- Independent Generation of Reactive Intermediates Leads to an Alternative Mechanism for Strand Damage Induced by Hole Transfer in Poly(dA-T) Sequences
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Purine radical cations (dA?+ and dG?+) are the primary hole carriers of DNA hole migration due to their favorable oxidation potential. Much less is known about the reactivity of higher energy pyrimidine radical cations. The thymidine radical cation (T?+) was produced at a defined position in DNA from a photochemical precursor for the first time. T?+ initiates hole transfer to dGGG triplets in DNA. Hole localization in a dGGG sequence accounts for ~26% of T?+ formed under aerobic conditions in 9. Reduction to yield thymidine is also quantified. 5-Formyl-2′-deoxyuridine is formed in low yield in DNA when T?+ is independently generated. This is inconsistent with mechanistic proposals concerning product formation from electron transfer in poly(dA-T) sequences, following hole injection by a photoexcited anthraquinone. Additional evidence that is inconsistent with the original mechanism was obtained using hole injection by a photoexcited anthraquinone in DNA. Instead of requiring the intermediacy of T?+, the strand damage patterns observed in those studies, in which thymidine is oxidized, are reproduced by independent generation of 2′-deoxyadenosin-N6-yl radical (dA?). Tandem lesion formation by dA? provides the basis for an alternative mechanism for thymidine oxidation ascribed to hole migration in poly(dA-T) sequences. Overall, these experiments indicate that the final products formed following DNA hole transfer in poly(dA-T) sequences do not result from deprotonation or hydration of T?+, but rather from deprotonation of the more stable dA?+, to form dA?, which produces tandem lesions in which 5′-flanking thymidines are oxidized.
- Sun, Huabing,Zheng, Liwei,Greenberg, Marc M.
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p. 11308 - 11316
(2018/09/13)
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- Independent Generation and Reactivity of Thymidine Radical Cations
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Thymidine radical cation (1) is produced by ionizing radiation and has been invoked as an intermediate in electron transfer in DNA. Previous studies on its structure and reactivity have utilized thymidine as a precursor, which limits quantitative product analysis because thymidine is readily reformed from 1. In this investigation, radical cation 1 is independently generated via β-heterolysis of a pyrimidine radical generated photochemically from an aryl sulfide. Thymidine is the major product (33%) from 1 at pH 7.2. Diastereomeric mixtures of thymidine glycol and the corresponding 5-hydroxperoxides resulting from water trapping of 1 are formed. Significantly lower yields of products such as 5-formyl-2′-deoxyuridine that are ascribable to deprotonation from the C5-methyl group of 1 are observed. Independent generation of the N3-methyl analogue of 1 (NMe-1) produces considerably higher yields of products derived from water trapping, and these products are formed in much higher yields than those attributable to the C5-methyl group deprotonation in NMe-1. N3-Methyl-thymidine is, however, the major product and is produced in as high as 70% yield when the radical cation is produced in the presence of excess thiol. The effects of exogenous reagents on product distributions are consistent with the formation of diffusively free radical cations (1, NMe-1). This method should be compatible with producing radical cations at defined positions within DNA.
- Sun, Huabing,Taverna Porro, Marisa L.,Greenberg, Marc M.
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p. 11072 - 11083
(2017/10/27)
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- Chemoenzymatic synthesis of 3′-O-acetal-protected 2′-deoxynucleosides as building blocks for nucleic acid chemistry
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We have developed a simple and convenient synthetic strategy for the preparation of tetrahydropyranyl, 4-methoxy-tetrahydropyranyl, and tetrahydrofuranyl ethers of 2′-deoxynucleosides, which are useful building blocks for nucleic acid chemistry. Enzymatic benzoylation provides an efficient alternative for protecting the 5′-hydroxy group of the parent nucleosides in a regioselective manner. Subsequently, tetrahydropyranylation and tetrahydrofuranylation of the 2′-deoxynucleosides at the 3′-hydroxy group were accomplished with p-toluensulfonic acid, MgBr2, or camphorsulfonic acid as catalysts. Deprotection of the 5′-O-benzoyl group furnished 3′-O-acetal-protected 2′-deoxynucleosides. The three-step process is expected to enable the large-scale synthesis of protected nucleosides.
- Rodriguez-Perez, Tatiana,Fernandez, Susana,Martinez-Montero, Saul,Gonzalez-Garcia, Tania,Sanghvi, Yogesh S.,Gotor, Vicente,Ferrero, Miguel
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experimental part
p. 1736 - 1744
(2010/06/13)
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- A mild, efficient and regioselective enzymatic procedure for 5′-O-benzoylation of 2′-deoxynucleosides
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Lipase from Candida antarctica B catalyzes the selective monobenzoylation at the 5′-hydroxyl group of 2′-deoxynucleosides using vinyl benzoate as acyl transfer reagent in quantitative yields. The industrial suitability of this process via the reclaim and reuse of enzyme and vinyl benzoate has been demonstrated.
- García, Javier,Fernández, Susana,Ferrero, Miguel,Sanghvi, Yogesh S.,Gotor, Vicente
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p. 1709 - 1712
(2007/10/03)
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- Direct measurement of pyrimidine C6-hydrate stability
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Pyrimidine C6-hydrates are produced via UV-irradiation and undergo dehydration upon standing. The stability of these compounds has a direct bearing on their genotoxicity. The rate constants for elimination from 5' -benzyoylated derivatives of 5,6-dihydro- 5-hydroxythymidine (6) and 5,6-dihydro-5-hydroxy-2' -deoxyuridine (9) were measured directly via HPLC. The rate constants for dehydration increase from pH 6.0 to 8.0. The half-lives for 6 and 9 at pH 7.4 and 37°C are 46.5 and 24.4 h, respectively. Deglycosylation is not observed, even upon heating at 90°C. These observations reinforce proposals that pyrimidine hydrates are sufficiently long-lived that they can exert significant effects on biological systems. Copyright
- Nolan Carter,Greenberg, Marc M.
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p. 2341 - 2346
(2007/10/03)
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