Efficient enzymatic synthesis and dual-colour fluorescent labelling of DNA probes using long chain azido-dUTP and BCN dyes

Lanthanide-Complex-Enhanced Bioorthogonal Branched DNA Amplification

Fluorescence in situ hybridization (FISH) is a widely used technique for detecting intracellular nucleic acids. However, its effectiveness in detecting low-copy nucleic acids is limited due to its low fluorescence intensity and background autofluorescence. To address these challenges, we present here an approach of lanthanide-complex-enhanced bioorthogonal-branched DNA amplification (LEBODA) with high sensitivity for in situ nuclear acid detection in single cells. The approach capitalizes on two levels of signal amplification. First, it utilizes click chemistry to directly link a substantial number of bridge probes to target-recognizing probes, providing an initial boost in signal intensity. Second, it incorporates high-density lanthanide complexes into each bridge probe, enabling secondary amplifications. Compared to the traditional "double Z" probes used in the RNAscope method, LEBODA exhibits 4 times the single enhancement for RNA detection signal with the click chemistry approach. Using SARS-CoV-2 pseudovirus-infected HeLa cells, we demonstrate the superiority in the detection of viral-infected cells in rare populations as low as 20% infectious rate. More encouragingly, the LEBODA approach can be adapted for DNA-FISH and single-molecule RNA-FISH, as well as other hybridization-based signal amplification methods. This adaptability broadens the potential applications of LEBODA in the sensitive detection of biomolecules, indicating promising prospects for future research and practical use.

Modified nucleic acid aptamers: development, characterization, and biological applications

Aptamers are single-stranded oligonucleotides that bind to their targets via specific structural interactions. To improve the properties and performance of aptamers, modified nucleotides are incorporated during or after a selection process such as systematic evolution of ligands by exponential enrichment (SELEX). We summarize the latest modified nucleotides and strategies used in modified (mod)-SELEX and post-SELEX to develop modified aptamers, highlight the methods used to characterize aptamer-target interactions, and present recent progress in modified aptamers that recognize different targets. We discuss the challenges and perspectives in further advancing the methodologies and toolsets to accelerate the discovery of modified aptamers, improve the throughput of aptamer-target characterization, and expand the functional diversity and complexity of modified aptamers.

Development of Fluorescent Isocoumarin-Fused Oxacyclononyne - 1,2,3-Triazole Pairs

Fluorescent isocoumarin-fused cycloalkynes, which are reactive in SPAAC and give fluorescent triazoles regardless of the azide nature, have been developed. The key structural feature that converts the non-fluorescent cycloalkyne/triazole pair to its fluorescent counterpart is the pi-acceptor group (COOMe, CN) at the C6 position of the isocoumarin ring. The design of the fluorescent cycloalkyne/triazole pairs is based on the theoretical study of the S1 state deactivation mechanism of the non-fluorescent isocoumarin-fused cycloalkyne IC9O using multi-configurational ab initio and DFT methodologies. The calculations revealed that deactivation proceeds through the electrocyclic ring opening of the α-pyrone cycle and is accompanied by a redistribution of electron density in the fused benzene ring. We proposed that the S1 excited state deactivation barrier could be increased by introducing a pi-acceptor group into a position that is in direct conjugation with the formed C=O group and has a reduced electron density in the transition state. As a proof of concept, we designed and synthesized two fluorescent isocoumarin-fused cycloalkynes IC9O-COOMe and IC9O-CN bearing pi-acceptors at the C6 position. The importance of the nature of a pi-acceptor group was shown by the example of much less fluorescent CF3 -substituted cycloalkyne IC9O-CF3 .

trans-Cyclooctene- and Bicyclononyne-Linked Nucleotides for Click Modification of DNA with Fluorogenic Tetrazines and Live Cell Metabolic Labeling and Imaging

