetic biology as well as medicine (diagnostics, antisense technology). ONs comprising natural nucleotides are usually unsuitable for application in the field of medicine due to their poor biostability and resistance to nucleases, e.g., blood serum nuclease. In order to improve ON's characteristics, e.g., stability, resistance, selectivity, catalytic activity, pharmacokinetic or pharmacodynamic characteristics, modified nucleotides are used. Moreover, limited chemical diversity of ONs plays an important role in various applications. As a result, ONs are modified to create a desirable diversity and to improve therapeutic ON's affinity, target specificity and distribution in the body, to control aggregation of ONs or to lower their toxicity.

Affinity and specificity are two major characteristics of any nucleic acid-based assay that determine its efficiency. Generally, sequence specificity and binding affinity anti-correlate with each other meaning that as affinity for the chosen target sequence increases, the likelihood of association with closely related but non-target sequences also increases (Lomakin and Frank-Kamenetskii, 1998). Such anti-correlation points out that the formation of nucleic acid duplexes and triplexes necessary for their functioning were either weak or nonspecific which in turn would contradict the methods and technologies which are based on nucleic acid interactions. This bizarre tendency can be explained using one of the two theories of biomolecular interactions. There are two modes of biomolecular interaction - shape complementarity or steric fit and nucleation-zipping. The shape complementarity concept means that biochemical recognition in enzyme-substrate, antigen-antibody and aptamer-small-molecule complexes is provided by a precise steric fit between surface regions of the binding ligand and the target (Koshland, 1995). The theory of steric fit is therefore considered to confirm the affinity-specificity correlation (von Hippel and Berg, 1986). The mechanism of nucleation zipping appears to be essential for the formation of nucleic acid duplexes and triplexes (Craig, Crothers, and Doty 1971 ; Alberti et al, 2002). By contrast to the steric fit, a strong zip with one irregular or missing link can still be fastened via bypassing this small mismatched part (Rougee et al, 1992). As a consequence, a problem of affinity-specificity is observed: both complexes correct and mismatched show comparable duplex formation efficiency.

Methods that are based on nucleic acid interactions can be divided into two major groups - the one which ignores several mismatches and the other that requires high specificity. For example, genome analysis is a technology which is based on a very high affinity and specificity. Genome analysis is a very important methodology in biochemical and biomedical research area as well as provides knowledge about various pathogens and diseases. The basis of genome analysis is often detection of trace amounts of specific DNA sequences and therefore both affinity and specificity are essential. In order to discriminate between two almost identical DNA sequences (e.g. single nucleotide polymorphism), high sequence specificity factor is the main one. Therefore, it is essential to deal with affinity-specificity problem as a major constraining factor to further develop growing high-throughput technologies such as genome analysis.

Synthesis of modified nucleic acids is a way to reduce affinity-specificity anti-correlation. Compared to natural ONs, modified ONs have certain advantages in affinity and specificity, stability or other characteristics. For example, synthetic DNA-RNA-like polymer peptide nucleic acid (PNA) surpasses DNA properties. PNA does not possess a phosphodiester backbone as it is replaced with pseudopeptide polymer (A/-(2-amino-ethyl)-glycine), which contains nucleobases connected to the backbone via methylencarbonyl bond. Compared to natural DNA, PNA shows enhanced affinity and specificity (Ratilainen et al, 2000) as well as it appears to be more stable on various surfaces (Kroger et al. 2002). PNA is uncharged and forms DNA-PNA duplexes without any need for additional ionic strength. As a consequence, contrary to natural ONs PNA easily hybridizes at a low salt concentration (Weiler et al. 1997). Hence favourable conditions for DNA-PNA duplex formation might be optimised by destabilizing DNA-DNA complexes. By doing so, PNA based detection probes would become superior compared to DNA probes (Pokorski et al. 2004).

Similarly to PNA, locked nucleic acid (LNA) also exhibits high affinity and specificity (Braasch and Corey 2001 ; Petersen and Wengel 2003; Rakesh N. Veedu and Wengel 2010; Veedu and Wengel 2009). The ribose moiety of an LNA nucleotide contains an additional covalent bridge connecting 2' oxygen and 4' carbon, thus conformational motion is repressed. As a result LNA possesses higher affinity and specificity for complementary DNA or RNA and forms duplexes more easily. In addition, carbocyclic LNA analogues such as ethylene- bridged nucleic acids (ENAs) are particularly advantageous in terms of resistance to blood serum nucleases, e.g. ENAs can circulate in blood more than 48 h compared to natural ONs (<3 h) or LNAs (>9 h).

In addition to PNA and LNA, other synthetic DNA/RNA-like oligomers show enhanced properties such as resistance to nucleases, stability, synthesis cost or diversity over natural counterparts. These artificial oligomers are morpholino nucleic acids (Summerton 1989), hexitol and 1 ,5-anhydrohexitol nucleic acids (Lescrinier et al. 2000), tricyclo-DNA (Renneberg and Leumann 2002), unlocked nucleic acids (UNAs) (Langkjaer, Pasternak, and Wengel 2009), arabinose nucleic acids (ANAs), treose nucleic acids (TNAs), etc. (Wilson and Keefe, 2006).

ONs are becoming increasingly important and promising molecular tool with widespread applications as nucleic acid therapeutics in the field of antisense therapy, RNA interference (RNAi), ribozyme or aptamer technology (Sharma, Rungta, and Prasad 2014; Rayburn and Zhang 2008). Antisense therapy and RNAi are very efficient processes regulating gene expression. Broadly speaking, the mechanism of action is based on interaction between synthetic ON and a complementary target messenger RNA (mRNA) so that gene silencing occurs. In order to use synthetic ON as a therapeutic agent it should meet several requirements. First of all, ON should cross cell membrane and specifically interact with its target only. Moreover, ON cannot be toxic to the host cell as well as stay stable both in the intercellular and extracellular environment. It should be noted that the cost of ON synthesis plays an important role. Consequently, ON should be described by superior features such as high affinity and specificity to the target, resistance to intra- and extra- cellular nucleases (endo- and exo- nucleases also). ONs cannot interact with other accessory biomolecules and should exhibit properties needed for trans-membrane transfer. As a result, to improve all this stringent properties and functions of a therapeutic ON a lot attention is being paid to the development of synthetic ONs.

Up to date, antisense ONs can be classified into 3 generations - first, second and third (Sharma, Sharma, and Singh, 2014; Chery, 2016). The first generation antisense ONs were developed to increase resistance to nucleases. The standard chemical modification to synthesize first generation antisense ON is replacement of the oxygen atom of the sugar- phosphate backbone with either a sulphur group (phosphorothioate ON (PS-ON)) (Xie et al. 2012; Rahman et al. 2012; E. De Clercq, Eckstein, and Merigan 1969) or methyl group (methyl phosphonates) (Monn and Schurch 2007; Shoji et al. 1991 ). These ONs have more resistance to nucleases; carry negative charges that ease their cell delivery and peripheral distribution (Yu et al. 2007). Compared to natural ONs, first generation ONs have advantages, however they still possess poor affinity and specificity. To eliminate shortcomings of the first generation ONs, second generation ONs were developed. Along with PS backbone, additional modifications of nucleobases and monosaccharides are present in the second generation ONs which help to increase affinity and specificity. The most commonly used second generation modifications are 2'-0-methyl ( 2'-OME) and 2'-0-methoxyethyl (2'-OMOE) modified ribose (Frank Bennett 2007). Other modifications include changes in C5-position of pyrimidines (Moulds et al. 1995; Flanagan, Kothavale, and Wagner 1996) and C7-position of purines (Buhr et al. 1996). Second generation antisense ONs are reported to have a higher affinity and specificity, better tissue uptake and lower toxicity.

Third generation ONs, for example PNAs, LNAs and phosphoroamidate-morpholino oligomers (PMO) are characterised by modifications of the furanose ring. The modifications were made to improve the nuclease resistance, biostability, target affinity and specificity, and other pharmacokinetic profiles of the third generation ONs.

Similarly to antisense therapy, the most commonly used modifications in RNAi are third generation backbone modifications such as boranophosphates and phosphorotioates as well as substitution of 2'-OH group of monosaccharide with OME-, OMOE-, F-, etc. (Bumcrot et al. 2006).

In addition to the widespread application of modified ONs, modified nucleosides or nucleotides are equally important molecular tools in modern medicine. Modified nucleosides/nucleotides come as one of the forms of reverse transcriptase inhibitors which in turn are a class of antiretroviral drugs. Unlike the natural nucleotides these analogues lack a 3'-OH group and thus act as chain terminators. Hence, viral DNA synthesis is halted. Modified nucleoside/nucleotide analogues are used for the treatment of AIDS and during highly active antiretroviral therapy (HAART). For example, acyclovir is a modified guanine based gold standard antiretroviral drug for the treatment of herpes virus infections (Elion et al. 1977; Schaeffer et al. 1978). 3'-substituted-2',3'-dideoxynucleosides are used for the treatment of HIV infections (Herdewijn et al. 1987; Balzarini et al. 1988), while L-nucleosides are known as specific hepatitis B virus inhibitors (Bryant et al. 2001 ). Plenty of other nucleoside and nucleotide analogues are used in the antiretroviral therapy (Erik De Clercq and Field 2006; Hurwitz and Schinazi 2013).

Currently there are three FDA approved ON-based drugs used in medicine. Pegaptanib (Macugen ^® ) is an ON that specifically interacts with vascular endothelial growth factor (VEGF) and is used for the treatment of age-related macular degeneration. It is the first FDA approved polyethylenglycol (PEG)-conjugated aptamer, bearing 2'-F and 2'-0-methyl modifications of pyrimidine nucleotide (Pat. No. US20130142796; Tucker et al. 1999). Fomivirsen (Vitravene™) is a PS-ON which binds to the complementary sequence of the cytomegalovirus (CMV) mRNA. It blocks translation of viral mRNA and thus is used for the treatment of CMV retinitis (Pat. No. US5595978; Marwick 1998). Mipomersen (Kynamro™) is also a PS-ON though with different modifications - 2'-OMOE and 5-methylcytosine (Pat. No. US2014243389). Mipomirsen targets mRNA of an apolipoprotein B (apo-B) and therefore apo-B synthesis is inhibited. This artificial ON is used to treat hypercholesterolemia (McGowan et al. 2012). There are many other synthetic ONs which are undergoing clinical trials (Chery 2016).

