Reactivity of the Excited States of the H-Cluster of FeFe Hydrogenases.

FeFe hydrogenases catalyze H oxidation and formation at an inorganic active site (the "H-cluster"), which consists of a [Fe(CO)(CN)(dithiomethylamine)] subcluster covalently attached to a FeS 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 H. 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 aspects of the photoreactivity of the H-cluster.