Synthesis of 5-substituted 2'-deoxycytidine 5'-(alpha-P-borano)triphosphates, their incorporationinto DNA and effects on exonuclease

Synthesis and Properties of α-Phosphate-Modified Nucleoside Triphosphates

This review article is focused on the progress made in the synthesis of 5'-α-P-modified nucleoside triphosphates (α-phosphate mimetics). A variety of α-P-modified nucleoside triphosphates (NTPαXYs, Y = O, S; X = S, Se, BH3, alkyl, amine, N-alkyl, imido, or others) have been developed. There is a unique class of nucleoside triphosphate analogs with different properties. The main chemical approaches to the synthesis of NTPαXYs are analyzed and systematized here. Using the data presented here on the diversity of NTPαXYs and their synthesis protocols, it is possible to select an appropriate method for obtaining a desired α-phosphate mimetic. Triphosphates' substrate properties toward nucleic acid metabolism enzymes are highlighted too. We reviewed some of the most prominent applications of NTPαXYs including the use of modified dNTPs in studies on mechanisms of action of polymerases or in systematic evolution of ligands by exponential enrichment (SELEX). The presence of heteroatoms such as sulfur, selenium, or boron in α-phosphate makes modified triphosphates nuclease resistant. The most distinctive feature of NTPαXYs is that they can be recognized by polymerases. As a result, S-, Se-, or BH3-modified phosphate residues can be incorporated into DNA or RNA. This property has made NTPαXYs a multifunctional tool in molecular biology. This review will be of interest to synthetic chemists, biochemists, biotechnologists, or biologists engaged in basic or applied research.

Polymerase Synthesis of DNA Containing Iodinated Pyrimidine or 7-Deazapurine Nucleobases and Their Post-synthetic Modifications through the Suzuki-Miyaura Cross-Coupling Reactions

All four iodinated 2'-deoxyribonucleoside triphosphates (dNTPs) derived from 5-iodouracil, 5-iodocytosine, 7-iodo-7-deazaadenine and 7-iodo-7-deazaguanine were prepared and studied as substrates for KOD XL DNA polymerase. All of the nucleotides were readily incorporated by primer extension and by PCR amplification to form DNA containing iodinated nucleobases. Systematic study of the Suzuki-Miyaura cross-coupling reactions with two bulkier arylboronic acids revealed that the 5-iodopyrimidines were more reactive and gave cross-coupling products both in the terminal or internal position in single-stranded oligonucleotides (ssONs) and in the terminal position of double-stranded DNA (dsDNA), whereas the 7-iodo-7-deazapurines were less reactive and gave cross-coupling products only in the terminal position. None of the four iodinated bases reacted in an internal position of dsDNA. These findings are useful for the use of the iodinated nucleobases for post-synthetic modification of DNA with functional groups for various applications.

Boron-containing nucleosides as tools for boron-neutron capture therapy

Despite the significant progress in cancer cure, the development of new approaches to cancer therapy is still of great importance since many deadly tumors remain untreatable. Boron neutron capture therapy (BNCT), proposed more than eighty years ago, is still considered a potentially advantageous approach. Irradiation of cells containing ¹⁰B isotopes with epithermal neutrons and the consequent decay of boron nuclei releases particles that deposit high energy along a very short path, inflicting heavy damage on the target cells but sparing the neighbouring tissue. Delivery and preferential accumulation of boron in cancer cells are the major obstacles that slow down the clinical use of BNCT. Since DNA damage caused by irradiation is the major reason for cell death, the incorporation of boron-containing nucleotides into the DNA of cancer cells may significantly increase the efficacy of BNCT. In this review, we discuss the current state of knowledge in the synthesis of boron-containing nucleosides and their application for BNCT with a special focus on their possible incorporation into genomic DNA.

Modified nucleoside triphosphates in bacterial research for in vitro and live-cell applications

Modified nucleoside triphosphates (NTPs) are invaluable tools to probe bacterial enzymatic mechanisms, develop novel genetic material, and engineer drugs and proteins with new functionalities. Although the impact of nucleobase alterations has predominantly been studied due to their importance for protein recognition, sugar and phosphate modifications have also been investigated. However, NTPs are cell impermeable due to their negatively charged phosphate tail, a major hurdle to achieving live bacterial studies. Herein, we review the recent advances made to investigate and evolve bacteria and their processes with the use of modified NTPs by exploring alterations in one of the three moieties: the nucleobase, the sugar and the phosphate tail. We also present the innovative methods that have been devised to internalize NTPs into bacteria for in vivo applications.

