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

Abstract

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.

Radiation-Induced Hydroxyl Addition to Purine Molecules: EPR and ENDOR Study of Hypoxanthine Hydrochloride Monohydrate Single Crystals

“…(Unfortunately, the presence of overlapping spectra in the Hx system prevented direct observation of the transformation during the controlled warming experiments.) This is like the behavior described by Shukla et al in solid DNA (15) and the behavior proposed by Reynisson and Steenken in a density functional theory-based computational examination of probable reactions between water and oxidized guanine or adenine (21). The latter group also found that calculations on a structure with H 2 O added to the C8 position of • Gua ϩ simply led to separation of the water from the • Gua ϩ .…”

“…In addition to reproducing the coupling values well for RIII, the computations based on the geometry-optimized Structure IV also predicted the dipolar vector for ␤-coupling RIII 2 paralleling that of the experimental result (in Table 3) within 3Њ. As well, a recent report on the • GOH species (with no evidence for protonation at N7) in DNA found the ␤-HC8 coupling to cover the range 17-28 G (ϳ48-78 MHz) (15), which includes the value reported here for Hx and previously for Gd. In summary, therefore, the body of evidence clearly indicates that RIII has the chemical form shown as Structure IV.…”

Increased ROS generation in subsets of OGG1 knockout fibroblast cells

“…Among DNA bases, guanine has the lowest redox potential, and so it is easily oxidized to 8-oxo-7,8-dihydroguanine (8-oxoG) via two-electron oxidation (Shukla et al, 2004). The key repair enzyme that excises 8-oxoG from cellular DNA (8-oxoG: C base pairs) is the 8-oxoguanine DNA glycosylase (OGG1) (Hazra et al, 1998).…”

Guanine Oxidation by Electron Transfer: One‐ versus Two‐Electron Oxidation Mechanism

“…[24][25][26] The proposed mechanism consists of attack of H 2 O at the C8 position of GC + (followed by deprotonation of the adduct). This leads to a neutral radical (GOH)C as an intermediate product [27] that is prone to further oxidation. A one-electron abstraction from (GOH)C leads to a nonradical cationic intermediate, (GOH) + , which transforms into an 8-oxoG residue after the loss of a second proton.…”