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 electrocatalytic production of H2 . All the proteins experience a non-diffusive electrochemical regime as electrode-bound species. The adsorbed Co-Mb construct was found to carry out the electrocatalytic reduction of water protons to H2 with a good efficiency under anaerobic conditions thus yielding a simple and tunable system for H2 production. Replacement of H2O as Co axial ligand by ammonia and imidazole significantly lowers the catalytic currents for H3O+/H2O reduction to H2. The E°’ 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 Å 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

Electrochemical and spectroscopic characterization of Co-neuroglobin: a bioelectrocatalyst for H2 production

The electronic absorption, MCD and RR spectra of the Co(III) and Co(II) derivatives of wild type human neuroglobin (Co-WT) and its C46A/C55A mutant (Co-C46AC55A) were thoroughly investigated and compared with those of the corresponding Fe species and of the few Co-substituted heme proteins characterized so far. In both oxidation states, Co-WT and Co-C46AC55A contain a low-spin six-coordinated Co ion, whose axial coordination positions appear to be occupied by the distal and proximal histidines and whose electronic properties are scarcely affected by the deletion of the C46-C55 disulfide bond. Both Co-WT and Co-C46AC55A feature negative E°’Co(III)/Co(II) values. Fe(III) to Co(III) swapping does not significantly alter the pH-dependence of their spectroscopic properties and E°’ values, indicating that no major changes occur in their regulating molecular factors. Most importantly, Co-WT and Co-C46AC55A can catalyze the reduction of H3O+ to H2, with onset potentials and overpotentials comparable to those of Co-porphyrin/polypeptide catalysts. The electrocatalytic efficiency of Co-WT and Co-C46AC55A for the development of H2 is slightly lower compared to six-coordinated aquo-His Co-Mb, although they are less affected by the presence of dioxygen