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Our Challenge
In the pursuit of a clean and renewable dihydrogen (H2) source, an obvious and historically prominent candidate is water. The reverse of the energy-releasing H2 oxidation reaction (2H2 + O2 → 2H2O), the splitting of water can be driven by an electrical potential in the process of electrolysis. Although the ability to generate this electrical potential from renewable resources is extant, a barrier to efficient electrolytic processes is the overpotential required for reduction of protons (H+) at the cathode to H2. Expensive precious metals such as platinum minimize this over-potential, but, by virtue of their scarcity, scale-up to commercial levels is hindered. Unfortunately, inexpensive and abundant materials have tended to require high over-potentials. This kinetic barrier can nonetheless, in principle, be overcome through an efficient catalyst, for which Nature has provided guidance in the [FeFe] hydrogenase.
An attractive solution would be a bio-hybrid cathode consisting of an inexpensive cathodic material coated with an engineered variant of the [FeFe] hydrogenase with enhanced catalytic activity and stability to O2-driven degradation. To produce such a variant, we must improve our understanding of hydrogenase structure/function relationships, interfacial catalytic properties, and practical performance in photoelectrochemical (PEC) cells. These topics include the limitations to proton and electron transfer rates in the enzyme; the molecular-scale interactions between hydrogenase and bulk or nanostructured cathode materials; and, changes in measured solution-phase catalytic behavior upon phase transfer to cathode materials and integration into experimental PEC devices.
Our Approach
This collaborative project involves the National Renewable Energy Laboratory (NREL) and Arizona State University in a broad spectrum of investigation, including protein mutagenesis, expression, and testing; nanomaterial synthesis and hydrogenase-nanomaterial complex characterization; integration studies with biofuel- and photochemistry-driven electrochemical cells; and, a variety of molecular modeling studies.
In the Scientific Computing group of NREL's Materials and Computational Sciences Center, we employ atomistic classical molecular dynamics, and pure quantum and mixed quantum/classical electronic structure studies to develop well-grounded models for proton transfer and electron transfer in the [FeFe] hydrogenase from Clostridium pasteurianum. Contemporary targets of investigation include the dynamic contributions of specific amino acid residues in proton transfer through the protein structure, and a theoretical description of electron transfer between the auxiliary iron-sulfur clusters of this enzyme. The former topic is complicated by distributions of accessible peptide sidechain and proton interactions, whereas the latter involves weakly magnetically coupled and highly correlated electronic systems, with large valence spaces that are difficult to describe fully using conventional electronic structure methods.
Our Results
We have developed a set of molecular mechanical parameters for the expected valence states of all the auxiliary clusters, and the H-cluster, present in [FeFe] hydrogenases. These facilitate mixed-mode calculations that treat a small subset of the protein quantum mechanically and the rest classically. Molecular dynamics on different redox states of the enzyme are yielding insight into protein responses to reduction during catalysis, and expected pathways of proton and electron transfer through the polypeptide framework. Furthermore, certain critical sidechain motions have been hypothesized for the proton transfer mechanism of hydrogenase based on classical and mixed-mode molecular dynamics, the contributions of which are being characterized.
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