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 O(2) 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 O(2) concentration, the 5',8-cyclo-2'-deoxyguanosine being produced more efficiently than the 5',8-cyclo-2'-deoxyadenosine for low O(2) 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 O(2) 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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