Formation of S-Cl 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 non-bridging oxygen atoms in the phosphate moiety of sugar-phosphate backbone is replaced by sulphur. In this work, electron spin resonance (ESR) studies of one-electron oxidation of several S-oligos by Cl2•⁻ at low temperatures are investigated. Electrophilic addition of Cl2•⁻ to phosphorothioate with elimination of Cl⁻ leads to the formation of a 2-center three-electron σ²σ*¹ bonded adduct radical (-P-S∸Cl). In AT S-oligomers with mutiple phosphorothioates, i.e., d[ATATAsTsAsT]2, -P-S∸Cl reacts with a neighboring phosphorothioate to form the σ²σ*¹ bonded disulphide 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 dsDNA [-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-]⁻ is 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.