A series of 2'-deoxyribonucleoside triphosphates (dNTPs) bearing 2- or 4-linked trans-cyclooctene (TCO) or bicyclononyne (BCN) tethered through a shorter propargylcarbamate or longer triethyleneglycol-based spacer were designed and synthesized. They were found to be good substrates for KOD XL DNA polymerase for primer extension enzymatic synthesis of modified oligonucleotides. We systematically tested and compared the reactivity of TCO- and BCN-modified nucleotides and DNA with several fluorophore-containing tetrazines in inverse electron-demand Diels-Alder (IEDDA) click reactions to show that the longer linker is crucial for efficient labeling. The modified dNTPs were transported into live cells using the synthetic transporter SNTT1, incubated for 1 h, and then treated with tetrazine conjugates. The PEG3-linked 4TCO and BCN nucleotides showed efficient incorporation into genomic DNA and good reactivity in the IEDDA click reaction with tetrazines to allow staining of DNA and imaging of DNA synthesis in live cells within time periods as short as 15 min. The BCN-linked nucleotide in combination with TAMRA-linked (TAMRA = carboxytetramethylrhodamine) tetrazine was also efficiently used for staining of DNA for flow cytometry. This methodology is a new approach for in cellulo metabolic labeling and imaging of DNA synthesis which is shorter, operationally simple, and overcomes several problems of previously used methods.

Internet of medical things-enabled CRISPR diagnostics for rapid detection of SARS-CoV-2 variants of concern

Previous studies have highlighted CRISPR-based nucleic acid detection as rapid and sensitive diagnostic methods for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here, we reported an optimized CRISPR-Cas12a diagnostic platform for the safe and rapid detection of SARS-CoV-2 variants of concern (VOCs). This platform, which was referred to as CALIBURN-v2, could complete the diagnosis on extracted RNA samples within 25 min in a closed-lid reaction mode and had 100-fold increase in detection sensitivity in comparison with previous platforms. Most importantly, by integrating a portable device and smartphone user interface, CALIBURN-v2 allowed for cloud server-based data collection and management, thus transforming the point-of-care testing (POCT) platform to internet of medical things (IoMT) applications. It was found that IoMT-enabled CALIBURN-v2 could achieve 95.56% (172 out of 180) sensitivity for SARS-CoV-2 wild type and 94.38% (84 out of 89) overall sensitivity for SARS-CoV-2 variants including Delta and Omicron strains. Therefore, our study provides a feasible approach for IoMT-enabled CRISPR diagnostics for the detection of SARS-CoV-2 VOCs.

N4 -allyldeoxycytidine: A New DNA Tag with Chemical Sequencing Power for Pinpointing Labelling Sites, Mapping Epigenetic Mark, and in situ Imaging

DNA tagging with base analogs has found numerous applications. To precisely record the DNA labelling information, it will be highly beneficial to develop chemical sequencing tags that can be encoded into DNA as regular bases and decoded as mutant bases upon a mild, efficient and bioorthognal chemical treatment. Here we reported such a DNA tag, N4-allyldeoxycytidine (a4dC), to label and identify DNA by in vitro assays. The iodination of a4dC led to fast and complete formation of 3, N4-cyclized deoxycytidine, which induced base misincorporation during DNA replication and thus could be located at single base resolution. We explored the applications of a4dC in pinpointing DNA labelling sites at single base resolution, mapping epigenetic mark N4-methyldeoxycytidine, and imaging nucleic acids in situ. In addition, mammalian cellular DNA could be metabolically labelled with a4dC. Together，our study sheds light on the design of next generation DNA tags with chemical sequencing power.

Advanced preparation of fragment libraries enabled by oligonucleotide-modified 2',3'-dideoxynucleotides

The ever-growing demand for inexpensive, rapid, and accurate exploration of genomes calls for refinement of existing sequencing techniques. The development of next-generation sequencing (NGS) was a revolutionary milestone in genome analysis. While modified nucleotides already were inherent tools in sequencing and imaging, further modification of nucleotides enabled the expansion into even more diverse applications. Herein we describe the design and synthesis of oligonucleotide-tethered 2',3'-dideoxynucleotide (dd^ONNTP) terminators bearing universal priming sites attached to the nucleobase, as well as their enzymatic incorporation and performance in read-through assays. In the context of NGS library preparation, the incorporation of dd^ONNTP fulfills two requirements at once: the fragmentation step is integrated into the workflow and the obtained fragments are readily labeled by platform-specific adapters. DNA polymerases can incorporate dd^ONNTP nucleotides, as shown by primer extension assays. More importantly, reading through the unnatural linkage during DNA synthesis was demonstrated, with 25-30% efficiency in single-cycle extension.

