Electron spin resonance (ESR) studies of radicals formed by radiation-induced multiple one-electron oxidations of guanine moieties in DNA are reported in this work. Annealing of gamma-irradiated DNA from 77 to 235 K results in the hydration of one electron oxidized guanine (G •+ ) to form the 8-hydroxy-7,8-dihydroguanin-7-yl-radical (•GOH) having one β-proton coupling of 17–28 G and an anisotropic nitrogen coupling, A‖ , of ∼20 G, A⊥ = 0 with g‖ = 2.0026 and g⊥ = 2.0037. Further annealing to 258 K results in the formation of a sharp singlet at g = 2.0048 with line-width of 5.3 G that is identified as the 8-oxo-7,8-dihydroguanine one-electron-oxidized radical (8-oxo-G •+ ). This species is formed via two one-electron oxidations of •GOH. These two one-electron oxidation steps leading to the formation of 8-oxo-G •+ from •GOH in DNA, are in accordance with the expected ease of oxidation of •GOH and 8-oxo-G. The incorporation of oxygen from water in G •+ leading to •GOH and to 8-oxo-G •+ is verified by ESR studies employing 17 O isotopically enriched water, which provide unambiguous evidence for the formation of both radicals. ESR analysis of irradiated-DNA in the presence of the electron scavenger, Tl 3+ , demonstrates that the cationic pathway leads to the formation of the 8-oxo-G •+ . In irradiated DNA–Tl 3+ samples, Tl 3+ captures electrons. Tl 2+ thus produced is a strong oxidant (2.2 V), which is metastable at 77 K and is observed to increase the formation of G •+ and subsequently of 8-oxo-G •+ upon annealing. We find that in the absence of the electron scavenger the yield of 8-oxo-G •+ is substantially reduced as a result of electron recombinations with G •+ and possible reaction with •GOH.
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Lata I. Shukla (2004) studied this question.