Chemistry Department, New York University, 31 Washington Place, New York, New York 10003-5180, United States.
J Am Chem Soc. 2012 Mar 14;134(10):4955-62. doi: 10.1021/ja212186w. Epub 2012 Feb 29.
The exposure of guanine in the oligonucleotide 5'-d(TCGCT) to one-electron oxidants leads initially to the formation of the guanine radical cation G(•+), its deptotonation product G(-H)(•), and, ultimately, various two- and four-electron oxidation products via pathways that depend on the oxidants and reaction conditions. We utilized single or successive multiple laser pulses (308 nm, 1 Hz rate) to generate the oxidants CO(3)(•-) and SO(4)(•-) (via the photolysis of S(2)O(8)(2-) in aqueous solutions in the presence and absence of bicarbonate, respectively) at concentrations/pulse that were ∼20-fold lower than the concentration of 5'-d(TCGCT). Time-resolved absorption spectroscopy measurements following single-pulse excitation show that the G(•+) radical (pK(a) = 3.9) can be observed only at low pH and is hydrated within 3 ms at pH 2.5, thus forming the two-electron oxidation product 8-oxo-7,8-dihydroguanosine (8-oxoG). At neutral pH, and single pulse excitation, the principal reactive intermediate is G(-H)(•), which, at best, reacts only slowly with H(2)O and lives for ∼70 ms in the absence of oxidants/other radicals to form base sequence-dependent intrastrand cross-links via the nucleophilic addition of N3-thymidine to C8-guanine (5'-GCT and 5'-TCG). Alternatively, G(-H)(•) can be oxidized further by reaction with CO(3)(•-), generating the two-electron oxidation products 8-oxoG (C8 addition) and 5-carboxamido-5-formamido-2-iminohydantoin (2Ih, by C5 addition). The four-electron oxidation products, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp), appear only after a second (or more) laser pulse. The levels of all products, except 8-oxoG, which remains at a low constant value, increase with the number of laser pulses.
寡核苷酸 5'-d(TCGCT) 中的鸟嘌呤暴露于单电子氧化剂最初会导致鸟嘌呤自由基阳离子 G(•+)、其质子化产物 G(-H)(•+)以及最终通过依赖于氧化剂和反应条件的途径形成各种二价和四价氧化产物的形成。我们利用单个或连续多个激光脉冲（308nm，1Hz 速率）在存在和不存在碳酸氢盐的情况下，分别通过水溶液中 S(2)O(8)(2-) 的光解来生成氧化剂 CO(3)(•-) 和 SO(4)(•-)（浓度/脉冲比 5'-d(TCGCT) 的浓度低约 20 倍）。单脉冲激发后进行的时间分辨吸收光谱测量表明，G(•+)自由基（pK(a) = 3.9）只能在低 pH 值下观察到，并且在 pH 2.5 下在 3ms 内水合，从而形成二价氧化产物 8-氧代-7,8-二氢鸟苷（8-氧代 G）。在中性 pH 值和单脉冲激发下，主要的反应性中间物是 G(-H)(•)，它在最佳情况下仅与 H(2)O 缓慢反应，并且在没有氧化剂/其他自由基的情况下，通过 N3-胸腺嘧啶与 C8-鸟嘌呤的亲核加成，形成碱基序列依赖性的链内交联，从而形成碱基序列依赖性的链内交联（5'-GCT和 5'-TCG）。或者，G(-H)(•)可以通过与 CO(3)(•-)反应进一步被氧化，生成二价氧化产物 8-氧代 G（C8 加成）和 5-羧酰胺基-5-甲酰胺基-2-亚氨基海因（2Ih，通过 C5 加成）。四价氧化产物胍基海因（Gh）和螺亚氨基二氢嘧啶（Sp）仅在第二个（或更多）激光脉冲后才出现。除了保持低恒定值的 8-氧代 G 之外，所有产物的水平都随着激光脉冲数的增加而增加。
