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FeFe hydrogenases catalyze H₂ oxidation and formation at an inorganic active site (the “H-cluster”), which consists of a [Fe₂(CO)₃(CN)₂(dithiomethylamine)] subcluster covalently attached to a Fe₄S₄ subcluster. This active site is photosensitive: visible light has been shown to induce the release of exogenous CO (a reversible inhibitor of the enzyme), shuffle the intrinsic CO ligands, and even destroy the H-cluster. These reactions must be understood because they may negatively impact the use of hydrogenase for the photoproduction of H₂. Here, we explore in great detail the reactivity of the excited states of the H-cluster under catalytic conditions by examining, both experimentally and using TDDFT calculations, the simplest photochemical reaction: the binding and release of exogenous CO. A simple dyad model can be used to predict which excitations are active. This strategy could be used for probing other …