Expanding the chemical functionality of DNA nanomaterials generated by rolling circle amplification

Rolling circle amplification (RCA) is a powerful tool for the construction of DNA nanomaterials such as hydrogels, high-performance scaffolds and DNA nanoflowers (DNFs), hybrid materials formed of DNA and magnesium pyrophosphate. Such DNA nanomaterials have great potential in therapeutics, imaging, protein immobilisation, and drug delivery, yet limited chemistry is available to expand their functionality. Here, we present orthogonal strategies to produce densely modified RCA products and DNFs. We provide methods to selectively modify the DNA component and/or the protein cargo of these materials, thereby greatly expanding the range of chemical functionalities available to these systems. We have used our methodology to construct DNFs bearing multiple surface aptamers and peptides capable of binding to cancer cells that overexpress the HER2 oncobiomarker, demonstrating their potential for diagnostic and therapeutic applications.

1,3-Diketone-Modified Nucleotides and DNA for Cross-Linking with Arginine-Containing Peptides and Proteins

Linear or branched 1,3-diketone-linked thymidine 5'-O-mono- and triphosphate were synthesized through CuAAC click reaction of diketone-alkynes with 5-azidomethyl-dUMP or -dUTP. The triphosphates were good substrates for KOD XL DNA polymerase in primer extension synthesis of modified DNA. The nucleotide bearing linear 3,5-dioxohexyl group (HDO) efficiently reacted with arginine-containing peptides to form stable pyrimidine-linked conjugates, whereas the branched 2-acetyl-3-oxo-butyl (PDO) group was not reactive. Reaction with Lys or a terminal amino group formed enamine adducts that were prone to hydrolysis. This reactive HDO modification in DNA was used for bioconjugations and cross-linking with Arg-containing peptides or proteins (e.g. histones).

A Hitchhiker's Guide to Click-Chemistry with Nucleic Acids

Click chemistry is an immensely powerful technique for the fast and efficient covalent conjugation of molecular entities. Its broad scope has positively impacted on multiple scientific disciplines, and its implementation within the nucleic acid field has enabled researchers to generate a wide variety of tools with application in biology, biochemistry, and biotechnology. Azide-alkyne cycloadditions (AAC) are still the leading technology among click reactions due to the facile modification and incorporation of azide and alkyne groups within biological scaffolds. Application of AAC chemistry to nucleic acids allows labeling, ligation, and cyclization of oligonucleotides efficiently and cost-effectively relative to previously used chemical and enzymatic techniques. In this review, we provide a guide to inexperienced and knowledgeable researchers approaching the field of click chemistry with nucleic acids. We discuss in detail the chemistry, the available modified-nucleosides, and applications of AAC reactions in nucleic acid chemistry and provide a critical view of the advantages, limitations, and open-questions within the field.

Nucleotide-Bearing Benzylidene-Tetrahydroxanthylium Near-IR Fluorophore for Sensing DNA Replication, Secondary Structures and Interactions

Thymidine triphosphate bearing benzylidene‐tetrahydroxanthylium near‐IR fluorophore linked to the 5‐methyl group via triazole was synthesized through the CuAAC reaction and was used for polymerase synthesis of labelled DNA probes. The fluorophore lights up upon incorporation to DNA (up to 348‐times) presumably due to interactions in major groove and the fluorescence further increases in the single‐stranded oligonucleotide. The labelled dsDNA senses binding of small molecules and proteins by a strong decrease of fluorescence. The nucleotide was used as a light‐up building block in real‐time PCR for detection of SARS‐CoV‐2 virus.

Targeted Delivery of Antisense Oligonucleotides Using Neurotensin Peptides

Despite recent advances, targeted delivery of therapeutic oligonucleotide to extra-hepatic tissues continues to be a challenging endeavor and efficient ligand-receptor systems need to be identified. To determine the feasibility of using neurotensin to improve the productive uptake of antisense oligonucleotides (ASO), we synthesized neurotensin-ASO conjugates and evaluated their cellular uptake and activity in cells and in mice. We performed a comprehensive structure-activity relationship study of the conjugates and determined the influence of ASO charge, ASO length, peptide charge, linker chemistry and ligand identity on receptor binding and internalization. We identified a modified neurotensin peptide capable of improving the cellular uptake and activity of gapmer ASOs in sortilin expressing cells (sixfold) and in spinal cord in mice (twofold). Neurotensin conjugation also improved the potency of morpholino ASO designed to correct splicing of survival motor neuron pre-mRNA in the cortex and striatum after intracerebroventricular injection. Neurotensin-mediated targeted delivery represents a possible approach for enhancing the potency of ASOs with diverse nucleic acid modifications.

