Abstract
A synthetic Fe2S2 complex in [FeFe] hydrogenase modeling system provides a datum point for us to clarify the catalytic mechanism in biological system. Taking into account of proton migration, varying the strength and the stoichiometric amount of acids enables us to observe the formation of several intermediates in catalytic cycle, e.g. iron-hydride species, doubly protonated species bearing S-proton and Fe-hydride, and hydrogen bonding species. All of these complexes were characterized by spectroscopy, first principle calculation or X-ray single crystal structural analysis. Furthermore, the investigation of the reversible proton relay process is essential in explaining the indispensable roles of the catalytic Fe center, the secondary coordination site, and the neighboring amino acid residue in the mechanism of the reversible H+/H2 catalysis by the active site. Moreover, a thorough study on electrocatalytic H2 production was done by applying different type/strength acids as proton source. The exhibition of concerted proton-electron transfer (CPET) step has the advantages on thermodynamics and kinetics over the stepwise one, which were identified by electrochemical study and theoretical calculation. Incorporating the effect of electric fields in ab initio calculation allows us to depict the detailed mechanism of CPET reactions. Different from proton tunneling process in traditional CPET reactions, the proton transfer is thermodynamic and kinetic favored under strong electric fields in this system. It leads to the change of electronic configuration from the conjugated acid-base complex, which turns out more positive catalytic potential and faster electron transfer rate. The CPET reactions could only occur in a limited range of acidic strength. It is consistent with the disappearance of CPET step from making certain protein residue mutation in enzymatic system of [FeFe] hydrogenase. Overall, these results allow us to build up a design principle for a more efficient catalytic system for H2 production.