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 NO solutions by Mb/ddab yields NO 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 for the irreversible loss of a nitroxyl group at pH 7 and 0.7 s at pH 10. The catalytic formation of NO 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 (NO)-Fe(II)- Mb intermediate.