Formation of 8-oxo-7,8-dihydroguanine-radicals in gamma-irradiated DNA by multiple one-electron oxidations.

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 beta-proton coupling of 17-28 G and an anisotropic nitrogen coupling, A(parallel), of approximately 20 G, A(perpendicular) = 0 with g(parallel) = 2.0026 and g(perpendicular) = 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 17O 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, Tl3+, demonstrates that the cationic pathway leads to the formation of the 8-oxo-G*+. In irradiated DNA-Tl3+ samples, Tl3+ captures electrons. Tl2+ 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.