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 (FeIII)Mb-doped electrode into aqueous solutions of NO results in a bulk chemical reductive nitrosylation forming NO−Mb−FeII, as indicated by the dissappearance of the FeIII/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 15NO solutions by Mb/ddab yields 15N2O as gaseous product. Cyclic voltammograms of films made of preformed nitrosyl myoglobin, MbFeII−NO, demonstrate a single, reversible reduction to the nitroxyl state, MbFeII−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 N2O during reduction of MbFeII−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 (N2O2 -)−FeII−Mb intermediate.