Wiring photosystem I for direct solar hydrogen production

The generation of H₂ by the use of solar energy is a promising way to supply humankind's energy needs while simultaneously mitigating environmental concerns that arise due to climate change. The challenge is to find a way to connect a photochemical module that harnesses the sun's energy to a catalytic module that generates H₂ with high quantum yields and rates. In this review, we describe a technology that employs a "molecular wire" to connect a terminal [4Fe-4S] cluster of Photosystem I directly to a catalyst, which can be either a Pt nanoparticle or the distal [4Fe-4S] cluster of an [FeFe]- or [NiFe]-hydrogenase enzyme. The keys to connecting these two moieties are surface-located cysteine residues, which serve as ligands to Fe - S clusters and which can be changed through site-specific mutagenesis to glycine residues, and the use of a molecular wire terminated in sulfhydryl groups to connect the two modules. The sulfhydryl groups at the end of the molecular wire form a direct chemical linkage to a suitable catalyst or can chemically rescue a [4Fe-4S] cluster, thereby generating a strong coordination bond. Specifically, the molecular wire can connect the F_B iron-sulfur cluster of Photosystem I either to a Pt nanoparticle or, by using the same type of genetic modification, to the differentiated iron atom of the distal [4Fe-4S] 3 (Cys)₃(Gly) cluster of hydrogenase.When electrons are supplied by a sacrificial donor, this technology forms the cathode of a photochemical half-cell that evolves H₂ when illuminated. If such a device were connected to the anode of a photochemical half-cell that oxidizes water, an in vitro solar energy converter could be realized that generates only O₂ and H₂ in the light. A similar methodology can be used to connect Photosystem I to other redox proteins that have surfacelocated [4Fe-4S] clusters. The controlled light-driven production of strong reductants by such systems can be used to produce other biofuels or to provide mechanistic insights into enzymes catalyzing multielectron, proton-coupled reactions.