The goal of this project to develop a hybrid biological/organic photo-electrochemical half-cell that couples a photochemical module, Photosystem I (PSI), which captures and stores energy derived from sunlight, with a catalytic module, hydrogenase (H2ase), which catalyzes H2 evolution with an input of two electrons and two protons. The challenge is to deliver electrons from PSI to the H2ase rapidly and at high quantum yield, thereby overcoming diffusion-based limits on electron transfer. The strategy is to employ molecular wire technology to directly connect PS I with a catalyst. The wire serves to tether the photochemical module to the catalytic module at a fixed distance so that an electron can quantum mechanically tunnel between surface-located [4Fe-4S] clusters of PSI and a H2ase at a rate faster than the competing charge recombination between P700+ and FB–. To link the photochemical and catalytic modules, a short aliphatic or aromatic dithiol molecule forms a coordination bond with an exposed Fe of the FB cluster of a PSI variant and with an exposed Fe of the distal [4Fe-4S] cluster of a H2ase variant. This is practically achieved by changing a ligating Cys residue of the [4Fe-4S] cluster of each protein to a Gly, thereby exposing the Fe atom for chemical rescue by the added dithiolate-containing molecular wire. By optimizing the reaction conditions, the PSI—wire—[FeFe]-H2ase construct evolves H2 at a rate of 2850 µmoles mg Chl-1 h-1, which is equivalent to a throughput of 5700 µmoles mg Chl-1 h-1, or 142 µmoles e– PSI-1 s-1. Putting this into perspective, cyanobacteria evolve O2 at a rate of ~400 µmoles mg Chl-1 h-1, which is equivalent to a throughput of 1600 µmoles mg Chl-1 h-1, or 49 e– PSI-1 s-1, given a PSI to PSII ratio of 1.8 as in the cyanobacterium Synechococcus sp. PCC 7002.