Ribozymes are RNA molecules that are capable of catalysing biochemical reactions. These catalytic RNA molecules mimic function of enzymes at the nucleic acid level. Since ribozymes are often used in medicine, various modifications are tested to improve ribozyme biostability and resistance to nucleases. In the case of ribozymes, selection of modifications is limited by the fact that catalytic activity can be disrupted. Modification of ribozyme can lead to either damaged or improved catalytic activity (Beigelman et al. 1995).

Aptamers are DNA or RNA ONs that form stable and unique tertiary structure and specifically bind to its target molecules, e.g., proteins, drugs, living cells, small organic or inorganic molecules (Patel and Suri 2000; Sun and Zu 2015). Systematic evolution of ligands by exponential enrichment (SELEX) is an in vitro selection technique for evolving aptamers (Pat. No. US5270163; Tuerk and Gold 1990; Ellington and Szostak 1990). A great interest in SELEX technology induced substantial progress towards more efficient, cheaper and easier- to-handle selection methods (Darmostuk et al. 2015). To obtain an aptamer of various desirable properties, e.g. high affinity to its target or selectivity, random ON library is subjected to repeated rounds of ONs selection and identification. Properties of selected aptamer, for example stability, overall charge of nucleic acid molecule, hydrophobicity or hydrophilicity, lipophilicity, thermostability or resistance to nucleases, can be further improved by post-SELEX technology that utilizes additional chemical modification of the selected ON (Gao et al. 2016; Kusser 2000). Unfortunately post-modification increases the risk to impair the tertiary structure of an aptamer thus changing essential ON characteristics (e.g. affinity, selectivity or catalytic activity) (Avino et al. 2012). To overcome this complication and to expand structural and functional diversity of ONs an alternative strategy referred to as mod- SELEX is performed using a library of ONs with the chemical substitutions already present (Keefe and Cload 2008). Various chemical modifications enrich starting ON pool, however, renders amplification step of SELEX challenging, as modified nucleotides are usually poor substrates for DNA and RNA polymerases (Lapa, Chudinov, and Timofeev 2016). A wide variety of upgraded approaches of conventional SELEX have been developed such as capillary electrophoresis, magnetic bead-based, cell, in vivo, one-round, post-, photo-, in silico, blended, mirror-image or Spiegelmer, chimeric, indirect, crossover, click SELEX, etc. There is no unique SELEX method to fulfil all requirements as each SELEX owns its benefits and drawbacks.

Aptamers have been used in numerous investigations, from acting as therapeutics in medicine to various applications in biotechnology, e.g. chromatography, mass spectrometry, capillary electrophoresis, as acoustic, optical or signalling aptamers, etc. (Tombelli, Minunni, and Mascini 2005). However it is believed that the major field of aptamer's application is clinical therapy. Besides clinical therapy, aptamers are used in many other applications; such as detection of viral or bacterial infections (Kiilerich-Pedersen et al. 2013; Rotherham et al. 2012), detection of cancer biomarkers (Chang, Donovan, and Tan 2013), detection of proteins in Western blot (Shin et al. 2010), as capture agents in microarrays and biosensors (Jung et al. 2013; Sosic et al. 2013; Q. Wang et al. 2014), chromatography and surface plasmon resonance (Zhao et al. 2008; H. Chen et al. 2014) or as in vivo imaging agents (Hong et al. 201 1 ).

Three types of nucleotide modifications are used to generate modified aptamers; the backbone, sugar or nucleobase modifications (Wilson and Keefe 2006). Similarly to therapeutic ONs, modification of sugar-phosphate backbone leads to an increased resistance to nucleases. However, the main task is to create three-dimensional architecture alternatives and expand variety. For example, aptamers with an F, NH ₂ or O-MOE residue at the 2' position of ribose are used (Wilson and Keefe 2006). To obtain high affinity and specificity aptamers with unique tertiary structure modification of nucleobases is performed. The most common modifications are C5-substituted pyrimidines (e.g. 5-pentynyl-, 5-/V- carbamoyl-, 5-borate-, 5-iodo-, 5-tyrosyl-, 5-imidazole-, 5-carboxamide-, 5- naphtylaminocarbonyl-, 5-benzylaminocarbonyl-) and purines bearing various residues at the C7 and C8 positions (Pat. No. US20160215013; Lapa, Chudinov, and Timofeev 2016). There are several reasons to select and modify C5 position of pyrimidines and C7 position of purines. It has been shown that both of these positions orient the modified residue towards the major groove of the double helix; this arrangement is commonly considered to minimize interference with aptamer activity. Moreover, dNTPs with C5/C7 substituents have demonstrated the best compatibility with most RNA and DNA polymerases. As a consequence such modified analogues can be amplified and used in SELEX.

Substitution of C5-position of pyrimidine nucleotide is one of the most commonly used base modifications in aptamer technology. SOMAmers (Slow Off-rate Modified Aptamers) are aptamers that distinguish themselves having very low dissociation rate constants and containing various functional groups at the 5-position of the uracil (J. C. Rohloff et al. 2014; Kuwahara et al. 2006). Functional moieties conjugated at the uracil ring mimic amino acid side chains and thus create unique intramolecular motifs and make direct contacts with proteins. This creates a somamer/aptamer which specifically interacts with extremely hydrophobic or charged molecule, e.g. protein. To increase C5-modified nucleotide repertoire copper(l)-catalysed azide-alkyne cycloaddition or Click chemistry may be applied (Tolle et al. 2015). Click chemistry is based on utilization of 5-ethynyl-dUTP that being a component of an ON may be further modified. This prevents incompatibility of modified substituent and polymerase. There are several aspects regulating the selection of nucleobase position to be modified. The most important rule is to preserve the Watson-Crick base geometry and complementarity with natural bases. Contrary to modified nucleotides, unnatural nucleotides are not limited to natural pair formation and may generate completely new unnatural base pairs. Thus unnatural base pairs can expand the genetic alphabet by creating a third base pair (next to A:T and C:G) (Yamashige et al. 2012; Seo et al. 201 1 ; Malyshev et al. 2012). Using six instead of four natural nucleotides, unnatural nucleotides could increase the functionality of aptamers by providing additional chemical and structural diversity. It has been demonstrated that during ExSELEX (genetic alphabet Expansion for SELEX) incorporation of unnatural base pairs into starting pool can yield aptamers with greatly augmented affinities to its targets (Kimoto et al. 2013; Georgiadis et al. 2015).

It should be specified that the compatibility of the modified nucleotides and ONs with DNA or RNA polymerases used in the SELEX process is a primary concern (Obeid et al. 2010; Bergen et al. 2012; Lam, Hipolito, and Perrin 2008). It is important that not only modified dNTPs should act as good substrates for polymerases but modified templates must also be further amplified. Since polymerases play a significant role in molecular biology it is obvious that a gold standard for polymerase (thermostability, efficiency, fast synthesis, accuracy, processing power, use of modified substrates) is being searched for. Before starting SELEX process with modified NTPs or dNTPs, commercially available polymerases are tested first. It is known that C5/C7-position of nucleotides are the most permissive sites for modification that are accepted by several known polymerases such as Vent, KOD Dash, KOD XL, Taq DNA and T7 RNA polymerases (Lipi et al. 2016). However, usually these polymerases are efficient only under specific optimal conditions. Investigation of structural and functional properties of polymerases leads to understanding of the nature of interactions between polymerase and its substrate as well as it helps to learn the mechanism of polymerization. Such knowledge in turn becomes a basis for generation of mutant polymerases following rational design principles (Kries, Blomberg, and Hilvert 2013; Khoury et al. 2014). Although rational design is based on the ability to predict crucial amino acids, it does not guarantee polymerase improvement (Samish et al. 201 1 ). An alternative strategy to upgrade proteins known as directed evolution mimics the process of Darwin's natural selection (Jackel, Kast, and Hilvert 2008). Directed evolution is based on repeated cycles of generation of huge molecular diversity and selection of proteins based on the desired properties. Directed evolution is analogous to natural selection since the link between genotype and phenotype is preserved (Leemhuis, Kelly, and Dijkhuizen 2009). So during in vitro evolution specific selection pressure is created, e.g., utilization of modified nucleotides instead of natural ones. In the directed molecular evolution of polymerases, selection by compartmentalized self- replication (CSR) has commonly been used (Pat. No. US7514210B2; Ghadessy, Ong, and Holliger 2001 ). By applying selection pressure, for instance replacing natural nucleotides with various analogues, mutant polymerases that are able to incorporate modified NTPs can be selected and become right for SELEX (T. Chen and Romesberg 2014; Laos, Thomson, and Benner 2014). CSR is based on simple feedback - only genes encoding active polymerases are replicated, whereas inactive variants fail to amplify their own genes and disappear from the gene pool. Using CSR a variety of polymerases have been evolved that differ in substrate specificity: DNA polymerase specific for modified dNTP (Obeid et al. 2010; Bergen et al. 2012; Meek, Rangel, and Heemstra 2016), RNA polymerases capable of incorporation of modified NTP (Chelliserrykattil and Ellington 2004), DNA polymerase acting as RNA polymerase (Xia et al. 2002), polymerase demonstrating DNA and RNA polymerase and reverse transcriptase activities (Ong et al. 2006).

