Interaction of nitric oxide with photoexcited Rose Bengal: Evidence for one-electron reduction of nitric oxide to nitroxyl anion

The interaction of nitric oxide ((NO)-N-.) with Rose Bengal (RE) in the presence of electron donors was investigated, Upon illumination of a mixture of RE and (NO)-N-. with visible light, an enhancement in the rate of (NO)-N-. consumption was observed that increased with increasing RE concentration, In the presence of electron donors (NADH, glutathione, or ascorbate), the rates of (NO)-N-. depletion increased further, NADH enhanced (NO)-N-. depletion to a greater extent than either glutathione or ascorbate, Photoactivated RE under anaerobic conditions reacts with NADH to form the RE anion radical (RB(.-)), which has a characteristic visible absorption band centered at 418 nm, Rose Bengal anion radical disproportionates to give RE and a colorless reduced form of RE, RBH(-). The net result of this process is the photobleaching of RE. The presence of (NO)-N-. during irradiation of RE and NADH introduced a lag time into the kinetics of RE photobleaching, The length of this lag time was proportional to the concentration of (NO)-N-., A similar lag time, which was also dependent on the (NO)-N-. concentration, was observed in the kinetics of formation of RB(.-), The three-line electron spin resonance (ESR) spectrum of RB(.-), with an intensity ratio 1:2:1, was obtained during irradiation of RE and NADH under anaerobic conditions, (NO)-N-. introduced a concentration-dependent lag time into the kinetics of the appearance of this ESR signal, We propose that (NO)-N-. oxidizes RB(.-) to regenerate RE and thus inhibit photobleaching until (NO)-N-. is consumed, This reaction predicts the formation of NO-, the one-electron reduced form of (NO)-N-., Nitrous oxide, a characteristic dimerization product of NO-, was detected by gas chromatography, This evidence indicates the occurrence of a Type I mechanism between photoactivated RE and (NO)-N-.. (C) 1995 Academic Press, Inc.