Sulfur-centered hemi-bond radicals as active intermediates in S-DNA phosphorothioate oxidation

Phosphorothioate (PS) modifications naturally appear in bacteria and archaea genome and are widely used as antisense strategy in gene therapy. But the chemical effects of PS introduction as a redox active site into DNA (S-DNA) is still poorly understood. Herein, we perform time-resolved spectroscopy to examine the underlying mechanisms and dynamics of the PS oxidation by potent radicals in free model, in dinucleotide, and in S-oligomer. The crucial sulphur-centered hemi-bonded intermediates -P–S∴S–P- were observed and found to play critical roles leading to the stable adducts of -P–S–S–P-, which are backbone DNA lesion products. Moreover, the oxidation of the PS moiety in dinucleotides d[G(PS)G], d[A(PS)A], d[G(PS)A], d[A(PS)G] and in S-oligomers was monitored in real-time, showing that PS oxidation can compete with adenine but not with guanine. Significantly, hole transfer process from A(+•) to PS and concomitant -P–S∴S–P- formation was observed, demonstrating the base-to-backbone hole transfer unique to S-DNA, which is different from the normally adopted backbone-to-base hole transfer in native DNA. These findings reveal the distinct backbone lesion pathway brought by the PS modification and also imply an alternative -P–S∴S–P-/-P–S–S–P- pathway accounting for the interesting protective role of PS as an oxidation sacrifice in bacterial genome.