Although CSR is a very powerful method for evolution of polymerases, it lacks versatility for evolution in vitro of other proteins. However insertion of an extra stage into the CSR cycle before the polymerase reaction allows its application to enzymes other than polymerases. Usually, in cooperative/coupled compartmentalized self-replication or otherwise known as compartmentalized partnered-replication (CPR), synthetic circuits are linked to the production of Taq DNA polymerase so that evolved circuits that most efficiently drive Taq DNA polymerase production are enriched by exponential amplification during a subsequent emulsion PCR step (Ellefson et al. 2014). Despite the fact that this approach is especially sensitive, examples of CPR are rare in literature, since it is complicated to design such a cooperative system that links activity of a random enzyme to its amplification by polymerase. DNA binding proteins regulate the majority of cellular processes (e.g. replication, transcription, epigenetic modification, DNA damage repair) therefore investigation of DNA- protein interactions is important for biotechnological progress as well as fundamentally. To investigate interactions between DNA and proteins a cross-linking strategy has been applied that is based on conversion of weak noncovalent interactions to covalent bonds by formation of covalent heteroconjugate. To characterize such conjugates properly DNA-protein complex must be stable, chemical bonds must connect nucleobases and amino acid side chains positioned at small distance only, and covalent complex must preserve native steric shape (Steen and Jensen 2002). Structural analysis of nucleic acid-protein heteroconjugates enables the understanding of cross-linking stoichiometry, finding out the domains of nucleic acid binding proteins and specific amino acid residues that interact with nucleic acid molecule.

Photochemical cross-linking (or photocrosslinking) is one of the most commonly used methods to analyse DNA-protein complexes and mechanisms of interaction. Since native proteins and nucleic acids absorb UV light at 250-280 nm, they can be connected photochemically but still maintain their tertiary structure. However, there are some drawbacks such as low yield and photochemical or oxidative degradation of covalent complex. To bypass such difficulties, photoactive chemical group that absorbs >300 nm UV light is used. This photoactive chemical group is introduced into nucleic acid or protein molecule by several approaches. One strategy allows incorporation of additional hetero-bifunctional photoactive cross-linking agents, e.g. 2-iminothiolane (Wower et al. 1981 ) or 4- mercaptobutyrimidate (Traut et al. 1973), for joining the DNA-protein complex. In order to apply this strategy, cross-linking agent is delivered after formation of ON-protein complex so that free amino acid side chain groups may interact. When complex is excited at 350-365 nm UV light, formation of covalent bonds occurs. An alternative approach to connect DNA to a protein is to utilize photosensitive DNA or protein analogues. Since both in vivo and in vitro synthesis of ONs is an advanced technology, photoactive group is introduced into nucleic acids more often. There is a vast amount of different photosensitive nucleotide analogues and photosensitive modifications. The most common nucleotide analogues are azido-, thio-, bromo- and iodo- substituted nucleobases bearing nucleotides (Meisenheimer and Koch 1997). Another way to photosensitize nucleotide is by incorporating photosensitive chemical group such as psoralen (S. S. Sastry et al. 1993), diazirine (Shigdel, Zhang, and He 2008) or benzophenone (S. Sastry ir Ross 1998).

Benzophenone is probably the most widely used and versatile photophore in organic chemistry, bioorganic chemistry, and material science due to its unique photochemical properties (Sergentu et al. 2014). The efficiency of photosensitization depends on two operations: ability of the agent to acquire and maintain excited triplet state and subsequent energy transfer which is strongly connected to interactions between photosensitive molecule and the surrounding chemical groups. It has been demonstrated that benzophenone absorbs at 365 nm UV light leading to triplet state transition that is characterized by long half-life. As a consequence benzophenone has a wide range of applications as a photoinitiator (Gyorgy Dorman et al. 2016; Khanum, Shashikanth, and Deepak 2004; Ranganatha et al. 2013). Due to its properties benzophenone is used in bioconjugation and immobilization, proteome and interactome profiling, surface and polymer chemistry, DNA photolabelling, etc. (Gyorgy Dorman et al. 2016). Application of benzophenone expanded to in vivo protein labelling by genetically encoding benzophenone-containing amino acid (Chin et al. 2002; Lang and Chin 2014; L. Wang et al. 2001 ). Moreover, since benzophenone initiates transformations of surface C-H bonds, it is a very important photophore used to develop surface technologies, and applied in biotechnology, optics, electronics and photonics (Turgeon, Harley, and Bailey 2014; G. Dorman and Prestwich 2000).

Benzophenone is activated in a UV range (350-365 nm) that does not damage cells, proteins or nucleic acids. Benzophenone photochemically reacts with sterically accessible C- H bonds found in the sugar-phosphate backbone of DNA or in the side chains of amino acids. As a result, benzophenone has been utilized along with various biomolecules for photoimmobilization of ONs and peptides for creating cell trapping systems (Herman et al. 201 1 ), functionalizing surfaces for the detection of antigens and pathogenic bacteria (Konry et al. 2005) or generating antimicrobial coating for textiles (Dhende et al. 201 1 ). DNA microarrays are a growing field of biotechnology and therefore different immobilization methods are being developed. Photochemical immobilization of ONs onto benzophenone- derivatives coated surface seems to be very promising in this field (Renberg et al. 2009; Marcon et al. 2010). Unfortunately, little is known about ONs bearing benzophenone modification, immobilization or cross-linking with various biomolecules (Nakatani, Dohno, and Saito 1999; Nakatani, Yoshida, and Saito 2002). Such ONs could be synthesized either by chemical pattern or during enzymatic catalysis. As a result benzophenone-modified specific ONs could be immobilized onto different supports and used for detection of complementary sequences in various samples.

Growing technologies of ONs stimulate generation of alternative nucleotide modifications. Although modifications of C5/C7/C8 nucleotide positions are analysed sufficiently, little is known about substitutions of other nucleotide positions. Furthermore, there still is a need to evolve new polymerases that not only incorporate modified nucleotides during PCR but also are efficient, accurate and productive and would improve mod-SELEX. CSR is a widespread approach to evolve mutant polymerases which incorporate modified nucleotides, yet there is a lack of alternatives to evolution in vitro of other proteins. Moreover, to investigate protein- protein, protein-nucleic acid interactions or to apply immobilization technologies conjugation/cross-linking technologies need to be further improved. The present invention provides knowledge about new nucleotide modifications by introducing A^-modified cytidine analogues and its applications.

SUMMARY OF THE INVENTION

The present invention describes A^-position modified cytidine triphosphates and the methods of making and using the same.

In one aspect, the invention describes A^-position modified cytidine nucleotides having common structural Formula I:

Formula I

wherein

R is independently selected from the group consisting of:

-(CH ₂ ) _n -CH ₃ , wherein n is 0, 1 , 2, 3, 4, 5, 6, 7, 8, 9 or 10;

wherein ^* is the point of attachment of the R group;

wherein R' is inde endently selected from the group consisting of:

wherein n is 0, 1 , 2, 3, 4, 5, 6, 7, 8, 9 or 10; and wherein ^* is the point of attachment of the R' group to the R group; and wherein

R" is independently selected from the group consisting of -H, -CH ₃ , -NH ₂ , -OH, -CI and - Br;

X is independently selected from the group consisting of -CH= and -NH=;

R ¹ is independently selected from the group consisting of -H, -OAc, -OBz, -Me and -Et;

R ² is independently selected from the group consisting of -H, -OH, -OMe and -OEt.

In another aspect, this invention describes nucleic acid molecule having any one of the compounds described above. In a related aspect, the nucleic acid molecule can be single stranded or double stranded. In another aspect, the nucleic acid molecule comprises DNA, RNA or a combination thereof.

In a related aspect, a nucleic acid molecule is from 10 to 4000 nucleotides in length.

In another aspect, a nucleic acid molecule is an aptamer.

In another aspect, the invention describes nucleic acid molecule having the structural Formula IA:

Formula ΙΑ

wherein

R is independently selected from the group consisting of:

-(CH ₂ ) _n -CH ₃ , wherein n is 0, 1 , 2, 3, 4, 5, 6, 7, 8, 9 or 10;

wherein ^* is the point of attachment of the R group;

wherein R' is inde endently selected from the group consisting of:

wherein n is 0, 1 , 2, 3, 4, 5, 6, 7, 8, 9 or 10; and wherein ^* is the point of attachment of the R' group to the R group; and wherein R" is independently selected from the group consisting of -H, -CH ₃ , -NH ₂ , -OH, -CI and - Br;

X is independently selected from the group consisting of -CH= and -NH=;

R ¹ is independently selected from the group consisting of -H, -OH, -OMe and -OEt.

In another aspect, the invention describes a method of using A^-modified cytidine nucleotides for the synthesis of modified oligonucleotides. In a related aspect, the nucleic acid molecule can be single stranded or double stranded. In another aspect, the nucleic acid molecule comprises DNA, RNA or a combination thereof.

In yet another aspect, the invention describes a method of synthesizing modified oligonucleotides by primer extension.

The present invention further provides for a method of usage of A^-modified cytidine nucleotides for enzyme selection. Enzymes can either i) remove modification; ii) or use modification for the synthesis of modified oligonucleotide. In another aspect, the invention describes an enzyme having a specification for the enzymatic activity of esterase, amidase, oxidoreductase, lyase or other enzymatic activity showing ability to convert modified cytidine nucleotide to cytidine nucleotide. In yet another aspect, the invention describes an enzyme capable of decomposing modified nucleotide to the natural one and either i) modified residue or ii) decomposed modified residue, or in other words an enzyme can further degrade modified residue yielding several constituents.

In another aspect, the invention describes a selection method of enzymes, namely polymerases, when an enzyme under selection carries out polymerization of nucleic acids using A^-modified cytidine analogues. Activity of such enzyme is unique by the fact that A/ - modified cytidine nucleotide is used instead of natural cytidine nucleotide, and the reaction product is a nucleic acid molecule comprising any one of the compounds described above. In yet another aspect, the invention describes a compartmentalized self-replication as a selection method covering selection of the gene encoding polymerase and a subsequent amplification of that gene, or a compartmentalized partnered-replication as a selection method covering selection of the gene encoding an enzyme that removes or otherwise alters the modification, and an additional polymerase performing compartmentalized amplification of the gene encoding enzymes under selection. In a related aspect, enzyme selection by compartmentalized (self)replication takes place in an emulsion, and amplification of the gene encoding an enzyme is performed during emulsion PCR.

