Formation of S-Cl phosphorothioate adduct radicals in dsDNA S-oligomers: hole transfer to guanine vs disulfide anion radical formation.

In phosphorothioate-containing dsDNA oligomers (S-oligomers), one of the two nonbridging oxygen atoms in the phosphate moiety of the sugar-phosphate backbone is replaced by sulfur. In this work, electron spin resonance (ESR) studies of one-electron oxidation of several S-oligomers by Cl2(•-) at low temperatures are performed. Electrophilic addition of Cl2(•-) to phosphorothioate with elimination of Cl(-) leads to the formation of a two-center three-electron σ(2)σ*(1)-bonded adduct radical (-P-S-̇Cl). In AT S-oligomers with multiple phosphorothioates, i.e., d[ATATAsTsAsT]2, -P-S-̇Cl reacts with a neighboring phosphorothioate to form the σ(2)σ*(1)-bonded disulfide anion radical ([-P-S-̇S-P-](-)). With AT S-oligomers with a single phosphorothioate, i.e., d[ATTTAsAAT]2, reduced levels of conversion of -P-S-̇Cl to [-P-S-̇S-P-](-) are found. For guanine-containing S-oligomers containing one phosphorothioate, -P-S-̇Cl results in one-electron oxidation of guanine base but not of A, C, or T, thereby leading to selective hole transfer to G. The redox potential of -P-S-̇Cl is thus higher than that of G but is lower than those of A, C, and T. Spectral assignments to -P-S-̇Cl and [-P-S-̇S-P-](-) are based on reaction of Cl2(•-) with the model compound diisopropyl phosphorothioate. The results found for d[TGCGsCsGCGCA]2 suggest that [-P-S-̇S-P-](-) undergoes electron transfer to the one-electron-oxidized G, healing the base but producing a cyclic disulfide-bonded backbone with a substantial bond strength (50 kcal/mol). Formation of -P-S-̇Cl and its conversion to [-P-S-̇S-P-](-) are found to be unaffected by O2, and this is supported by the theoretically calculated electron affinities and reduction potentials of [-P-S-S-P-] and O2.