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 deletion of the C46-C55 disulfide bond. Both Co-WT and Co-C46AC55A feature negative <i>E</i>°'_{Co(III)/Co(II)} values. Fe(III) to Co(III) swapping does not significantly alter the pH dependence of their spectroscopic properties and <i>E</i>°' 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 that of six-coordinated aquo-His Co-Mb, although they are less affected by the presence of dioxygen.