The present invention further describes the method of targeted labelling of a modified biomolecule, including labelling of the reporter group that leads to the identification of a labelled biomolecule among unlabelled.

In another aspect, the invention describes a site-directed mutagenesis. The invention further provides for a method of attachment of modified nucleotide to an oligonucleotide or longer DNA fragment, and subsequent amplification of modified nucleic acid molecule when a modified nucleobase is coupled to the other modified nucleobase regardless of complementarity principle (A couples with T and C couples with G), for example, instead of interaction with guanine modified cytidine nucleobase interacts with other nucleobase of nucleic acid molecule.

The present invention further describes a method of application of A^-modified cytidine nucleotides as a cross-linking agent. In a related aspect, the invention provides for an incorporation of A^-modified cytidine nucleotide analogues into the structure of an oligonucleotide and subsequent cross-linking to various surfaces. As used herein, the term "surface" refers to a biomolecule such as protein or nucleic acid surface as well as appropriately pre-treated or natural surfaces (e.g. polyethylene, polystyrene, etc.).

In yet another aspect, the invention describes a method of using A^-modified cytidine nucleotides for the detection of DNA fragmentation. Fragmented cellular DNA molecules composed of modified nucleotides described above could be further labelled using additional dye molecule. This detection method would allow labelling of cells that have been already initiated signalling cascades leading to apoptosis.

BRIEF DESCRIPTION OF THE FIGURES

In order to illustrate the major features of the invention two accompanying figures are presented:

Fig. 1 . A polyacrylamide gel image of a primer extension assay using template-dependent Taq DNA polymerase as described in the Materials and Methods section Example 8.

Fig. 2. A polyacrylamide gel image of a primer extension assay using template-independent terminal deoxynucleotidyl transferase as described in the Materials and Methods section Example 9.

DETAILED DESCRIPTION OF THE INVENTION

List of abbreviations used herein:

2'-OME - 2'-0-methyl-

2'-OMOE - 2'-0-metoxyethyl-

Apo-B - apolipoprotein B

BSA - bovine serum albumin

CFU - colony-forming unit

DCC - A/,A/'-dicyclohexylcarbodiimide

DEAE - diethylaminoethyl

DMF - A/JV-dimethylformamide

DTT - dithiothreitol

EDTA - ethylendiaminetetraacetic acid EtOAc - ethylacetate

IPTG - isopropyl^-D-1 -thiogalactopyranoside

L ₀ - initial library

LNA - locked nucleic acid

mRNR - messenger RNR

NMR - nuclear magnetic resonance

ON - oligonucleotide

PAGE - polyacrylamide gel electrophoresis

PEG - polyethylene glycol

PMO - phosphoroamidate-morpholino oligomer

PNA - peptide nucleic acid

POCI ₃ - phosphorus oxychloride

PS - phosphorothioate

RNRi - RNR interference

SDS - sodium dodecylsulfate

SELEX - systematic evolution of ligands by exponential enrichment)

TBA - tributylamine

TBAPF - tributylamonium pyrophosphate

TBE - Tris-borate-EDTA buffer solution

TdT - terminal deoxynucleotidyl transferase

TLC - thin-layer chromatography

Scheme I for the chemical synthesis of compounds provided in the present invention is described herein:

3 (a-f)

1. P0C1 ₃ , TBA

15-45 %

2. TBAPP

4 (a-f)

P = -(CH ₂ ) ₄ -CH ₃ (a); -CH ₃ (b);

Scheme I. Synthesis of /V*-acyl-2'-deoxycytidine-5'-triphosphates 4 (a-f).

Modified at 4-position of heterocyclic base 2'-deoxycytidines were prepared by acylation of 2'-deoxycytidine with appropriate activated ester of carboxylic acid. After purification of synthesized nucleosides by column chromatography, /V*-acyl-2'-deoxycytidines were isolated in 65-84 % yields. The synthesized nucleosides were phosphorylated with phosphorus oxychloride and the obtained monophosphates were treated with tributylammonium pyrophosphate. The synthesized modified nucleosides triphosphates were purified by ion exchange chromatography on DEAE Sephadex A-25 columns with a linear gradient of LiCI as a mobile phase. The yields of synthesized /V*-acyl-2'-deoxycytidine-5'-triphosphates were 15-45 %. The structures of new compounds were proved by NMR spectroscopy and HPLC- MS analysis. As used herein, the term "nucleotide" refers to a ribonucleoside or deoxyribonucleoside triphosphate, or its modified variations or analogues thereof. Nucleotides comprise one of purine (e.g. adenine, guanine, hypoxanthine or any variations thereof) or pyrimidine (cytosine, thymine, uracil or any variations thereof) nucleobases.

As used herein, the term "nucleic acid" refers to DNA, RNA, DNA-RNA hybrid, modified DNA or modified RNA molecule. The term "oligonucleotide" includes oligoribonucleotide or oligodeoxyribonucleotide.

As used herein, the terms "modified", "modification", "modify", and any variations thereof, when used in reference to an oligonucleotide means that the nucleotide contains additional chemical group that is not present in the natural nucleosides (i.e., adenosine, guanosine, cytidine, thymidine, uridine), and at least one of the four constituent nucleotide bases (i.e., adenosine, guanosine, cytidine, thymidine, uridine) of the oligonucleotide is an analogue. Additional modifications can include sugar-phosphate backbone modifications, methylations, unusual base-pairing combinations, 3' and 5' modifications (e.g., capping), etc. Further, any of the hydroxyl groups ordinary present on the sugar (e.g., ribose, deoxyribose or other) of a nucleotide may be replaced by a phosphonate group or a phosphate group; protected by standard protecting groups; or activated by additional bonds. The 3' and 5' terminal OH groups can be phosphorylated or substituted with amines or other organic functional groups. Oligonucleotides can also contain analogous forms of ribose or deoxyribose sugars, including 2'-0-methyl-, 2'-0-allyl-, 2'-0-ethyl-, 2'-0-propyl-, 2'-metoxyethyl-, 2'-0-fluoro-, 2'-0- amino-, 2'-0-azido- pentoses, a-anomeric sugars, other aldopentoses (i.e., arabinoses, xyloses, lyxoses), pyranoses, acyclic analogues.

Modification can be performed on a nucleotide or an ON. ON can be additionally modified, for example, conjugating with other molecules.

As used herein, the term "aptamer" refers to a non-naturally occurring nucleic acid that has a desirable action on a target molecule. A desirable action includes, but is not limited to, i) binding of the target, ii) catalytically changing the target, iii) reacting with the target in a way that modifies or alters the target or the functional activity of the target, iv) covalently attaching to the target (as in a suicide inhibitor), and v) facilitating the reaction between the target and another molecule.

As used herein, the term "compartmentalized self-replication" refers to a selection method of evolution in vitro, that includes self-replication of active polymerase encoding genes only under selection pressure, and the amplification occurs in different compartments. The term "compartment" refers to a miniature drop of water that is present in an emulsion, i.e., mixture of water and oil solutions. Emulsion can be prepared from various water-based and oil-based solutions. It is considered that such emulsion contains 10 ¹⁰ /ml_ water drops/compartments on average. Further, it is assumed that statistically one water droplet contains a single cell. Except for the cell, water drop is also provided with reagents for emulsion PCR such as buffer solution, nucleotides, gene encoding polymerase, primers and other supplementary enzymes or reagents. Supplementary enzymes or reagents may refer to various components necessary for PCR or optional selection reactions. Additional selection steps and necessary components are mentioned in Examples section but are not limited to Examples and may contain other variations.

The term "selective pressure" refers to an individual experimental condition such as, but not limited to, higher or lower temperature, different reaction components, number and duration of wash steps that is used during selection procedure and that leads to amplification of those variants which stay active under selective pressure. As used herein, the terms "selective pressure" and "evolutionary pressure" are used interchangeably.

As used herein, the term "compartmentalized partnered-replication" refers to a selection or screening method of evolution in vitro, when only active protein variants out of entire protein library generate specific conditions for additional polymerase to amplify genes encoding active protein variants, and when amplification occurs in compartments.

The term "enzyme library" refers to a collection of plasmids that are composed of a vector and gene of interest. An enzyme library can be a metagenomic library or mutant library. Metagenomic library is used for screening of novel enzymes that are found in microorganisms originated from different environmental places such as soil, seawater, sludge, etc. Mutant library is used for the selection of variants of a known protein/enzyme that contains novel mutations which improve desirable properties or functions.

As used herein, the term "cross-linking" refers to a formation of a covalent bond between two polymers. The term "polymer" refers to any kind of biological molecule (protein, nucleic acid, lipid, etc.) or non-biological polymer, for example, polyester, polystyrene, polyethylene, poly(methylmethacrylate), and others.

EXAMPLES

The following examples are provided to illustrate certain particular features and/or embodiments. These examples should not be construed to limit the invention to the particular features or embodiments described.

Example 1

Preparation of V'-hexanoyl^'-deoxycytidine-S'-triphosphate (compound 4a)

This example provides the methods for making the modified triphosphate according to the Scheme II.

4 a

Scheme II. Synthesis of /V*-hexanoyl-2'-deoxycytidine-5'-triphosphate (4a).

