Unusual voltammetry of manganese-substituted myoglobin in surfactant film: evidence for two redox pathways.

The surfactant film methodology is used to examine the electrochemistry of manganese-substituted myoglobin. Cyclic voltammograms at different scan rates depict a dynamic exchange between two redox couples, E1 (-0.25 V vs. SCE) and E2 (-0.41 V). Similar behavior is seen for Mn-substituted cytochrome c peroxidase, but the free cofactor, Mn(protoporphyrin IX) yields a single couple (-0.32 V) under the same conditions. A square scheme is proposed which describes equilibration between two different redox pathways associated with different forms of the protein. Overlapping oxidative currents from these two couples can be deconvoluted, and a pseudo first-order rate constant of 2.3 s(-1) is obtained for the reaction following reduction of Mn(III)Mb. Experiments have been performed to probe possible mechanisms for this equilibrium, such as ligand dissociation or reversible adsorption at the electrode surface. A cofactor-induced reorganization of the protein structure is suggested as the basis of the behavior.