Published online 15 December 2004
Nucleic Acids Research, Vol. 32 No. 22 © Oxford University Press 2004; all rights reserved
Formation of 8-oxo-7,8-dihydroguanine-radicals in 
-irradiated DNA by multiple one-electron oxidations
1 Department of Chemistry, Oakland University, Rochester, MI 38309, USA and 2 Department of Chemistry, Rajdhani College, University of Delhi, Raja Garden, Delhi 110 015, India
* To whom correspondence should be addressed. Tel: +1 248 370 2328; Fax: +1 248 370 2321; Email: sevilla{at}oakland.edu
Received August 10, 2004; Revised November 2, 2004; Accepted November 23, 2004
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 ß-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 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.
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