Electrochemical Reduction of NO by Myoglobin in Surfactant Film:  Characterization and Reactivity of the Nitroxyl (NO⁻) Adduct

We have examined the electrochemical reduction of NO by myoglobin (Mb) contained within a dimethyldidodecylammonium bromide (ddab) film on pyrolytic graphite electrodes. Immersion of the (Fe III )Mb-doped electrode into aqueous solutions of NO results in a bulk chemical reductive nitrosylation forming NO−Mb−Fe II, as indicated by the dissappearance of the Fe III/II −Mb couple in voltammograms. At more negative potentials, a catalytic reduction wave appears at ca. −0.7 V/SCE, which remains catalytic in solutions from pH 5.5 to pH 10 and is NO-concentration dependent. Bulk electrolysis at −0.8 V/SCE of 15 NO solutions by Mb/ddab yields 15 N 2 O as gaseous product. Cyclic voltammograms of films made of preformed nitrosyl myoglobin, MbFe II −NO, demonstrate a single, reversible reduction to the nitroxyl state, MbFe II −NO -, at E ° = −0.87 V/SCE. The reversibility of the nitrosyl reduction is pH dependent; digital simulation yields a rate of 22.5 s - 1 for the irreversible loss of a nitroxyl group at pH 7 and 0.7 s - 1 at pH 10. The catalytic formation of N 2 O during reduction of MbFe II −NO in the presence of exogenous NO implies that an N−N coupling reaction occurs at the active site between the Fe-bound nitroxyl and a free NO. A mechanism is proposed for the catalysis involving decomposition of an (N 2 O 2 - )−Fe II −Mb intermediate.