Stable immobilization of aldehyde ketone reductase mutants containing nonstandard amino acids on an epoxy resin via strain-promoted alkyne–azide cycloaddition

To avoid random chemical linkage and achieve precisely directed immobilization, mutant enzymes were obtained and immobilized using an incorporated reactive nonstandard amino acid (NSAA). For this purpose, aldehyde ketone reductase (AKR) was used as a model enzyme, and 110Y, 114Y, 143Y, 162Q and 189Q were each replaced with p-azido-l-phenylalanine (pAzF). Then, the mutant AKR was coupled to the functionalized support by strain-promoted alkyne–azide cycloaddition (SPAAC). The effects of the incorporation number and site of NSAAs on the loading and thermal stability of the immobilized AKR were examined. The results show that the mutant enzymes presented better specific activity than the wild type, except for AKR-110Y, and AKR-114Y showed 1.16-fold higher activity than the wild type. Moreover, the half-life (t_1/2) of the five-point immobilized AKR reached 106 h and 45 h, 13 and 7 times higher than that of the free enzyme at 30 °C and 60 °C, respectively. Comparison of these three types of enzymes shows that multi-point immobilization provides improved loading and thermal stability and facilitates one-step purification. We expect this platform to facilitate a fundamental understanding of precisely oriented and controllable covalent immobilization and enable bio-manufacturing paradigms for fine chemicals and pharmaceuticals.

Stable immobilization of aldehyde ketone reductase mutants containing non-standard amino acids on an epoxy resin via strain-promoted alkyne–azide cycloaddition.

Enzymatic Synthesis of Base-Functionalized Nucleic Acids for Sensing, Cross-linking, and Modulation of Protein-DNA Binding and Transcription

Protein-DNA interactions are important in replication, transcription, repair, as well as epigenetic modifications of DNA, which involve methylation and demethylation of DNA resulting in regulation of gene expression. Understanding of these processes and chemical tools for studying and perhaps even modulating them could be of great relevance and importance not only in chemical biology but also in real diagnostics and treatment of diseases. In the past decade, we have been working on development of synthesis of base-modified 2'-deoxyribo- or ribonucleoside triphosphates (dNTPs or NTPs) and their use in enzymatic synthesis of modified nucleic acids using DNA or RNA polymerases. These synthetic and enzymatic methods are briefly summarized with focus on recent development and outlining of scope, limitations, and further challenges. The main focus of this Account is on applications of base-modified nucleic acids in sensing of protein-DNA interactions, in covalent cross-linking to DNA-binding proteins ,and in modulation of protein-DNA binding and transcription. Several environment-sensitive fluorescent nucleotides were incorporated to DNA probes which responded to protein binding by light-up, changing of color, or lifetime of fluorescence. Using a cyclodextrin-peptide transporter, fluorescent nucleotides can be transported through the cell membrane and incorporated to genomic DNA. Several dNTPs bearing reactive groups (i.e., vinylsulfonamide or chloroacetamide) were used for polymerase synthesis of DNA reactive probes which cross-link to Cys, His, or Lys in peptides or proteins. An attractive challenge is to use DNA modifications and bioorthogonal reactions in the major groove of DNA for modulation and switching of protein-DNA interactions. We have systematically explored the influence of major-groove modifications on recognition and cleavage of DNA by restriction endonucleases and constructed simple chemical switches of DNA cleavage. Systematic study of the influence of major-groove modifications on transcription with bacterial RNA polymerases revealed not only that some modified bases are tolerated, but also that the presence of 5-hydroxymethyluracil or -cytosine can even enhance the transcription (350 or 250% compared to native DNA). Based on these results, we have constructed the first chemical switch of transcription based on photocaging of hydroxymethylpyrimidines in DNA by 2-nitrobenzyl protection (transcription off), photochemical deprotection of the DNA (transcription on), and enzymatic phosphorylation (only for 5-hydroxymethyluracil, transcription off). Although it has been so far demonstrated only in vitro, it is the proof-of-principle first step toward chemical epigenetics.