Reaction of Boranephosphonate Diesters with Pyridines or Tertiary Amines in the Presence of Iodine: Synthetic and Mechanistic Studies

Boranephosphonate diesters react with heteroaromatic and certain tertiary amines in the presence of an oxidant (I₂) to afford the boron-modified phosphodiester analogues containing a P-B-N structural motif. Our multinuclear ³¹P and ¹¹B NMR spectroscopy studies lend support for a two-step mechanism involving generation of a λ³-boranephosphonate intermediate that immediately coordinates an amine in the solvent cage, leading to B-pyridinium or B-ammonium boranephosphonate betaine derivatives. We found that the type of the solvent used (e.g., dichloromethane vs acetonitrile) significantly affected the course of the reaction, resulting in either formation of boron-modified derivatives or loss of the boron group with a subsequent oxidation of the phosphorus atom. In aprotic, electron-donating, polar solvents., e.g., acetonitrile (ACN) and tetrahydrofuran (THF), a λ³-boranephosphonate intermediate can also coordinate solvent molecules forming P-B-ACN or P-B-THF complexes that may influence the type of the products formed.

The Toolbox for Modified Aptamers

Aptamers are nucleic acid-based scaffolds that can bind with high affinity to a variety of biological targets. Aptamers are identified from large DNA or RNA libraries through a process of directed molecular evolution (SELEX). Chemical modification of nucleic acids considerably increases the functional and structural diversity of aptamer libraries and substantially increases the affinity of the aptamers. Additionally, modified aptamers exhibit much greater resistance to biodegradation. The evolutionary selection of modified aptamers is conditioned by the possibility of the enzymatic synthesis and replication of non-natural nucleic acids. Wild-type or mutant polymerases and their non-natural nucleotide substrates that can support SELEX are highlighted in the present review. A focus is made on the efforts to find the most suitable type of nucleotide modifications and the engineering of new polymerases. Post-SELEX modification as a complementary method will be briefly considered as well.

Aptamer-Functionalized Nanoparticles as "Smart Bombs": The Unrealized Potential for Personalized Medicine and Targeted Cancer Treatment

Conventional delivery of chemotherapeutic agents leads to multiple systemic side effects and toxicity, limiting the doses that can be used. The development of targeted therapies to selectively deliver anti-cancer agents to tumor cells without damaging neighboring unaffected cells would lead to higher effective local doses and improved response rates. Aptamers are single-stranded oligonucleotides that bind to target molecules with both high affinity and high specificity. The high specificity exhibited by aptamers promotes localization and uptake by specific cell populations, such as tumor cells, and their conjugation to anti-cancer drugs has been explored for targeted therapy. Advancements in the development of polymeric nanoparticles allow anti-cancer drugs to be encapsulated in protective nonreactive shells for controlled drug delivery with reduced toxicity. The conjugation of aptamers to nanoparticle-based therapeutics may further enhance direct targeting and personalized medicine. Here we present how the combinatorial use of aptamer and nanoparticle technologies has the potential to develop "smart bombs" for targeted cancer treatment, highlighting recent pre-clinical studies demonstrating efficacy for the direct targeting to particular tumor cell populations. However, despite these pre-clinical promising results, there has been little progress in moving this technology to the bedside.

Transcription of 4′-thioDNA templates to natural RNA in vitro and in mammalian cells

Synthetic chemically modified nucleic acids, which are compatible with DNA/RNA polymerases, have great potential as a genetic material for synthetic biological studies.

