Photocatalytic and free radical interactions of the heterocyclic N-oxide resazurin with NADH, GSH, and Dopa

Electron donating free radicals NAD(.), (.)CO2(-), MV(.)+, and e(aq)-, generated by pulse radiolysis, reduce resazurin (RNO) with rate constants of 1.9 x 10(9), 2.8 x 10(9), 4.8 x 10(9), and 2.3 x 10(10) M(-1) s(-1), respectively, 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 N3(.) interact with RNO via complex consecutive processes, probably by addition-elimination reactions. Stable products generated upon oxidation of RNO by N3(.) exhibit a red-shifted absorption, but GS(.) and (.)OH also cause partial reduction to RN. Neither O2(.)- 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 stoichiometry near two (gamma-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.