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

Abstract

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.