The synthesis of 2'-methylseleno adenosine and guanosine 5'-triphosphates

Modified nucleoside triphosphates (NTPs) represent powerful building blocks to generate nucleic acids with novel properties by enzymatic synthesis. We have recently demonstrated the access to 2′-SeCH₃-uridine and 2′-SeCH₃-cytidine derivatized RNAs for applications in RNA crystallography, using the corresponding nucleoside triphosphates and distinct mutants of T7 RNA polymerase. In the present note, we introduce the chemical synthesis of the novel 2′-methylseleno-2′-deoxyadenosine and -guanosine 5′-triphosphates (2′-SeCH₃-ATP and 2′-SeCH₃-GTP) that represent further candidates for the enzymatic RNA synthesis with engineered RNA polymerases.

Protection-free one-pot synthesis of 2'-deoxynucleoside 5'-triphosphates and DNA polymerization

By differentiating the functional groups on nucleosides, we have designed and developed a one-pot synthesis of deoxyribonucleoside 5'-triphosphates without any protection on the nucleosides. A facile synthesis is achieved by generating an in situ phosphitylating reagent that reacts selectively with the 5'-hydroxyl groups of the unprotected nucleosides. The synthesized triphosphates are of high quality and can be effectively incorporated into DNAs by DNA polymerase. This novel approach is straightforward and cost-effective for triphosphate synthesis.

Convenient synthesis of nucleoside 5'-triphosphates for RNA transcription

By generating a selective phosphitylating reagent in situ, nucleoside 5'-triphosphates can be conveniently synthesized in one pot. This novel strategy without nucleoside protection has been developed to largely simplify synthesis of the nucleoside triphosphates. This demonstrated principle can be applied to the 5'-triphosphate synthesis of both native and modified nucleosides.

Recent progress toward the templated synthesis and directed evolution of sequence-defined synthetic polymers

Biological polymers such as nucleic acids and proteins are ubiquitous in living systems, but their ability to address problems beyond those found in nature is constrained by factors such as chemical or biological instability, limited building-block functionality, bioavailability, and immunogenicity. In principle, sequence-defined synthetic polymers based on nonbiological monomers and backbones might overcome these constraints; however, identifying the sequence of a synthetic polymer that possesses a specific desired functional property remains a major challenge. Molecular evolution can rapidly generate functional polymers but requires a means of translating amplifiable templates such as nucleic acids into the polymer being evolved. This review covers recent advances in the enzymatic and nonenzymatic templated polymerization of nonnatural polymers and their potential applications in the directed evolution of sequence-defined synthetic polymers.

Transcription and reverse transcription of artificial nucleic acids involving backbone modification by template-directed DNA polymerase reactions

Oligodeoxyribonucleotides (ODN) where the phosphodiester linkage had been replaced with an amide-type linker [-CH(2)C=ONH-] or an amine-type linker [-CH(2)CH(2)NH-] were synthesized to investigate the effect of these backbone modifications on polymerase reactions. In addition, a triphosphate analogue of thymidine dinucleotide with the amide-type linker was synthesized and enzymatic insertion of the amide linkage into ODN was attempted using this analogue for the polymerase reaction. Primer extension reactions using three types of thermostable DNA polymerases, KOD(exo-), Vent(exo-) and Taq were performed for the assays. Analysis of these data indicate that (i) the polymerase reaction tends to be affected much more by insertion of the cationic flexible amine-type linker than by insertion of the neutral rigid amide-type linker; (ii) the backbone modification has a greater effect on the polymerase reaction when it is adjacent to the 3'-end of a primer as the elongation terminus than when it is on the template, as well as in base or sugar modification; (iii) although the modified linker in the modified DNA template is passed beyond by the polymerase, it still affects the extension reaction several bases downstream from its location; (iv) the modified linker in the template, in some cases, also affects the extension reaction upstream from its location; (v) further improvement of the chemical structure is required for dinucleotide-mimic incorporation.

