| Abstract
| - We have examined the electrochemical reduction of NO by myoglobin (Mb) contained within adimethyldidodecylammonium 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 formingNO−Mb−FeII, as indicated by the dissappearance of the FeIII/II−Mb couple in voltammograms. At morenegative potentials, a catalytic reduction wave appears at ca. −0.7 V/SCE, which remains catalytic in solutionsfrom pH 5.5 to pH 10 and is NO-concentration dependent. Bulk electrolysis at −0.8 V/SCE of 15NO solutionsby Mb/ddab yields 15N2O as gaseous product. Cyclic voltammograms of films made of preformed nitrosylmyoglobin, 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 rateof 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 formationof N2O during reduction of MbFeII−NO in the presence of exogenous NO implies that an N−N couplingreaction occurs at the active site between the Fe-bound nitroxyl and a free NO. A mechanism is proposed forthe catalysis involving decomposition of an (N2O2-)−FeII−Mb intermediate.
|
