Pulse-radiolysis study of the reaction of nitric oxide with superoxide

Using the pulse-radiolysis technique, nitric oxide (NO) was produced by the reaction of an hydrated electron with NO2–. This was verified by the formation of the NO complex of iron(II) myoglobin (Mb) after pulse radiolysis of iron(III) myoglobin in the presence of NO2– under anaerobic conditions. By employing this technique, the reaction of superoxide (O2–) with NO was followed directly in an oxygen-saturated solution containing NO2–. It was found that O2– reacted with NO with a second-order rate constant of 3.8 × 109 dm3 mol–1 s–1 at pH 7.5 to form oxoperoxonitrate(III)NO(O2)– which has an absorption maximum around 300 nm. Oxoperoxonitrate(III) thus formed decomposed with a first-order rate constant of 0.8 s–1 at pH 7.5. A similar decay process was observed in the reaction of O2– with NO by monitored stopped-flow, where one part KO2 in dimethyl sulfoxide was mixed with nine parts of a NO-saturated solution in water. The formation of oxoperoxonitrate(III) was complete within the dead-time of the stopped-flow system.