Multi-edge X-ray absorption spectroscopy and electronic structure calculations of biomimetic model complexes of the H-cluster of [FeFe]-hydrogenase
Giles, Logan James.
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FeFe-hydrogenases are members of a family of metalloenzymes that catalyze the conversion of protons and electrons to dihydrogen at a remarkable rate. The catalytic center of this enzyme, the H-cluster, contains a classical [4Fe-4S] cluster that is covalently and magnetically coupled through a cysteine residue to a 2Fe-subcluster. The 2Fe-subcluster contains normally biotoxic carbonyl and cyanide ligands and a dithiolate ligand that is unique in biology. Many biomimetic model complexes have been synthesized that attempted to mimic the H-cluster reactivity, but none have been successful at as low of a reduction potential and as high of a reaction rate as the metalloenzyme. Thus the goal of this research is to develop a blueprint for understanding the electronic structure of the H-cluster, through functionally analogous model complexes. The first step towards this goal is to carry out multi-edge X-ray absorption spectroscopic measurements and electronic structure calculations. We first developed the multi-edge X-ray absorption spectroscopy method for a prototypical biomimetic complex, Fe 2(u-S(CH 2) 3S)(CO) 6. This allowed for the complete definition of the orbital composition for the unoccupied frontier orbitals. We used this information to calibrate our computational results in order to accurately describe similar biomimetic model complexes. We used the multi-edge X-ray absorption spectroscopic approach and the calibrated computational models to analyze four structural features of the 2Fe-subcluster of the H-cluster through representative biomimetic model complexes. We find unique trends for each series that helped to develop an understanding of how each compositional feature contribute to structure. These insights can be used for optimizing model complexes with potential to match the reactivity of the FeFe-hydrogenase enzymes. We also used our calibrated electronic structure method to analyze the spin density at the bridgehead position of the unique dithiolate ligand and dissect the intricate details of the electronic structure for the protein-environment embedded H-cluster model.