Stereospecificity, substrate, and inhibitory properties of nucleoside diphosphate analogs for creatine and pyruvate kinases

Antiviral alpha-P-borano substituted NTPs are promising chain terminators targeting HIV reverse transcriptase (RT). Activation of antiviral nucleoside diphosphates (NDPs) to NTPs may be carried out by pyruvate kinase (PK) and creatine kinase (CK). Herein, are presented the effects of nucleobase, ribose, and alpha-phosphate substitutions on substrate specificities of CK and PK. Both enzymes showed two binding modes and negative cooperativity with respect to substrate binding. The stereospecificity and inhibition of ADP phosphorylation by alpha-P-borano substituted NDP (NDPalphaB) stereoisomers were also investigated. The Sp-ADPalphaB isomer was a 70-fold better substrate for CK than the Rp isomer, whereas PK preferred the Rp isomer of NDPalphaBs. For CK, the Sp-ADPalphaB isomer was a competitive inhibitor; for PK, the Rp-ADPalphaB isomer was a poor competitive inhibitor and the Sp-ADPalphaB isomer was a poor non-competitive inhibitor. Taken together, these data suggest that, although the Rp-NDPalphaB isomer would be minimally phosphorylated by CK or PK, it should not inhibit either enzyme.

Enzymatic incorporation of a third nucleobase pair

DNA polymerases are identified that copy a non-standard nucleotide pair joined by a hydrogen bonding pattern different from the patterns joining the dA:T and dG:dC pairs. 6-Amino-5-nitro-3-(1′-β-d-2′-deoxyribofuranosyl)-2(1H)-pyridone (dZ) implements the non-standard ‘small’ donor–donor–acceptor (pyDDA) hydrogen bonding pattern. 2-Amino-8-(1′-β-D-2′-deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one (dP) implements the ‘large’ acceptor–acceptor–donor (puAAD) pattern. These nucleobases were designed to present electron density to the minor groove, density hypothesized to help determine specificity for polymerases. Consistent with this hypothesis, both dZTP and dPTP are accepted by many polymerases from both Families A and B. Further, the dZ:dP pair participates in PCR reactions catalyzed by Taq, Vent (exo⁻) and Deep Vent (exo⁻) polymerases, with 94.4%, 97.5% and 97.5%, respectively, retention per round. The dZ:dP pair appears to be lost principally via transition to a dC:dG pair. This is consistent with a mechanistic hypothesis that deprotonated dZ (presenting a pyDAA pattern) complements dG (presenting a puADD pattern), while protonated dC (presenting a pyDDA pattern) complements dP (presenting a puAAD pattern). This hypothesis, grounded in the Watson–Crick model for nucleobase pairing, was confirmed by studies of the pH-dependence of mismatching. The dZ:dP pair and these polymerases, should be useful in dynamic architectures for sequencing, molecular-, systems- and synthetic-biology.

Nucleoside alpha-thiotriphosphates, polymerases and the exonuclease III analysis of oligonucleotides containing phosphorothioate linkages

The use of DNA polymerases to incorporate phosphorothioate linkages into DNA, and the use of exonuclease III to determine where those linkages have been incorporated, are re-examined in this work. The results presented here show that exonuclease III degrades single-stranded DNA as a substrate and digests through phosphorothioate linkages having one absolute stereochemistry, assigned (assuming inversion in the polymerase reaction) as S, but not the other absolute stereochemistry. This contrasts with a general view in the literature that exonuclease III favors double-stranded nucleic acid as a substrate and stops completely at phosphorothioate linkages. Furthermore, not all DNA polymerases appear to accept exclusively the (R) stereoisomer of nucleoside alpha-thiotriphosphates [and not the (S) diastereomer], a conclusion inferred two decades ago by examination of five Family-A polymerases and a reverse transcriptase. This suggests that caution is appropriate when extrapolating the detailed behavior of one polymerase from the behaviors of other polymerases. Furthermore, these results provide constraints on how exonuclease III–thiotriphosphate–polymerase combinations can be used to analyze the behavior of the components of a synthetic biology.

Stereoselective synthesis of dinucleoside boranophosphates by an oxazaphospholidine method

A stereoselective synthesis of dinucleoside boranophosphates by using nucleoside 3'-oxazaphospholidine derivatives is described. The diastereoselectivity of the internucleotidic bond formation reactions varied with the nucleobase used. (Rp)- and (Sp)-dithymidine boranophosphates were synthesized with excellent diastereoselectivity both in solution and on a solid-support, whereas a loss of diastereopurity was observed for the 2'-deoxycytidine derivative having an unprotected nucleobase amino group. On the other hand, complete chemoselectivity of the 3'-oxazaphospholidine derivatives toward hydroxy groups over amino groups was serendipitously found during the study. This unique chemoselectivity of the 3'-oxazaphospholidine derivatives was investigated by comparing them with the conventional nucleoside 3'-phosphoramidite.

