Lifetimes and Reaction Pathways of Guanine Radical Cations and Neutral Guanine Radicals in an Oligonucleotide in Aqueous Solutions

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• 作者：Yekaterina Rokhlenko ; Nicholas E. Geacintov ; Vladimir Shafirovich
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：March 14, 2012
• 年：2012
• 卷：134
• 期：10
• 页码：4955-4962
• 全文大小：434K
• 年卷期：v.134,no.10(March 14, 2012)
• ISSN：1520-5126

文摘

The exposure of guanine in the oligonucleotide 5鈥?d(TCGCT) to one-electron oxidants leads initially to the formation of the guanine radical cation G^鈥?, its deptotonation product G(-H)^{鈥?/sup>, 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 CO₃鈥⑩€?/sup> and SO₄鈥⑩€?/sup> (via the photolysis of S₂O₈2鈥?/sup> in aqueous solutions in the presence and absence of bicarbonate, respectively) at concentrations/pulse that were 20-fold lower than the concentration of 5鈥?d(TCGCT). Time-resolved absorption spectroscopy measurements following single-pulse excitation show that the G鈥? 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)鈥?/sup>, which, at best, reacts only slowly with H₂O 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鈥?G*CT* and 5鈥?T*CG*). Alternatively, G(-H)鈥?/sup> can be oxidized further by reaction with CO₃鈥⑩€?/sup>, 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.}