Synthesis of A ^-hexanoyl^'-deoxycytidine 3a. Hexanoic acid (58 mg, 0.5 mmol), N- hydroxysuccinimide (NHS, 63 mg, 0.55 mmol) and Λ/,Λ/'-dicyclohexylcarbodiimide (DCC, 1 13 mg, 0.55 mmol) were dissolved in 15 mL of ethyl acetate (EtOAc) and stirred at room temperature for 20 h. The formed precipitate was filtered and activated acid 2a remained in the filtrate. The ethyl acetate was evaporated under reduced pressure. The activated hexanoic acid was dissolved in 1 .5 mL of A/JV-dimethylformamide (DMF), and 103 mg (0.45 mmol) of 2'-deoxycytidine was added. The mixture was stirred at 30-35 °C temperature for 24-48 h. Completion of the reaction was determined by thin-layer chromatography (TLC, chloroform/methanol, 9/1 ). After the reaction was completed (TLC), DMF was evaporated under reduced pressure. The residue was dissolved in chloroform and purified by column chromatography (silica gel, chloroform/methanol mixture, 10:0→10:1 ). Yield 122 mg (75 %). MS (EST): m/z 326.10 [M+H] ⁺ ; 324.10 [M-H]-. UV A _max 247; 298 nm. ¹ H-NMR (DMSO-cf ₆ ): δ = 0.86 (t, 3H, J = 6.8 Hz, CH ₃ ); 1 .22 (m, 4H, CH ₂ ); 1.52 (m, 2H, CH ₂ ); 2.02 (m, 1 H, CH ₂ ); 2.29 (m, 1 H, CH ₂ ); 2.39 (t, 2H, J = 7.3 Hz, CH ₂ ); 3.61 (m, 1 H, CH ₂ ); 3.74 (m, 1 H, CH ₂ ); 3.93 (m, 2H, CH); 5.04 (s, 1 H, OH); 5.26 (s, 1 H, OH); 6.1 1 (t, 1 H, J = 6.0 Hz, CH); 7.23 (d, 1 H, J = 7.5 Hz, CH=CH); 8.32 (d, 1 H, J = 7.5 Hz, CH=CH); 10.83 (s, 1 H, NH). ¹³ C-NMFt (DMSO-cf ₆ ): δ =14.26; 22.29; 24.61 ; 25.69; 31 .18; 36.24; 61 .42; 70.40; 86.60; 88.37; 95.72; 145.41 ; 154.93; 162.77; 174.39.

Synthesis of A ⁴ -hexanoyl-2'-deoxycytidine-5'-triphosphate 4a. Synthesized /V*-hexanoyl-2'-deoxycytidine (98 mg, 0.3 mmol) 3a, tributylamine (TBA, 143 μΙ_, 0.6 mmol) were suspended in 1 .5 mL of trimethylphosphate and cooled to 0-4 °C temperature. Phophorous oxychloride (POCI ₃ , 56 μΐ, 0.6 mmol) was added to the cooled mixture and stirred at 0-4 °C temperature for 60-120 min. Completion of the reaction was determined by TLC, 1 ,4-dioxane/2-propanol/water/NH ₄ OH, 4/2/2/1 . After the formation of N ⁴ - hexanoyl-2'-deoxycytidine monophosphate, 72 μΐ (0.3 mmol) of TBA and 3 mL of 0.5 M tributylammonium pyrophosphate solution (TBAPP, 1 .5 mmol) in acetonitrile were added dropwise. After stirring for 10-15 min the reaction mixture was poured into ice-water and neutralized with saturated sodium bicarbonate solution to pH 6-7. The reaction mixture was purified by ion exchange chromatography on DEAE-Sephadex A25 column (30 mL) with a linear gradient (0.05-0.4 M) of LiCI as the mobile phase. The product was eluted with 0.25- 0.3 M LiCI, the purity of fractions were monitored by TLC (1 ,4-dioxane/2- propanol/water/NH ₄ OH, 4/2/5/1 ). The pure fractions were combined. The solution was concentrated under reduced pressure to several millilitres and poured into a 30^0 mL mixture of acetone/methanol, 4/1 . The formed precipitate was collected by centrifugation (4000 rpm, 10 min) and twice washed with a mixture of acetone/methanol, 4/1 . The nucleotide was dissolved in 2-3 mL of water, evaporated under reduced pressure and neutralized with 1 M sodium hydroxide solution to pH 7.0. The synthesized nucleotide after purification by ion exchange chromatography was purified once again by reverse phase chromatography (C-18 cartridges, water/methanol mixture, 10:0→10:2). The triphosphate was eluted with 15-20 % methanol/water mixture, and the purity of fractions were monitored by TLC (1 ,4-dioxane/2-propanol/water/NH ₄ OH, 4/2/5/1 ). The fractions with pure /V*-hexanoyl- 2'-deoxycytidine triphosphate were combined, and the solvents were removed under reduced pressure. The triphosphate was dissolved in 2-3 mL of water, neutralized with 1 M sodium hydroxide solution to pH 7.0 and filtered (0.45 μηι PTFE). The absorption of purified nucleotide was measured, the concentration and amount of synthesized compound was calculated. Yield 5.2 mL, 26 mM, 135 mmol (45 %). MS (EST): m/z 566.05 [M+H] ⁺ ; 564.05 [M-H]-. UV A _max 246; 298 nm. ¹ H-NMR (D ₂ 0): δ = 0.75 (t, 3H, J = 7.0 Hz, CH ₃ ); 1.20 (m, 4H, CH ₂ ); 1 .54 (m, 2H, CH ₂ ); 2.24 (m, 1 H, CH ₂ ); 2.38 (t, 2H, J = 7.4 Hz, CH ₂ ); 2.40 (m, 1 H, CH ₂ ); 4.09 (m, 1 H, CH); 4.14 (m, 2H, CH ₂ ); 4.54 (m, 1 H, CH); 6.15 (t, 1 H, J = 6.2 Hz, CH); 7.26 (d, 1 H, J= 7.5 Hz, CH=CH); 8.28 (d, 1 H, J= 7.5 Hz, CH=CH). ³¹ P-NMR (D ₂ 0): δ = -20.54 (t, J = 19.7 Hz, Ρβ); -9.23 (d, J = 18.7 Hz, Pa), -7.19 (d, J = 19.8 Hz, Ργ).

Example 2

Synthesis of V-acetyl^'-deoxycytidine-S'-triphosphate (compound 4b)

The compound was prepared as described in example 1 (compound 4a) from acetic acid 1 b. Af-Acetyl^'-deoxycytidine, 3b. Yield 105 mg, (78 %). MS (ESI ⁺ ): m/z 270.10 [M+H] ⁺ ; 268.10 [M-H] ^" . UV A _max 246; 298 nm. ¹ H-NMR (DMSO-cf ₆ ): δ = 2.04 (m, 3H, CH ₃ ); 2.1 1 (m, 1 H, CH ₂ ); 2.41 (m, 1H, CH ₂ ); 4.12 (m, 3H, CH, CH ₂ ); 4.46 (m, 1H, CH); 5.04 (s, 1H, OH); 5.27 (s, 1 H, OH); 6.12 (t, 1 H, J = 6.0 Hz, CH); 7.20 (d, 1 H, J = 7.5 Hz, CH=CH); 8.26 (d, 1 H, J = 7.5 Hz, CH=CH); 10.81 (s, 1H, NH). ¹³ C-NMR (DMSO-c( ₆ ): δ = 23.98; 39.72; 63.59; 69.73; 85.35; 87.10; 98.35; 145.57; 156.96; 162.57; 173.34.

V-Acetyl-2'-deoxycytidine-5'-tNphosphate, 4b. Yield 3.2 mL, 23 mM, 74 mmol (25 %). MS (ESI ⁺ ): m/z 510.00 [M+H] ⁺ ; 508.00 [M-H] ^" . UV A _max 243; 296 nm. ¹ H-NMR (D ₂ 0): δ = 2.08 (m, 3H, CH ₃ ); 2.21 (m, 1 H, CH ₂ ); 2.40 (m, 1 H, CH ₂ ); 4.11 (m, 3H, CH, CH ₂ ); 4.47 (m, 1 H, CH); 6.11 (t, 1H, J= 6.0 Hz, CH); 7.20 (d, 1H, J= 7.5 Hz, CH=CH); 8.26 (d, 1H, J= 7.5 Hz, CH=CH. ³¹ P- NMR (D ₂ 0): δ = -20.61 (t, J = 18.8 Hz, Ρβ); -10.94 (d, J = 18.7 Hz, Pa); -5.24 (d, J = 18.9 Hz, Ργ).

Example 3

Synthesis of V-benzoyl^'-deoxycytidine-S'-triphosphate (compound 4c)

The compound was prepared as described in example 1 (compound 4a) from benzoic acid 1c.

Af-Benzoyl- -deoxycytidine, 3c. Yield 132 mg, (80 %). MS (ESI ⁺ ): m/z 332.10 [M+H] ⁺ ; 330.10 [M-H]-. UV A _max 256; 303 nm. ¹ H-NMR (DMSO-cf ₆ ): δ = 2.34 (m, 2H, CH ₂ ); 3.74 (m, 2H, CH ₂ );4.05 (m, 1 H, CH); 4.54 (m, 1H, CH); 5.03 (s, 1H, OH); 5.26 (s, 1 H, OH); 6.12 (t, 1H, J =6.1 Hz, CH); 7.26 (d, 1H, J= 7.5 Hz, CH=CH); 7.40 (t, 2H, J= 7.6 Hz, CH); 7.65 (t, 1H, J= 7.5 Hz, CH); 7.87 (dd, 2H, J= 8.4; 1.2 Hz, CH); 8.29 (d, 1H, J= 7.5 Hz, CH=CH); 10.64 (s, 1H, NH). ¹³ C-NMR (DMSO-cie): δ = 38.77; 61.78; 70.31; 85.90; 86.86; 96.25; 128.13; 128.60; 131.75; 135.04; 143.10; 155.02; 168.20; 169.17.

