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, isopropyloxyl, tert -butoxyl, and benzoyloxyl) radicals, which damage DNA through strand cleavage and guanine oxidation. Unexpectedly, the N -hydroxy derivative 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 pyridinethiones. Detailed mechanistic investigations have revealed that the guanine oxidation by the pyridinethiones 1b − d is not caused by oxyl radicals but by a type-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 guanine oxidation is effectively inhibited by the N, N ‘-dioxide 7, which is produced only in the photolysis of the N -hydroxypyridine-2-thione ( 1a ) and not from 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 -alkoxy and N -benzoyloxy derivatives 1b − d are more effective DNA-photooxidizing reagents than the N -hydroxypyridinethione 1a, because 1b − d oxidize DNA photocatalytically through sensitization by the betain 8, while 1a oxidizes DNA mainly through the stoichiometrically photogenerated hydroxyl radicals.