Formation of S-CI Phosphorothioate Adduct Radicals in dsDNA S-Oligomers: Hole Transfer to Guanine vs Disulfide Anion Radical Formation

摘要

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 Cl_2~(·-) at low temperatures are performed. Electrophilic addition of Cl_2~(·-) to phosphorothioate with elimination of Cl~- leads to the formation of a two-center three-electron σ~2σ~(*1)-bonded adduct radical (-P-S-C1). In AT S-oligomers with multiple phosphorothioates, i.e., d[ATATAsTsAsT]_2 -P-S-Cl reacts with a neighboring phosphorothioate to form the σ~2σ~(*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 Cl_2~(·-) 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 O_2, and this is supported by the theoretically calculated electron affinities and reduction potentials of [-P-S-S-P-] and O_2.