The present work is aimed at gaining conclusive mechanistic insights into the radiation-induced formation of the 5′R and 5′S diastereomers of both adenine and guanine 5′,8-cyclo-2′-deoxyribonucleosides, with emphasis on the delineation of the inhibitory effect of O2 in isolated and cellular DNA. The levels of purine 5′,8-cyclo-2′-deoxyribonucleosides as assessed by HPLC-MS/MS were found to decrease steadily with the increase of O2 concentration, the 5′,8-cyclo-2′-deoxyguanosine being produced more efficiently than the 5′,8-cyclo-2′-deoxyadenosine for low O2 concentrations. A high stereoselectivity was observed in the intramolecular addition of the C5′ radical to the C8 of the purine leading, after the creation of the C5′–C8 bond and a subsequent oxidation step, to the predominant formation of the 5′R diastereomer for both purine 5′,8-cyclonucleosides. The reduced formation yield of the 4 tandem lesions in the presence of O2 explains, at least partly, the low efficiency of radiation-induced yields of the purine 5′,8-cyclo-2′-deoxyribonucleosides in cellular DNA, which are about two orders of magnitude lower than the previously reported data obtained from HPLC-MS analysis.
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