Biomimetic Studies on the Reactivity of Sulfur-Centered Radicals with Purine Moieties of DNA

The reaction of the HS^•/S^•- radical (pKa ~3.4), generated selectively from H₂S by γ-irradiated N₂-flushed aqueous solutions at pH 5, with purine nucleosides (dG or dA), a 10-mer double-stranded oligodeoxynucleotide (ds-ODNs), and calf thymus (ct) DNA was investigated, under various experimental conditions. Concurrent quantification of the four purine 5',8-cyclo-2'-deoxynucleosides (cPu) and two 8-oxo-7,8-dihydro-2'-deoxypurines (8-oxo-Pu) by LC-MS/MS analysis using isotopomeric internal standards was achieved. The formation of 8-oxo-Pu is several tens of times larger than cPu. Mechanistic schemes for the formation of the two product groups are proposed. Hydrogen atom abstraction from C5'-H by S^•- produces the cPu via cyclization of the C5' radical onto C8, forming a new covalent bond, C5'-C8. The unexpected formation of 8-oxo-Pu should be mechanistically more complex. We propose that an S^•- (coupled with H⁺) adds to the base rings, followed by the elimination of HS^- to form the corresponding radical cation; subsequent reactions with H₂O and radical disproportionation with another S^•- lead to 8-oxo-Pu. A comparison of S^•- with the available literature data for HO^• reactivity towards ct-DNA in de-oxygenated aqueous solutions is also presented. Before the present findings, cPu lesions were attributed exclusively to HO^• reactivity toward ct-DNA. The reaction of the thiyl radical (HOCH₂CH₂S^•) with ct-DNA was also investigated, yielding results similar to those of S^•- obtained under comparable experimental conditions. Our results contributed to a better understanding of DNA damage induced by reactive sulfur species (RSS), particularly the formation of purine lesions and the relative abundance of cPu versus 8-oxo-Pu.