Synthesis of 5-(1-propynyl)-2'-deoxyuridine 5'-(alpha-P-borano)triphosphate and kinetic characterization as a substrate for mmlv reverse transcriptase

In order to introduce pyrimidine C5-propynyl modification into boranophosphate oligodeoxyribonucleotides (BP- ODNs), 5-(1-propynyl)-2'-deoxyuridine 5'-(alpha-P-borano) triphosphate (d5PUTPalphaB) was synthesized. The two diastereomers were separated by reverse-phase HPLC. Kinetic studies showed that the Rp isomer was a slightly better substrate for MMLV reverse transcriptase than thymidine triphosphate or Rp-thymidine 5'-(alpha-P-borano)triphosphate. Using the Rp isomers of d5PUTPalphaB and the other three 5'-(alpha-P-borano) triphosphates, a DNA primer could be extended to the full length of the template.

Kinetic analysis of an efficient DNA-dependent TNA polymerase

α-l-Threofuranosyl nucleoside triphosphates (tNTPs) are tetrafuranose nucleoside derivatives and potential progenitors of present-day β-d-2‘-deoxyribofuranosyl nucleoside triphosphates (dNTPs). Therminator DNA polymerase, a variant of the 9°N DNA polymerase, is an efficient DNA-directed threosyl nucleic acid (TNA) polymerase. Here we report a detailed kinetic comparison of Therminator-catalyzed TNA and DNA syntheses. We examined the rate of single-nucleotide incorporation for all four tNTPs and dNTPs from a DNA primer−template complex and carried out parallel experiments with a chimeric DNA−TNA primer−DNA template containing five TNA residues at the primer 3‘-terminus. Remarkably, no drop in the rate of TNA incorporation was observed in comparing the DNA−TNA primer to the all-DNA primer, suggesting that few primer-enzyme contacts are lost with a TNA primer. Moreover, comparison of the catalytic efficiency of TNA synthesis relative to DNA synthesis at the downstream positions reveals a difference of no greater than 5-fold in favor of the natural DNA substrate. This disparity becomes negligible when the TNA synthesis reaction mixture is supplemented with 1.25 mM MnCl₂. These results indicate that Therminator DNA polymerase can recognize both a TNA primer and tNTP substrates and is an effective catalyst of TNA polymerization despite changes in the geometry of the reactants.

Mechanistic insights into the suppression of drug resistance by human immunodeficiency virus type 1 reverse transcriptase using alpha-boranophosphate nucleoside analogs

A class of amino acid substitutions in drug-resistant HIV-1 reverse transcriptase (RT) is responsible for the selectively impaired incorporation of the nucleotide analog inhibitor into DNA. We have shown previously that alpha-boranophosphate nucleoside analogs suppress RT-mediated resistance when the catalytic rate is responsible for drug resistance such as in the case of K65R and dideoxy (dd)NTPs, and Q151M toward AZTTP and ddNTPs. Here, we extend this property to BH3-d4TTP and BH3-3TCTP toward their clinically relevant mutants Q151M and M184V, respectively. Pre-steady-state kinetics on mutants of the Q151M RT family reveal a 3-5-fold resistance to d4TTP. This resistance is suppressed using BH3-d4TTP. Likewise, resistance to 3TCTP by M184V RT (30-fold) and K65R/M184V RT (180-fold) is suppressed using BH3-3TCTP because of a 160-fold acceleration of the catalytic constant kpol. Mechanistic insights into the rate enhancement were obtained using various alpha-boranophosphate nucleotides. The presence of the BH3 group renders kpol independent of amino acid substitutions present in RT. Indeed, the approximately 100-fold decrease in polymerase activity caused by the R72A substitution is restored to wild-type levels using BH3-dTTP. Metal ion titration studies show that alpha-boranophosphate nucleoside analogs enhance 3-8-fold the binding of Mg2+ ions to the active site of the RT.DNA.dNTP complex and alleviate the requirement of critical amino acids involved in phosphodiester bond formation. To our knowledge, this is the first example of rescue of polymerase activity by means of a nucleotide analog.

