Enhanced photocatalytic hydrogen evolution by covalent attachment of plastocyanin to photosystem I

A simple photocatalytic hydrogen-evolving system is reported based on intermolecular electron transfer using isolated Photosystem I (PSI) reaction centers as the photoactive element. The system is composed of platinized PSI covalently linked to plastocyanin (PC). Water-soluble sodium ascorbate is the electron donor. PC was attached to PSI by formation of peptide bonds with the cross-linking reagent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride. Compared to the unlinked proteins, cross-linking of PC and PSI resulted in a substantial increase in light-driven reduction of hexachloroplatinate ions (PtCl₆^2- + 4e^- → Pt↓ + 6Cl^-). Hydrogen photoevolution by cross-linked PC-platinized PSI was increased 3-fold both in initial rate and total yield. Analysis of the reaction indicates that covalent linkage of PC to PSI results in a greater rate of total electron throughput from sodium ascorbate to light-activated hydrogen evolution. In addition, although photocatalytic hydrogen-evolving activity was easily demonstrated in the cross-linked system, the native pathway of electron flow yielding enzymatic NADP⁺ reduction activity was not observed upon addition of the natural PSI electron-accepting system, ferredoxin plus ferredoxin: NADP⁺ oxidoreductase.