Influence of the local helical conformation on the guanine modifications generated from one-electron DNA oxidation.

Two major products, 2,2-diamino-4-[(2-deoxy-beta-D-erythro-pentafuranosyl)amino]-5-(2H )-oxazolone and its imidazole derivative have been generated from one-electron oxidation of the free 2'-deoxyguanosine. The formation of 7,8-dihydro-8-oxoguanine (8-oxodG), not detected in this case, has been observed from DNA exposed to oxidizing agents. Since these compounds are thought to reflect, respectively, either deprotonation or hydration of the transient guanyl radical cation, these findings suggested that the helical structure could influence the chemical decomposition pathway of the guanine moiety. In the present study, we have photoionized DNA sequences by exposure to high-intensity UV (266 nm) laser pulses. Homo- or heteroduplexes, including guanines in various environments as well as Gn runs, were used as templates. Lesions were analyzed, at the nucleotide level, by taking advantage of the specific removal of 8-oxodG from DNA by the formamidopyrimidine DNA glycosylase (Fpg protein) and of the differential sensitivity of 8-oxodG and oxazolone to piperidine. Variations were observed in the relative yield of each type of lesion at individual guanines of the DNA sequences. We found that the Fpg lesions predominate in regions of stable double helix but are decreased in favor of the piperidine ones in regions of destabilization of the helix. Results are discussed in terms of a relationship between intramolecular rearrangements of the guanyl radical cation and the DNA helical conformation and dynamics.