Light-Driven Hydrogen Evolution Reaction Catalyzed by a Molybdenum-Copper Artificial Hydrogenase.

Orange protein (Orp) is a small bacterial metalloprotein of unknown function that harbors a unique molybdenum/copper (Mo/Cu) heterometallic cluster, [SMoSCuSMoS]. In this paper, the performance of Orp as a catalyst for the photocatalytic reduction of protons into H has been investigated under visible light irradiation. We report the complete biochemical and spectroscopic characterization of -Orp containing the [SMoSCuSMoS] cluster, with docking and molecular dynamics simulations suggesting a positively charged Arg, Lys-containing pocket as the binding site. -Orp exhibits excellent photocatalytic activity, in the presence of ascorbate as the sacrificial electron donor and [Ru(bpy)]Cl as the photosensitizer, for hydrogen evolution with a maximum turnover number of 890 after 4 h irradiation. Density functional theory (DFT) calculations were used to propose a consistent reaction mechanism in which the terminal sulfur atoms are playing a key role in promoting H formation. A series of dinuclear [SMSM'SMS] clusters, with M = Mo, W and M' = Cu, Fe, Ni, Co Zn, Cd were assembled in Orp, leading to different M/M'-Orp versions which are shown to display catalytic activity, with the Mo/Fe-Orp catalyst giving a remarkable turnover number (TON) of 1150 after 2.5 h reaction and an initial turnover frequency (TOF°) of 800 h establishing a record among previously reported artificial hydrogenases.