Clickable PNA Probes for Imaging Human Telomeres and Poly(A) RNAs

The ability to bind strongly to complementary nucleic acid sequences, invade complex nucleic acid structures, and resist degradation by cellular enzymes has made peptide nucleic acid (PNA) oligomers as very useful hybridization probes in molecular diagnosis. For such applications, the PNA oligomers have to be labeled with appropriate reporters as they lack intrinsic labels that can be used in biophysical assays. Although solid-phase synthesis is commonly used to attach reporters onto PNA, development of milder and modular labeling methods will provide access to PNA oligomers labeled with a wider range of biophysical tags. Here, we describe the establishment of a postsynthetic modification strategy based on bioorthogonal chemical reactions in functionalizing PNA oligomers in solution with a variety of tags. A toolbox composed of alkyne- and azide-modified monomers were site-specifically incorporated into PNA oligomers and postsynthetically click-functionalized with various tags, ranging from sugar, amino acid, biotin, to fluorophores, by using copper(I)-catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne cycloaddition, and Staudinger ligation reactions. As a proof of utility of this method, fluorescent PNA hybridization probes were developed and used in imaging human telomeres in chromosomes and poly(A) RNAs in cells. Taken together, this simple approach of generating a wide range of functional PNA oligomers will expand the use of PNA in molecular diagnosis.

Self-Priming Enzymatic Fabrication of Multiply Modified DNA

The self-priming synthesis of multiply modified DNA by the extension of repeating unit duplex "oligoseeds" provides a source of versatile DNA. Sterically-demanding nucleotides 5-Br-dUTP, 7-deaza-7-I-dATP, 6-S-dGTP, 5-I-dCTP as well as 5-(octadiynyl)-dCTP were incorporated into two extending oligoseeds; [GATC]₅ /[GATC]₅ and [A₄ G]₄ /[CT₄ ]₄ . The products contained modifications on one or both strands of DNA, demonstrating their recognition by the polymerase as both template (reading) and substrate (writing). Nucleobase modifications that lie in the major groove were reliably read and written by the polymerase during the extension reaction, even when bulky or in contiguous sequences. Repeat sequence DNA over 500 bp long, bearing four different modified units was produced by this method. The number, position and type of modification, as well as the overall length of the DNA can be controlled to yield designer DNA that offers sequence-determined sites for further chemical adaptations, targeted small molecule binding studies, or sensing and sequencing applications.

dUTPs conjugated with zwitterionic Cy3 or Cy5 fluorophore analogues are effective substrates for DNA amplification and labelling by Taq polymerase

To develop structural modifications of dNTPs that are compatible with Taq DNA polymerase activity, we synthesized eight dUTP derivatives conjugated with Cy3 or Cy5 dye analogues that differed in charge and charge distribution throughout the fluorophore. These dUTP derivatives and commercial Cy3- and Cy5-dUTP were studied in Taq polymerase-dependent polymerase chain reactions (PCRs) and in primer extension reactions using model templates containing one, two and three adjacent adenine nucleotides. The relative amounts of amplified DNA and the kinetic parameters Km and Vmax characterizing the incorporation of labelled dUMPs have been estimated using fluorescence measurements and analysed. The dUTPs labelled with electroneutral zwitterionic analogues of Cy3 or Cy5 fluorophores were used by Taq polymerase approximately one order of magnitude more effectively than the dUTPs labelled with negatively charged analogues of Cy3 or Cy5. The nucleotidyl transferase activity of Taq polymerase was also observed and resulted in the addition of dUMPs labelled with electroneutral or positively charged fluorophores to the 3' ends of DNA. The introduction of mutually compensating charges into fluorophores or other functional groups conjugated to dNTPs can be considered a basis for the creation of PCR-compatible modified nucleoside triphosphates.

Synthesis of Dihydroxyalkynyl and Dihydroxyalkyl Nucleotides as Building Blocks or Precursors for Introduction of Diol or Aldehyde Groups to DNA for Bioconjugations

(3,4-Dihydroxybut-1-ynyl)uracil, -cytosine and -7-deazaadenine 2'-deoxyribonucleoside triphosphates (dNTPs) were prepared by direct aqueous Sonogashira cross-coupling of halogenated dNTPs with dihydroxybut-1-yne and converted to 3,4-dihydroxybutyl dNTPs through catalytic hydrogenation. Sodium periodate oxidative cleavage of dihydroxybutyl-dUTP gave the desired aliphatic aldehyde-linked dUTP, whereas the oxidative cleavage of the corresponding deazaadenine dNTP gave a cyclic aminal. All dihydroxyalkyl or -alkynyl dNTPs and the formylethyl-dUTP were good substrates for DNA polymerases and were used for synthesis of diol- or aldehyde-linked DNA. The aldehyde linked DNA was used for the labelling or bioconjugations through hydrazone formation or reductive aminations.

