Formation of Aminyl Radicals on Electron Attachment to AZT: Abstraction from the Sugar Phosphate Backbone versus One-Electron Oxidation of Guanine
Formation of Aminyl Radicals on Electron Attachment to AZT: Abstraction from the Sugar Phosphate Backbone versus One-Electron Oxidation of Guanine
复制标题
DOI：
10.1021/jp103403p
复制
发表时间：
2010-07-22
影响因子：
3.3
通讯作者：
Sevilla, Michael D.
中科院分区：
文献类型：
--
作者：
Adhikary, Amitava;Khanduri, Deepti;Sevilla, Michael D.
利用电子自旋共振（ESR）谱，我们对3 '-叠氮基-3'-脱氧胸苷（3 '-AZT）及其类似物5'-叠氮基-5 '-脱氧胸苷（5'-AZT）在γ射线辐照的水溶液（H2O或D2 O）玻璃态（7.5M LiCl）体系中电子附着后形成的自由基进行了表征。ESR谱研究和理论计算表明，3 '-AZT和5'-AZT的主要电子捕获位点是叠氮基，而不是胸腺嘧啶部分。因此，叠氮胸苷中的叠氮基比最亲电子的DNA碱基胸腺嘧啶更亲电子。电子附着在3 '-AZT和5'-AZT上产生不稳定的叠氮阴离子自由基中间体（RN 3中心点（-）），即使在77 K下，该中间体的寿命也太短，无法在我们的工作中观察到。在77 K下，我们观察到中性胺基自由基（RNH中心点）在从RN 3中心点（-）失去N-2之后，接着氮烯阴离子自由基（RN中心点（-））质子化，得到RNH中心点。没有观察到预期的RN中心点（-）中间体，因为即使在高碱性条件下，水的质子化也在77 K下完成。D2 O溶液中RND中心点的形成证实了水是RNH中心点中NH质子的来源。我们的任务，这些自由基的DFT计算的超精细耦合常数，密切匹配的实验值。在退火到更高的温度（约。160-170 K），RNH。从胸腺嘧啶甲基和糖部分进行双分子夺氢反应，导致形成胸腺嘧啶烯丙基（UCH 2中心点）和两个糖基，C3'中心点和C5'中心点。RNH中心点还导致3 '-AZG中鸟嘌呤碱基的单电子氧化。这项工作提供了一个潜在的机制，报告的放射增敏作用的AZT。
Employing electron spin resonance (ESR) spectroscopy, we have characterized the radicals formed in 3'-azido-3'-deoxythymidine (3'-AZT) and in its 5'-analog 5'-azido-5'-deoxythymidine (5'-AZT) after electron attachment in gamma-irradiated aqueous (H2O or D2O) glassy (7.5 M LiCl) systems. ESR spectral studies and theoretical calculations show that the predominant site of electron capture in 3'-AZT and in 5'-AZT is at the azide group and not at the thymine moiety. The azide group in AZT is therefore more electron affinic than the most electron affinic DNA base, thymine. Electron attachment to 3'-AZT and 5'-AZT results in an unstable azide anion radical intermediate (RN3 center dot(-)) that is too short-lived to be observed in our work even at 77 K. At 77 K, we observe the neutral aminyl radical (RNH center dot) after loss of N-2 from RN3 center dot(-) followed by protonation of nitrene anion radical (RN center dot(-)) to give RNH center dot. The expected RN center dot(-) intermediate is not observed as protonation from water is complete at 77 K even under highly basic conditions. Formation of RND center dot in D2O solutions confirms water as the source of the NH proton in the RNH center dot. Our assignments to these radicals are aided by DFT calculations for hyperfine coupling constants that closely match the experimental values. On annealing to higher temperatures (ca. 160-170 K), RNH. undergoes bimolecular hydrogen abstraction reactions from the thymine methyl group and the sugar moiety resulting in the formation of the thymine allyl radical (UCH2 center dot) and two sugar radicals, C3'center dot and C5'center dot. RNH center dot also results in one-electron oxidation of the guanine base in 3'-AZG. This work provides a potential mechanism for the reported radiosensitization effects of AZT.