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) with Rose Bengal (RB) in the presence of electron donors was investigated. Upon illumination of a mixture of RB and^·NO with visible light, an enhancement in the rate of^·NO consumption was observed that increased with increasing RB concentration. In the presence of electron donors (NADH, glutathione, or ascorbate), the rates of^·NO depletion increased further. NADH enhanced^·NO depletion to a greater extent than either glutathione or ascorbate. Photoactivated RB under anaerobic conditions reacts with NADH to form the RB anion radical (RB^·—), which has a characteristic visible absorption band centered at 418 nm. Rose Bengal anion radical disproportionates to give RB and a colorless reduced form of RB, RBH—. The net result of this process is the photobleaching of RB. The presence of^·NO during irradiation of RB and NADH introduced a lag time into the kinetics of RB photobleaching. The length of this lag time was proportional to the concentration of^·NO. A similar lag time, which was also dependent on the^·NO 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 RB and NADH under anaerobic conditions.^·NO introduced a concentration-dependent lag time into the kinetics of the appearance of this ESR signal. We propose that^·NO oxidizes RB^·—to regenerate RB and thus inhibit photobleaching until^·NO is consumed. This reaction predicts the formation of NO—, the one-electron reduced form of^·NO. 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 RB and^·NO.