A/^-Benzoyl^'-deoxycytidine-S'-triphosphate, 4c. Yield 6.5 mL, 17 mM, 110 mmol (37 %). MS (EST): m/z 572.05 [M+H] ⁺ ; 570.00 [M-H] ^" . UV A _max 257; 303 nm. ¹ H-NMR (D ₂ 0): δ = 2.28 (m, 1H, CH ₂ ); 2.50 (m, 1H, CH ₂ ); 4.18 (m, 3H, CH, CH ₂ ); 4.54 (m, 1H, CH); 6.19 (t, 1H, J= 6.3 Hz, CH); 7.39 (d, 1H, J= 7.5 Hz, CH=CH); 7.48 (t, 2H, J= 7.8 Hz, CH); 7.61 (t, 1H, J= 7.5 Hz, CH); 7.82 (dd, 2H, J= 8.4; 1.2 Hz, CH); 8.34 (d, 1H, J= 7.5 Hz, CH=CH). ³¹ P-NMR (D ₂ 0): δ = -20.69 (t, J= 19.1 Hz, Ρβ); -10.95 (d, J= 18.7 Hz, Pa); -5.33 (d, J= 19.1 Hz, Ργ).

Example 4

Synthesis of V-benzoyl^-acetyl^'-deoxycytidine-S'-triphosphate (compound 4d)

The compound was prepared as described in example 1 (compound 4a) from 2- acetylbenzoic acid 1d.

Af-Benzoyl^-acetyl- -deoxycytidine, 3d. Yield 121 mg, (65 %). MS (ESI ⁺ ): m/z 374.05 [M+H] ⁺ ; 372.10 [M-H] ^" . UV A _max 265; 307 nm. ¹ H-NMR (DMSO-cf ₆ ): δ = 2.09 (s, 3H, CH ₃ ); 2.35 (m, 2H, CH ₂ ); 3.63 (m, 2H, CH ₂ ); 3.90 (m, 1H, CH); 4.25 (m, 1H, CH); 5.09 (s, 1H, OH); 5.30 (s, 1H, OH); 6.16 (t, 1H, J= 6.2 Hz, CH); 6.85 (d, 1H, J= 7.5 Hz, CH=CH); 7.53 (m, 2H, CH); 7.62 (m, 1H, CH); 7.80 (m, 1H, CH); 8.44 (d, 1H, J= 7.5 Hz, CH=CH); 10.57 (s, 1H, NH). ¹³ C-NMR (DMSO-cfe): δ = 28.25; 39.10; 61.67; 70.32; 86.07; 86.86; 96.25; 127.18; 131.68; 131.98; 135.50; 142.76; 154.60; 168.40; 171.88; 177,60.

A^-Benzoyl^-acetyl^'-deoxycytidine-S'-triphosphate, 4d. Yield 2,8 mL, 17 mM, 48 mmol (16 %). MS (EST): m/z 614.00 [M+H] ⁺ ; 612.00 [M-H] ^" . UV A _max 267; 307 nm. ¹ H-NMR (D ₂ 0): δ = 1.97 (s, 3H, CH ₃ ); 2.29 (m, 1H, CH ₂ ); 2.49 (m, 1H, CH ₂ ); 4.14 (m, 3H, CH, CH ₂ ); 4.32 (m, 1H, CH); 6.19 (t, 1H, J= 6.0 Hz, CH); 7.39 (d, 1H, J= 7.6 Hz, CH=CH); 7.57 (m, 1H, CH); 7.66 (m, 2H, CH); 7.78 (m, 1H, CH); 8.33 (d, 1H, J= 7.6 Hz, CH=CH). ³¹ P-NMR (D ₂ 0): δ = -20.26 (t, J = 18.5 Hz, Ρβ); -10.80 (d, J = 18.5 Hz, Pa); -^.91 (d, J = 18.2 Hz, Ργ).

Example 5

Synthesis of A^-benzoyl-S-benzoyl^'-deoxycytidine-S'-triphosphate (compound 4e)

The compound was prepared as described in example 1 (compound 4a) from 3- benzoylbenzoic acid 1e.

A'-Benzoyl-S-benzoyl^'-deoxycytidine, 3e. Yield 152 mg, (70 %). MS (EST): m/z 436.10 [M+H] ⁺ ; 434.10 [M-H]-. UV A _max 255; 303 nm. ¹ H-NMR (DMSO-cf ₆ ): δ = 2.07 (m, 1 H, CH ₂ ); 2.33 (m, 1H, CH ₂ ); 3.63 (m, 2H, CH ₂ ); 3.89 (m, 1H, CH); 4.25 (m, 1H, CH); 5.06 (s, 1H, OH); 5.29 (s, 1H, OH); 6.15 (t, 1H, J= 6.3 Hz, CH); 7.38 (d, 1H, J= 7.0 Hz, CH=CH); 7.60 (m, 2H, CH); 7.73 (m, 1H, CH); 7.77 (m, 2H, CH); 7.83 (d, 2H, J= 8.2 Hz, CH); 8.16 (d, 2H, J= 8.2 Hz, CH); 8.44 (d, 1H, J= 7.0 Hz, CH=CH); 11.45 (s, 1H, NH). ¹³ C-NMR (DMSO-cf ₆ ): δ = 41.10; 61.42; 70.39; 86.78; 88.45; 107.18; 129.12; 129.22; 129.84; 130.19; 133.62; 136.96; 140.46; 145.21; 163.02; 168.56; 188.32; 195.80.

A/^-Benzoyl-S-benzoyl^'-deoxycytidine-S'-triphosphate, 4e. Yield 6 mL, 20 mM, 120 mmol (40 %). MS (ESI ⁺ ): m/z 676.05 [M+H] ⁺ ; 674.05 [M-H] ^" . UV A _max 257; 302 nm. ¹ H-NMR (D ₂ 0): δ = 2.25 (m, 1H, CH ₂ ); 2.45 (m, 1H, CH ₂ ); 4.17 (m, 3H, CH, CH ₂ ); 4.53(m, 1H, CH); 6.15 (t, 1H, J= 6.0 Hz, CH); 7.,31 (d, 1H, J= 7.0 Hz, CH=CH); 7.47 (m, 2H, CH); 7.63 (m, 4H, CH); 7.90 (m, 1H, CH); 8.06 (m, 2H, CH); 8.31 (d, 1H, J= 7.0 Hz, CH=CH). ³¹ P-NMR (D ₂ 0): δ = - 20.58 (t, J= 18.5 Hz, Ρβ); -10.91 (d, J= 18.4 Hz, Pa); -5.28 (d, J= 18.4 Hz, Ργ).

Example 6

Synthesis of A/^-nicotinoyl^'-deoxycytidine-S'-triphosphate (compound 4f)

The compound was prepared as described in example 1 (compound 4a) from nicotinic acid 1f.

Af-Nicotinoyl- -deoxycytidine, 3f. Yield 140 mg, (84 %). MS (EST): m/z 333.05 [M+H] ⁺ ; 331.05 [M-H] ^" . UV A _max 253; 307 nm. ¹ H-NMR (DMSO-c/ ₆ ): δ = 2.07 (m, 1H, CH ₂ ); 2.33 (m, 1H, CH ₂ ); 3.62 (m, 2H, CH ₂ ); 3.89 (m, 1H, CH); 4.25 (m, 1H, CH); 5.09 (t, 1H, J= 5.1 Hz, OH); 5.30 (d, 1H, J= 4.2 Hz, OH); 6.14 (t, 1H, J= 6.3 Hz, CH); 7.33 (s, 1H, CH=CH); 7.55 (dd, 1H, J=7.9; 4.9 Hz, CH); 8.33 (d, 1H, J=8.0Hz, CH); 8.42 (d, 1H, J= 7.3 Hz, CH=CH); 8.77 (d, 1H, J=4.9 Hz, CH); 9.1 1 (s, 1 H, CH); 1 1 .50 (s, 1 H, NH). ¹³ C-NMR (DMSO-c( ₆ ): δ = 41 .39; 49.06; 61 .41 ; 70.38; 86.73; 88.45; 96.53; 128.89; 129.68; 136.61 ; 145.58; 149.84; 158.38; 163.17; 173.48. Ar'-Nicotinoyl^'-deoxycytidine-S'-triphosphate, 4f. Yield 3 mL, 15 mM, 45 mmol (15 %). MS (EST): m/z 573.00 [M+H] ⁺ ; 571 .00 [M-H]-. UV A _max 256; 305 nm. ¹ H-NMR (D ₂ 0): δ = 2.26 (m, 1 H, CH ₂ ); 2.47 (m, 1 H, CH ₂ ); 4.42 (m, 4H, CH ₂ , CH); 6.16 (t, 1 H, J = 6.1 Hz, CH); 7.37 (d, 1 H, J = 7.5 Hz, CH=CH); 7.51 (dd, 1 H, J = 7.8; 5.1 Hz, CH); 8.22 (d, 1 H, J = 8.1 Hz, CH); 8.35 (d, 1 H, J = 7.5 Hz, CH=CH); 8.64 (s, 1 H, CH); 8.89 (s, 1 H, CH). ³¹ P-NMR (D ₂ 0): δ = -20.67 (t, J = 18.5 Hz, Ρβ); -10.92 (d, J = 18.5 Hz, Pa); -6.06 (d, J = 18.2 Hz, Ργ).

Example 7

Selection of enzymes hydrolysing amide bond of modified dNTP

The present invention describes a method of enzyme selection in vitro. In vitro enzyme selection takes place in an emulsion during compartmentalized replication. Enzymes undergoing selection catalyse the hydrolysis of an amide bond present in a modified dNTP (4 a-e) yielding natural dNTP. After hydrolysis step, all four natural dNTPs necessary for PCR are formed in water droplets (or compartments) of an emulsion. An additional DNA polymerase amplifies gene encoding enzyme that is undergoing selection.

Metagenomic library (L ₀ ) used for the selection contained a set of plasmids that have been constructed using pET28b expression vector and DNA fragments which were obtained by hydrolysing chromosomal DNA with restriction endonucleases. L ₀ library was transformed into E. coli BL21 (DE3) bacteria, bacterial cell number was estimated and was equated to the size of L ₀ . L ₀ was evaluated to contain -5x10 ⁷ variants.

