Solvent exposure associated with single abasic sites alters the base sequence dependence of oxidation of guanine in DNA in GG sequence contexts

The effect of exposure of guanine in double‐stranded oligonucleotides to aqueous solvent during oxidation by one‐electron oxidants was investigated by introducing single synthetic tetrahydrofuran‐type abasic sites (Ab) either adjacent to or opposite tandem GG sequences. The selective oxidation of guanine was initiated by photoexcitation of the aromatic sensitizers riboflavin and a pyrene derivative, and by the relatively small negatively charged carbonate radical anion. The relative rates of oxidation of the 5′‐ and 3′ side G in runs of 5′???GG??? (evaluated by standard hot alkali treatment of the damaged DNA strand followed by high resolution gel electrophoresis of the cleavage fragments) are markedly affected by adjacent abasic sites either on the same or opposite strand. For example, in fully double‐stranded DNA or one with an Ab adjacent to the 5′‐G, the 5′‐G/3′‐G damage ratio is ≥4, but is inverted (<1.0) with the Ab adjacent to the 3′‐G. These striking effects of Ab are attributed to the preferential localization of the “hole” on the most solvent‐exposed guanine regardless of the size, charge, or reduction potential of the oxidizing species.