Metal-sulfur proteins, such as hydrogenases, ferredoxins and rubredoxin play an important role in many enzymatic reactions. The presence of transition metals with quasidegenerate d orbitals in the active sites of metal-sulfur proteins is responsible for existence of multiple low-lying electronic states with different spin multiplicities. The relative energies of these electronic states depend on the structure of the active site, which is controlled by the protein backbone. We demonstrate that small structural changes in the active sites can lead to intersection between the low lying electronic states with different spin multiplicities opening new multistate reaction mechanisms involving fast intersystem crossings. The rates of intersystem crossings in the active sites of [NiFe]-hydrogenase, rubredoxin and 2Fe-2S ferredoxin are calculated with nonadiabatic transition state theory using Landau-Zener and Delos equations for spin-forbidden transition probabilities. The cross sections of potential energy surfaces are obtained with density functional theory, while spin-obit couplings are calculated using complete active space configuration interaction method and multireference perturbation theory. We speculate that multistate nonadiabatic reaction mechanisms could play an important role in electron transfer and enzymatic catalysis involving metal-sulfur proteins.

Theoretical study of dynamics at intersystem crossings in the active sites of metal-sulfur proteins

251st National Meeting (2016)

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