Metagenomic library was then prepared for the enzyme selection procedure. 5x10 ⁹ metagenomic library cells were grown in 50 mL LB medium with appropriate antibiotic. E. coli BL21 (DE3) bacteria were incubated at 37 °C on a rotator until OD ₆₀₀ reached 0.7. Then gene expression was induced by adding IPTG to a final concentration of 0.5 mM. Bacteria were grown for 2 hours at 37 °C, then the culture was chilled on ice. Spectrophotometric quantification of bacterial density (colony forming units (CFU/mL)) was performed. The cell culture was aliquoted into 1 mL and cells were harvested by centrifugation at 3300g for 1 min at 4 °C. Cells were suspended in 1 mL of 0.9 % NaCI solution, centrifuged and once again re-suspended in 0.5 mL of 1 X PCR buffer solution. Freshly prepared lysozyme solution (20 μg/μL) was added to the cell suspension and incubated for 10 min at 37 °C. Lysozyme- processed cell suspension was then used for emulsion PCR.

Oil-surfactant mixture (50 mL) was prepared by thoroughly mixing mineral oil (97.95 %), ABIL EM 90 (2 %) and Triton X-100 (0.05 %).

PCR mixture was prepared by mixing the following components to the final concentration: 1 X HF PCR buffer solution, 0.33 μΜ T7prom primer, 0.33 μΜ T7ter primer, 2 mM natural dNTP's each (dATP, dTTP, dGTP), 3 mM modified dCTP, 0.5 μg mL BSA, 0.6 units of Phusion DNA polymerase (Thermo Fisher Scientific). Primer sequences: forward primer (T7prom) SEQ ID No. 8: 5'-TAATACG ACTCACTATAGGG AG A-3' ; reversed primer (T7ter) SEQ ID No. 9: 5'- CTAGTTATTGCTCAGCGGTG-3'. 1 x10 ⁷ of lysozyme-processed cells were added into prepared PCR mixture and the mixture was emulsified using modified Williams protocol (Williams et al. 2006). PCR mixture (300 μΙ_) was added to the oil-surfactant mixture (700 μΙ_) over a period of 2 min and stirring was continued for 5 min. Stirring was carried out at 1 ,700 rpm on the magnetic stirrer at 4 °C.

The lysis of the cells present in an emulsion was conducted using four freeze (10 min at -70 °C)-thaw (10 min at 37 °C) cycles. Emulsion was pipeted into PCR vials as aliquots of 50 μΙ_ and PCR was carried out. Additional non-emulsified PCR control was performed. PCR was conducted using the following program: 30 s at 98 °C, 35 cycles of - 10 s at 98 °C, 15 s at 55 °C, 45 s at 72°C, and 3 min at 72 °C.

After PCR, emulsion was broken using modified Williams method (Williams et al. 2006). Emulsified PCR reactions were pooled into tubes and centrifuged at 16,000g for 10 min at 37 °C. Three extraction steps were performed - two extractions with diethyl ether and one (the middle) extraction with water-saturated ethyl acetate. Residual solvent was vaporized under vacuum.

Nucleic acids were purified using "GeneJET™ PCR Purification Kit" (Thermo Fisher Scientific) using manufacturer's recommended protocol. In order to remove residual plasmid DNA the purified mixture was treated with double-stranded DNA hydrolysing deoxyribonuclease (dsDNase). The mixture was incubated for 5 min at 37 °C. Then PCR products were analysed by agarose gel electrophoresis.

The mixture of PCR products was then hydrolysed with appropriate restriction endonucleases and cloned into pre-treated pET28b vector. Ligation mixture was used to transform E. coli DH5a bacteria, single colony plasmid DNA was extracted and selected DNA sequence(s) were identified. Three DNA fragments (EST, D6 ir YqfB) were selected.

Then recombinant proteins (EST, D6 and YqfB) having polyhystidine-tag at the N-terminus were purified by affinity chromatography, enzymatic activities of each protein were estimated. It was determined that EST, D6 and YqfB hydrolyse the amide bond of modified dCTP (activity of amidase) as well as behave as esterases.

All three selected proteins are able to hydrolyse the amide bond of A^-modified cytidine nucleotide. The hydrolysis reaction proceeds in various buffer solutions at 37 °C. Depending on the concentration of purified protein enzymatic reaction lasts from several minutes to several hours. His tag present in the N-terminus of the protein does not interfere with enzymatic activity. Moreover, these proteins are capable of hydrolysing not only the amide bond of modified dCTP but also the amide bond of modified cytidine.

Conversion of modified cytidine to natural nucleotide is important for several reasons. One of the most promising applications of various modified nucleotides is aptamer technology. During generation of modified aptamers there is often a need to remove modification group, for example, to amplify aptamer sequences using polymerases which do not interact with modified nucleotides. As a result, aptamers composed of natural nucleotides can be created by using these selective amide bond hydrolysing enzymes (the same approach would be valid for different modification and an appropriate enzyme). Aptamers composed of natural nucleotides can further be easily amplified. This approach eliminates the need to screen or select for mutant polymerase that amplifies modified template.

Detoxification is a very perspective field of medicine that can be closely related to proteins of this invention. For a while, chronic inflammation has been associated with many diseases of aging, but the mechanisms responsible for the production of this inflammation remain unclear. An inflammasome which is an intracellular multiprotein oligomer is responsible for activation of inflammatory processes. The inflammasome triggers the maturation of the inflammatory cytokine interleukin 1 β ( I L- 1 β) that in conjunction with other molecules and proteins participates in immune and inflammatory response. It was recently determined that A^-acetyl-cytidine which is an intracellular nucleotide-derived metabolite is detectable in the blood of patients that suffer from nucleotide metabolism dysfunction, elevated oxidative stress, high rates of hypertension and arterial stiffness (Furman et al. 2017). It was demonstrated that A^-acetyl-cytidine activates the NLRC4 inflammasome, induces the production of IL-1 β, activates platelets and neutrophils and elevates blood pressure in mice. Furthermore, in individuals over 85 years of age, the elevated expression of inflammasome gene modules was associated with all-cause mortality. Thus, targeting inflammasome components and various related signalling molecules may ameliorate chronic inflammation and various other age-associated conditions. The fact that A^-acetyl-cytidine is involved in the action of inflammatory response promotes generation of drugs that are based on recombinant proteins capable of converting A^-acetyl-cytidine to native cytidine. Therefore, esterases of this invention may be utilized for detoxification of older patients.

Amino acid sequences of enzymes of this invention:

EST (SEQ ID No. 1 ):

MSSLFIGQVFAKTPEVQTSDLTGNTTCSNLVGMVIPADEIGLPTSGATITSATLKIVEDG AIKD

AEYCEVLGAIHPVDPTAPDINFQVNLPTNWNKKFLQFGGGYFNGTVRTGLGNPPAGD RKLG

KNTPLAQGYVTFGSDSGNSTAPLDASFGMNDEALKNFAGDQLKKTKDVALALANVRY NAV

PDQVYFAGGSEGGREGLFIVQNFPDEYDGVISVYPVLNWIPKALKDNRDAQALYKND GEG

WISPEENDLINETVFKACDSLDGVKDGIISNTSECAEKEDKILDTLSESLSEKQIEV IKSFNGP

MEFDIQLANDFTTMPGYSQLQGADIGRLFGTRPIPGVPPVVSESVGHVIDEQDALMG VYSD

QVIRYKITRNPDFNTLTFDPNEYREEILKASNLLDVTDPNISEFRENGGKLILVHGT EDEMVAP

QGTSDYYSKLVNEFGQESLDEFAQYYLVPGFSHGGGNFTMSANLLGALDAWVVNGDV PSN

LVAEDQNSATFGRTRPLCEYPTYPQYNGSGDVNSAASFTCLKADKDKDISASDIQKL IEKFE VDGEFANHGTARSLQAHLDILIKLESQERETVDQIVKHTQKFIKLLDNHKKNGKITDHAY NTLK ELAESYIKQIK

D6 (SEQ ID No. 2):

MEQLKFQKNWNNKCSCDFFTTIRLKGPKYTVGKELEMRIYKGGVFQNHGMIRVASLRPIQ L HQINEWISRLDSGLSPEELRSELFYMYKDKVADVNKVDFYLILCERVKSKPIQNALFSTE STP AHD

YqfB (SEQ ID No. 3):

MQPNDITFFQRFQDDILAGRKTITIRDESESHFKTGDVLRVGRFEDDGYFCTIEVTATST VTL DTLTEKHAEQENMTLTELKKVIADIYPGQTQFYVIEFKCL

Example 8

Primer extension reaction using template-dependent polymerase

This example describes a method for the synthesis of nucleic acids containing nucleotide modifications provided in this invention inside the nucleic acid sequence.

