We performed a KS-DFT study of the catalytic cycle of FeNi Hydrogenase where hydrogen is reduced to protons and electrons. We have previously shown that the major effect on the catalytic cycle of this system is the rigidity of the local structure of the active site which is modulated by the surrounding enzyme environment, in addition to the electronic effects of the iron and nickel atoms. Recent experimental work has been performed to attempt to replicate the activity of this enzyme using biomimetic analogues; however, the turn over frequencies and overpotentials of these analogues differ significantly from the native enzyme. Therefore, we performed a series of calculations using biomimetic inspired changes to the active site of FeNi hydrogenase to determine the effect these changes have on the geometries and energetics of the reaction intermediates. Popular biomimetic changes include replacing the CN ligands coordinated to the Fe atom with cyclopentadienyl based ligands, however we found that this causes significant structural changes in the active site, specifically the Fe-Ni bond distance, that increases the energetics of the reaction profile and inhibits the creation of the µ-H bound between the metal centers. To further study the ligand environment on Fe, we also compared how the electron withdrawing or donating nature influences the reaction. By replacing one CN ligand with a more electron withdrawing group (NN), the energetics are either consistent with or slightly decreased when compared to the native ligand; when the same change is made with a more electron donating group (NH<sub>2</sub>), we see a net increase in the energetics of the reaction profile. These results indicate that the electron withdrawing nature of the CN ligands is integral for efficient catalysis. Additionally, the effect of other active site changes such as replacing the bridging cysteine residues with a 1,3-propanedithiol (pdt) ligand and the terminal cysteine residues with a 1,2-bis(diphenylphosphino)ethane (dppe) ligand among others are discussed.  These results provide a comprehensive picture of how ligand modifications influence the reaction profile and structure of the active site.

Computational biomimetic-inspired modifications onthe reaction and geometry of the FeNi-hydrogenaseactive site

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