Photocatalytic and Free Radical Interactions of the HeterocyclicN-Oxide Resazurin with NADH, GSH, and Dopa

Electron donating free radicals NAD•,[formula]MV•+, and[formula]generated by pulse radiolysis, reduce resazurin (RNO) with rate constants of 1.9 × 109, 2.8 × 109, 4.8 × 109, and 2.3 × 1010M−1s−1, respectively, in neutral solution. The semireduced dye (RN•-O−) disproportionates slowly to RN (resorufin) and RNO. There was little evidence that RN•-O−behaves as an oxidizing species capable of initiating chain reactions, for instance via oxidation of NADH to NAD•. The oxidizing radicals GS•,•OH, and[formula]interact with RNO via complex consecutive processes, probably by addition–elimination reactions. Stable products generated upon oxidation of RNO by[formula]exhibit a red-shifted absorption, but GS•and•OH also cause partial reduction to RN. Neither[formula]nor dopa semiquinone nor tyrosine phenoxyl radicals appear to interact with RNO. Radicals formed by reaction of•OH with (Gly)3 reduce RNO to RN with a stoichiometry near two (γ-radiolysis), and there is evidence (pulse radiolysis) for direct slow O-atom transfer from RNO to these species. Resazurin is highly photosensitive under anaerobic conditions in presence of H-atom donors like NADH, GSH, or dopa. Under aerobic conditions RNO becomes an efficient catalyst of red light induced photooxidation of these donors; the RN•-O−intermediate, formed in the photooxidative process, is apparently recycled to RNO by O2, and by other electron acceptors. Our results suggest that RNO can behave as a photoactive, free radical generating xenobiotic compound.