Incorporation of (alpha-P-borano)-2',3'-dideoxycytidine 5'-triphosphate into DNA by drug-resistant MMLV reverse transcriptase and Taq DNA polymerase

The Rp-stereoisomer of 5'-(alpha-P-borano)triphosphates of 2'-deoxycytidine (Rp-dCTPalphaB) and 2',3'-dideoxycytidine (Rp-ddCTPalphaB) were synthesized. Their steady-state kinetics of incorporation by ddNTP-resistant enzymes, e.g., MMLV reverse transcriptase (RT) and Taq DNA polymerase, were investigated and compared with incorporation of dCTP and ddCTP. The alpha-boranophosphate substitution in ddCTP results in a 28-fold increase in efficiency of incorporation of the Rp-ddCTPalphaB isomer by MMLV RT, yet has minimal effect on the efficiency of incorporation by Taq DNA polymerase.

Synthesis of acyclothymidine triphosphate and alpha-P-boranotriphosphate and their substrate properties with retroviral reverse transcriptase

[reaction: see text] The first example of an acyclonucleoside alpha-P-boranotriphosphate has been synthesized via a phosphoramidite approach in a one-pot reaction with good yield. The presence of the alpha-P-BH(3) in 5b results in a 9-fold increase in efficiency of incorporation by MMLV retroviral reverse transcriptase relative to non-boronated 5a in pre-steady-state conditions. The preliminary results indicate that acyclonucleoside alpha-P-boranotriphosphates may have promising applications as a probe of enzyme mechanisms and in the design of new antiviral drugs.

3'-Exonuclease resistance of DNA oligodeoxynucleotides containing O6-[4-oxo-4-(3-pyridyl)butyl]guanine

Tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is a chemical carcinogen thought to be involved in the initiation of lung cancer in smokers. NNK is metabolically activated to methylating and pyridyloxobutylating species that form promutagenic adducts with DNA nucleobases, e.g. O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine (O(6)-POB-dG). O(6)-POB-dG is a strongly mispairing DNA lesion capable of inducing both G-->A and G-->T base changes, suggesting its importance in NNK mutagenesis and carcinogenesis. Our earlier investigations have identified the ability of O(6)-POB-dG to hinder DNA digestion by snake venom phosphodiesterase (SVPDE), a 3'-exonuclease commonly used for DNA ladder sequencing and as a model enzyme to test nuclease sensitivity of anti-sense oligonucleotide drugs. We now extend our investigation to three other enzymes possessing 3'-exonuclease activity: bacteriophage T4 DNA polymerase, Escherichia coli DNA polymerase I, and E.coli exonuclease III. Our results indicate that, unlike SVPDE, 3'-exonuclease activities of these three enzymes are not blocked by O(6)-POB-dG lesion. Conformational analysis and molecular dynamics simulations of DNA containing O(6)-POB-dG suggest that the observed resistance of the O(6)-POB-dG lesion to SVPDE-catalyzed hydrolysis may result from the structural changes in the DNA strand induced by the O(6)-POB group, including C3'-endo sugar puckering and the loss of stacking interaction between the pyridyloxobutylated guanine and its flanking bases. In contrast, O(6)-methylguanine lesion used as a control does not induce similar structural changes in DNA and does not prevent its digestion by SVPDE.

Introduction of the alpha-P-borano-group into deoxynucleoside triphosphates increases their selectivity to HIV-1 reverse transcriptase relative to DNA polymerases

A series of 2'-deoxynucleoside 5'-triphosphates (dNTPs) and their alpha-P-thio or alpha-P-borano analogues, i.e., (Sp-dNTPalphaS), (Rp-dNTPalphaB) and (Sp-dNTPalphaB) were studied as substrates for DNA dependent DNA polymerases and HIV-1 reverse transcriptase (RT). For HIV-1 RT the Rp-dNTPalphaB isomers are 1.2-fold better substrates than natural dNTPs. For DNA polymerases their efficiencies of incorporation are 3-fold (Klenow, Sequenase) and 5-fold (Taq) lower than for dNTPs. Thus, introduction of the alpha-boranophosphate group into dNTPs increases their selectivity to HIV-1 RT relative to bacterial DNA polymerases.