Enzymatic synthesis and reverse transcription of RNAs incorporating 2'-O-carbamoyl uridine triphosphate

Enzymatic synthesis and the reverse transcription of RNAs containing 2'-O-carbamoyl uridine were evaluated. A mild acidic deprotection procedure allowed the synthesis of 2'-O-carbamoyl uridine triphosphate (U_cmTP). U_cmTP was incorporated correctly into long RNAs, and its fidelity during reverse transcription using SuperScript III was sufficient for RNA aptamer selection.

Site-specific one-pot triple click labeling for DNA and RNA

We report site-specific triple click labeling for DNA and RNA in a one-pot setup by performing inverse electron demand Diels–Alder reaction and strain-promoted and copper catalyzed click reactions sequentially.

Synthesis of chemically modified DNA

Naturally occurring DNA is encoded by the four nucleobases adenine, cytosine, guanine and thymine. Yet minor chemical modifications to these bases, such as methylation, can significantly alter DNA function, and more drastic changes, such as replacement with unnatural base pairs, could expand its function. In order to realize the full potential of DNA in therapeutic and synthetic biology applications, our ability to 'write' long modified DNA in a controlled manner must be improved. This review highlights methods currently used for the synthesis of moderately long chemically modified nucleic acids (up to 1000 bp), their limitations and areas for future expansion.

2-Substituted dATP Derivatives as Building Blocks for Polymerase-Catalyzed Synthesis of DNA Modified in the Minor Groove

2'-Deoxyadenosine triphosphate (dATP) derivatives bearing diverse substituents (Cl, NH₂ , CH₃ , vinyl, ethynyl, and phenyl) at position 2 were prepared and tested as substrates for DNA polymerases. The 2-phenyl-dATP was not a substrate for DNA polymerases, but the dATPs bearing smaller substituents were good substrates in primer-extension experiments, producing DNA substituted in the minor groove. The vinyl-modified DNA was applied in thiol-ene addition and the ethynyl-modified DNA was applied in a CuAAC click reaction to form DNA labelled with fluorescent dyes in the minor groove.

5-Substituted Pyrimidine and 7-Substituted 7-Deazapurine dNTPs as Substrates for DNA Polymerases in Competitive Primer Extension in the Presence of Natural dNTPs

A complete series of 5-substituted uracil or cytosine, as well as 7-substituted 7-deazaadenine and 7-deazaguanine 2'-deoxyribonucleoside triphosphates (dNTPs) bearing substituents of increasing bulkiness (H, Me, vinyl, ethynyl, and phenyl) were systematically studied in competitive primer extension in the presence of their natural counterparts (nonmodified dNTPs), and their kinetic data were determined. The results show that modified dNTPs bearing π-electron-containing substituents (vinyl, ethynyl, Ph) are typically excellent substrates for DNA polymerases comparable to or better than natural dNTPs. The kinetic studies revealed that these modified dNTPs have higher affinity to the active site of the enzyme-primer-template complex, and the calculations (semiempirical quantum mechanical scoring function) suggest that it is due to the cation-π interaction of the modified dNTP with Arg629 in the active site of Bst DNA polymerase.

Chloroacetamide-Linked Nucleotides and DNA for Cross-Linking with Peptides and Proteins

Nucleotides, 2'-deoxyribonucleoside triphosphates (dNTPs), and DNA probes bearing reactive chloroacetamido group linked to nucleobase (cytosine or 7-deazadaenine) through a propargyl tether were prepared and tested in cross-linking with cysteine- or histidine-containing peptides and proteins. The chloroacetamide-modifed dNTPs proved to be good substrates for DNA polymerases in the enzymatic synthesis of modified DNA probes. Modified nucleotides and DNA reacted efficiently with cysteine and cysteine-containing peptides, whereas the reaction with histidine was sluggish and low yielding. The modified DNA efficiently cross-linked with p53 protein through alkylation of cysteine and showed potential for cross-linking with histidine (in C277H mutant of p53).