| Described here is the development and refinement of a hybrid biological/organic photo-electrochemical half-cell that couples a photochemical module, Photosystem I (PS I), 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 design philosophy is based on the biological paradigm in which two independent photochemical halfcells function in series to generate O2 and NADPH. The work described in this dissertation focuses on the optimization of the half-cell reaction: 2H+ + 2e- + 2hnu → H 2. The challenge is to deliver electrons from PS I to H2ase rapidly and at high quantum yield, thereby overcoming diffusion-based limits on electron transfer. To accomplish this, a technology has been devised that is based on a molecular wire, which serves to tether the photochemical module to the catalytic module at a fixed distance so that an electron can quantum mechanically tunnel between the FB cluster of PS I and the distal [4Fe-4S] cluster of a H2ase enzyme at a rate faster than the charge recombination between P700+ and F B - To link the photochemical and catalytic modules of the half-cell, a short aliphatic or aromatic dithiol molecule forms a coordination bond with an exposed Fe of the FB cluster of a PS I 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 surface-located [4Fe-4S] cluster of each protein to a Gly, thereby exposing the Fe atom for chemical rescue by the added dithiolate-containing molecular wire. |