Boron-containing aptamers to ATP

Boron neutron capture therapy (BNCT), an experimental treatment for certain cancers, destroys only cells near the boron; however, there is a need to develop highly specific delivery agents. As nucleic acid aptamers recognize specific molecular targets, we investigated the influence of boronated nucleotide analogs on RNA function and on the systematic evolution of ligands by exponential enrichment (SELEX) process. Substitution of guanosine 5'-(alpha-P-borano) triphosphate (bG) for GTP or uridine 5'-(alpha-P-borano) triphosphate (bU) for UTP in several known aptamers diminished or eliminated target recognition by those RNAs. Specifically, ATP-binding aptamers containing the zeta-fold, which appears in several selections for adenosine aptamers, became inactive upon bG substitution but were only moderately affected by bU substitution. Selections were carried out using the bG or bU analogs with C8-linked ATP agarose as the binding target. The selections with bU and normal NTP yielded some zeta-fold aptamers, while the bG selection yielded none of this type. Non-zeta aptamers from bU and bG populations tolerated the borano substitution and many required it. The borano nucleotide requirement is specific; bU could not be used in bG-dependent aptamers nor vice versa. The borano group plays an essential role, as yet undefined, in target recognition or RNA structure. We conclude that the bG and bU nucleotides are fully compatible with SELEX, and that these analogs could be used to make boronated aptamers as therapeutics for BNCT.

A commented dictionary of techniques for genotyping

Several tools, differing in their technical and practical parameters, are available for the detection of point mutations as well as small deletions and insertions. In this article, a dictionary featuring over fifty methods for detection of mutation is presented. The distinguishing principle for each method is briefly explained. Sorting of and discussion on the methods give the reader a brief introduction to the field of genotyping.

Synthesis and separation of diastereomers of uridine 2',3'-cyclic boranophosphate

The first boron-containing 2',3'-cyclic phosphate-modified analogue, uridine 2',3'-cyclic boranophosphate (2',3'-cyclic-UMPB), was synthesized. 5'-O-Protected uridine was cyclophosphorylated by diphenyl H-phosphonate to yield uridine 2',3'-cyclic H-phosphonate, which upon silylation followed by boronation and subsequent acid treatment gave 2',3'-cyclic-UMPB in high yield. The two diastereomers of 2',3'-cyclic-UMPB were separated by HPLC. An alternative method for synthesis of uridine 2',3'-cyclic phosphorothioate (2',3'-cyclic-UMPS) via H-phosphonate was also described.

Synthetically modified DNAs as substrates for polymerases

DNA polymerase enzymes process their natural substrates with very high specificity. Yet recent experiments have shown that these enzymes can also process DNA in which the backbone or bases are modified to a surprising degree. Such experiments have important implications in understanding the mechanisms of DNA replication, and suggest important biotechnological uses as well.

Structural basis for activation of alpha-boranophosphate nucleotide analogues targeting drug-resistant reverse transcriptase

AIDS chemotherapy is limited by inadequate intracellular concentrations of the active triphosphate form of nucleoside analogues, leading to incomplete inhibition of viral replication and the appearance of drug-resistant virus. Drug activation by nucleoside diphosphate kinase and inhibition of HIV-1 reverse transcriptase were studied comparatively. We synthesized analogues with a borano (BH(3)(-)) group on the alpha-phosphate, and found that they are substrates for both enzymes. X-ray structures of complexes with nucleotide diphosphate kinase provided a structural basis for their activation. The complex with d4T triphosphate displayed an intramolecular CH.O bond contributing to catalysis, and the R(p) diastereoisomer of thymidine alpha-boranotriphosphate bound like a normal substrate. Using alpha-(R(p))-boranophosphate derivatives of the clinically relevant compounds AZT and d4T, the presence of the alpha-borano group improved both phosphorylation by nucleotide diphosphate kinase and inhibition of reverse transcription. Moreover, repair of blocked DNA chains by pyrophosphorolysis was reduced significantly in variant reverse transcriptases bearing substitutions found in drug-resistant viruses. Thus, the alpha-borano modification of analogues targeting reverse transcriptase may be of generic value in fighting viral drug resistance.