Primer extension reactions were carried out using modified nucleoside triphosphates (4 a-f) and DNA polymerases: Taq DNA polymerase (Thermo Scientific), Klenow (exo ) polymerase (Thermo Scientific), Pfu DNA polymerase (Thermo Scientific), KOD DNA polymerase (Merck Millipore), KOD XL DNA polymerase (Merck Millipore), Bsm DNA polymerase (Thermo Scientific). Four different DNA templates were used that contained four natural nucleotides (A, G, C, T) in a row. Sequences of DNA templates:

TempA: 5'-CCGGAATTAAAAtctccctatagtgagtcgtatta-3' (SEQ ID No. 4) (Metabion)

TempG: 5'-CCGGAATTGGGGtctccctatagtgagtcgtatta-3' (SEQ ID No.5) (Metabion)

TempC: 5'-CCGGAATTCCCCtctccctatagtgagtcgtatta-3' (SEQ ID No.6) (Metabion)

TempT: 5'-CCGGAATTTTTTtctccctatagtgagtcgtatta-3' (SEQ ID No.7) (Metabion)

Primer sequence:

T7pr: 5'-taatacgactcactatagggaga-3' (SEQ ID No. 8) (Metabion)

The primer was labelled at the 5'-end using ³³ Ρ-γΑΤΡ (TriLink Biotechnologies). 20 pmol of ³³ Ρ-γΑΤΡ was used to label 20 pmol of primer using T4 polynucleotide kinase (Thermo Scientific). Reaction was incubated in a buffer solution containing 50 mM Tris-HCI (pH 7.6 at 25 °C), 10 mM MgCI ₂ , 5 mM DTT and 0.1 mM spermidine for 20 min at 37 °C. Reaction was inhibited by adding 1 μΙ_ 0.5 M EDTA (pH 8.0) and heating for 10 min at 80 °C. For the primer extension assay using various DNA polymerases single-stranded labelled primer was annealed to the single-stranded DNA template. Annealing was performed using equal quantities (5 pmol) of primer and DNA template and heating for 1 min at 95 °C and slowly cooling to room temperature. Desalting of primer and DNA template hybrid was performed using Zeba™ Spin 7K MWCO columns (Thermo Scientific). The primer extension assay was carried out in 20 mM glutamate (pH 8.2 at 25 °C), 10 mM DTT, 0.5 % Triton X-100, 20 mM NaCI, 1 mM MgCI ₂ buffer solution or other polymerase specific buffer solution using 5 nM of primer and template (TempA, TempT, TempG or TempC) hybrid, 50 nM of DNA polymerase, 0.01 units of inorganic pyrophosphatase (Thermo Scientific) and 10 μΜ each nucleotide (dATP, dGTP, dCTP, dTTP/dUTP or its modified analogues). Reactions were incubated for 5-60 min at 37 °C using Taq, Klenow (exo ), Pfu, KOD, KOD XL polymerases or at 60 °C for Bsm polymerase. Reactions were inhibited by adding double amount (v/v) of STOP solution (95 % (v/v) formamide, 0.5 M EDTA, 0.6 % (w/v) bromphenol blue and xylene cyanol).

After reactions were completed, samples were incubated for 2-5 min at 95 °C and chilled on ice. Samples were separated on 15 % denaturing (8 M urea) polyacrylamide gel in TBE buffer solution (89 mM Tris, 89 mM boric acid, 2 mM EDTA (pH 8.3)). After electrophoresis gel was soaked for 15 min in 10 % acetic acid solution and washed for 15 min under running tap water. Gel was placed on a 3 MM CHR Whatman™ chromatographic paper (GE, Healthcare Life Sciences) and dried in vacuum dryer. Finally, the gel was kept on imaging plate for -16 hours and the results were visualized using Fujifilm FLA-5100 imaging system.

Example 9

Primer extension reaction using template-independent polymerase

This example describes a method for the synthesis of nucleic acids containing nucleotide modifications provided in this invention at the 3'-end of nucleic acid sequence.

Primer extension reactions were carried out using modified nucleoside triphosphates (4 a-f) and terminal deoxynucleotidyl transferase (TdT) (Thermo Scientific). DNA primer sequence: T7pr: 5'-taatacgactcactatagggaga-3' (SEQ ID No. 8) (Metabion)

The primer was labelled at the 5'-end using ³³ Ρ-γΑΤΡ (TriLink Biotechnologies). 20 pmol of ³³ Ρ-γΑΤΡ was used to label 20 pmol of primer using T4 polynucleotide kinase (Thermo Scientific). Reaction was incubated in buffer solution containing 50 mM Tris-HCI (pH 7.6 at 25 °C), 10 mM MgCI ₂ , 5 mM DTT and 0.1 mM spermidine for 20 min at 37 °C. Reaction was inhibited by adding 1 μΐ 0.5 M EDTA (pH 8.0) and heating for 10 min at 80 °C. For the primer extension assay using TdT labelled primer was diluted to the concentration of 100 nM and desalted using Zeba™ Spin 7K MWCO columns (Thermo Scientific).

The primer extension assay using TdT was carried out in 20 mM glutamate (pH 8.2 at 25 °C), 10 mM DTT, 0.5 % Triton X-100, 20 mM NaCI, 1 mM MgCI ₂ buffer solution or commercial TdT buffer solution (200 mM potassium cacodilate (pH 7.2 at 25 °C), 25 mM Tris, 0.01 % Triton X-100, 1 mM MgCI ₂ ) using 5 nM of primer, 50 nM of TdT and 10 μΜ of one nucleotide (dATP, dGTP, dCTP, dTTP/dUTP or its modified analogues). Reactions were incubated for 5-15 min at 37 °C. Reactions were inhibited by adding double amount (v/v) of STOP solution (95 % (v/v) formamide, 0.5 M EDTA, 0.6 % (w/v) bromphenol blue and xylene cyanol). After reactions were completed, samples were incubated for 2-5 min at 95 °C and chilled on ice. Samples were separated on 15 % denaturing (8 M urea) polyacrylamide gel (PAGE) in TBE buffer solution (89 mM Tris, 89 mM boric acid, 2 mM EDTA (pH 8.3)). After electrophoresis gel was soaked for 15 min in 10 % acetic acid solution and washed for 15 min under running tap water. Gel was placed on a 3 MM CHR Whatman™ chromatographic paper (GE, Healthcare Life Sciences) and dried in a vacuum dryer. Finally, the gel was kept on imaging plate for -16 hours and the results were visualized using Fujifilm FLA-5100 imaging system.

Example 10

Cross-linking of benzophenone containing oligonucleotide to protein

This example demonstrates that benzophenone modification containing oligonucleotides can be covalently cross-linked to a protein that interacts with modified section of an oligonucleotide, and it can be used to study DNA-protein interactions.

First of all, 5'- ³³ P-labelled primers are used to synthesize 3'-modified oligonucleotides. Λ/ - benzoyl-3-benzoyl-dCTP (4e) and TdT were used for the synthesis as described in Example 9. Following reaction, TdT was inactivated by heating as recommended by manufacturer. UV cross-linking equipment was assembled with slight modifications as described previously (Sontheimer 1994). It consisted of an ice container, 96-well plate, a sheet of parafilm and a UV light source. A sheet of parafilm was placed over the top of 96-well plate, and taped to the plate on all four sides. Each well was pressed to create a shallow groove. Reaction mixtures were transferred as 10 μΙ_ drops to the wells on the parafilm tape. The ice container was placed underneath a 365 nm UV, so that the samples were 1 cm from the surface of the light source. Samples were irradiated at 365 ± 5 nm for 5 min.

After irradiation the samples were supplemented with SDS loading dye, heated for 5 min at 95 °C, and analysed by electrophoresis on a 14% w/v SDS-PAGE gel. Proteins were stained with Coomasie Briliant Blue staining solution. To examine TdT-oligonucleotide cross-links generated by irradiation, the gel was placed on a 3 MM CHR Whatman™ chromatographic paper (GE, Healthcare Life Sciences) and dried in vacuum dryer. Finally, the gel was kept on imaging plate for -16 hours and the results were visualized using Fujifilm FLA-5100 imaging system.

PAGE analysis revealed a new band with molecular mass corresponding to the sum of molecular masses of TdT and modified oligonucleotide.

Example 11

Cross-linking of benzophenone containing oligonucleotide to polymeric surfaces

This example shows that A^-benzophenone-modified cytidine containing ON can be covalently cross-linked to a polymeric surface, and it can be used for photoimmobilization of nucleic acids. As used herein, the term "polymeric surface" refers to polystyrene, polypropylene, polylactate, polydimethylsiloxane or polystyrene-, polypropylene-, polylactate- , polydimethylsiloxane- based materials.

Initially, 5'- ³³ P-labeled primers are used to synthesize 3'-modified ONs. A^-acetyl-S-benzoyl- dCTP (4b) or A^-benzoyl-S-benzoyl-dCTP (4c) and TdT were used for the synthesis as described in Example 6. Following reaction, TdT was inactivated by heating as recommended by manufacturer.

After TdT inactivation, the reaction mixtures were chilled on ice and transferred as 2 μΙ_ drops onto a slide of polymeric solid support. Then the specimens were placed under the UV light source (5 mm away from the surface of the light source) and irradiated for 5 min (365±5 nm). Immediately after irradiation, the specimens were rinsed with 500 μΙ_ of wash buffer (50 mM potassium phosphate, 1 % Triton X-100 (pH 7.0)) following incubation (1 h at room temperature) in 1 .5 mL of wash buffer vigorously shaking. Then the specimens were rinsed with 200 μΙ_ of distilled water and air-dried. Photochemical immobilization of modified ONs to polymeric solid supports was then visualized by phosphor imaging.

Example 12

Cross-linking of benzophenone containing oligonucleotide to inorganic surfaces

This example demonstrates that A^-benzophenone-modified cytidine containing ON can be covalently cross-linked to an inorganic surface, and it can be used for photo-immobilization of nucleic acids. As used herein, the term "inorganic surface" refers to silicate glass or other glass-based materials.

Initially, 5'- ³³ P-labeled primers are used to synthesize 3'-modified ONs. A^-acetyl-S-benzoyl- dCTP (4b) or A^-benzoyl-S-benzoyl-dCTP (4c) and TdT were used for the synthesis as described in Example 6. Following reaction, TdT was inactivated by heating as recommended by manufacturer.

After TdT inactivation, the reaction mixtures were chilled on ice and transferred as 2 μΙ_ drops onto a slide of solid glass support. Then the specimens were placed under the UV light source (5 mm away from the surface of the light source) and irradiated for 5 min (365±5 nm). Immediately after irradiation, the specimens were rinsed with 500 μΙ_ of wash buffer (50 mM potassium phosphate, 1 % Triton X-100 (pH 7.0)) following incubation (1 h at room temperature) in 1 .5 mL of wash buffer vigorously shaking. Then the specimens were rinsed with 200 μΙ_ of distilled water and air-dried. Photochemical immobilization of modified ONs to polymeric solid supports was then visualized by phosphor imaging. References

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