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.