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, E 1 (–0.25 V vs. SCE) and E 2 (–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 MnIIIMb. 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.