Are pyridinethiones reliable photochemical oxyl-radical sources for photobiological studies? The importance of secondary photolysis products in the guanine oxidation of 2 '-deoxyguanosine and cell-free DNA

The photolysis of the pyridinethiones 1a-d releases oxyl (hydroxyl, isoprop yloxyl, tert-butoxyl, and benzoyloxyl) radicals, which damage DNA through s trand cleavage and guanine oxidation. Unexpectedly, the N-hydroxy derivativ e 1a is significantly less effective in the oxidation of cell-free DNA than the alkoxy and benzoyloxy ones 1b-d. A similar trend was observed for the photooxidative consumption of 2'-deoxyguanosine (dG) by these pyridinethion es. Detailed mechanistic investigations have revealed that the guanine oxid ation by the pyridinethiones 1b-d is not caused by oxyl radicals but by a t ype-I photooxidation process through the novel betain intermediate 8, which is formed in small amounts (ca. 5%) during the photolysis of these thiones from the intermediary photoproduct disulfide 3. This photosensitized guani ne oxidation is effectively inhibited by the N,N'-dioxide 7, which is produ ced only in the photolysis of the N-hydroxypyridine-2-thione (1a) and not f rom the N-alkoxy and N-benzoyloxy derivatives 1b-d. Thus, for the N-hydroxy derivative 1a hydroxyl radicals are the main DNA-damaging species. The N-a lkoxy and N-benzoyloxy derivatives 1b-d are more effective DNA-photooxidizi ng reagents than the N-hydroxypyridinethione 1a, because 1b-d oxidize DNA p hotocatalytically through sensitization by the betain, while 1a oxidizes DN A mainly through the stoichiometrically photogenerated hydroxyl radicals.