Efficient Electrocatalytic H₂ Production by Immobilized Co(III)-myoglobin

The thermodynamics and kinetics of heterogeneous electron transfer (ET) for Co-substituted horse myoglobin (Co-Mb) and its derivatives with ammonia and imidazole as heme axial ligands were studied with cyclic voltammetry on a pyrolytic graphite electrode along with their ability to mediate the electro-catalytic production of H-2. All the proteins experience a non-diffusive electrochemical regime as electrode-bound species. The adsorbed Co-Mb construct carries out the electro-catalytic reduction of water protons to H-2 with a good efficiency under anaerobic conditions thus yielding a simple and tunable system for H-2 production. Replacement of H2O as Co axial ligand by ammonia and imidazole significantly lowers the catalytic currents for H3O+/H2O reduction to H-2. The E degrees' values of the Co(III)/Co(II) redox couple for all species are mainly determined by the enthalpic contribution. Differences were found in the kinetics of ET for the different protein adducts due to changes in the activation enthalpies. However, all species share the same distance of about 14 & Aring; from the electrode surface to the Co(III)/Co(II) center determined using the Marcus model, consistent with a non-denaturing adsorption of the protein.