Photosystem II and the Bio-mimetic Production of Hydrogen

  • Dr Ron Pace, Department of Chemistry, The Australian National University, Canberra, Australia
  • Rob Stranger, , Department of Chemistry, The Australian National University, Canberra, Australia
  • Simon Petrie, Department of Chemistry, The Australian National University, Canberra, Australia

    • Many reactions in Photosynthesis occur with efficiencies unmatched by man-made catalytic systems. A key such step is the electro-chemical splitting of water into H+ and molecular Oxygen in Photosystem II (PSII). This 'solves' the most chemically demanding reaction in the electrolytic decomposition of water into H2- and O2, vis the anodic oxidation of water (or OH-) to molecular oxygen and protons. The Mn containing water splitting catalytic site in PSII performs this reaction at close to thermodynamically limiting efficiency (< 0.2V over-voltage), at a high turnover rate (~103 s-1), under mild external pH and in the presence of significant concentrations of environmentally common anions, such as Cl-. A bio-mimetic electrolysis system based on the natural PSII catalytic site would have substantial thermodynamic and kinetic advantage.
    The PS II oxygen evolving complex (OEC) contains 4 Mn and 1 Ca in a compact, exchange coupled cluster. While full structural detail of the site is yet to be resolved, most of the protein ligands, which define the cluster geometry, are located in a very small region near the C terminus of the D1 polypeptide of the PSII reaction centre. This suggests that functioning catalytic site analogs may be assembled from small model peptides or other synthetic constructs. Proposals for such a structures have been made and some existing Mn complex OEC models show promise of useful catalytic function. (for details, see [1])
    Recently computational chemistry has been employed to explore possible intermediate structures and catalytic pathways to water oxidation which the Mn cluster might utilize [2]. This approach shows great promise and reveals aspects of the system essentially inaccessible by other techniques. Here the latest results of this and other work will be discussed and the possibilities for a practical, fully bio-mimetic cathodic water oxidiser will be assessed.

    [1] R. Pace in " Artificial Photosynthesis: From Basic Biology to Industrial Application" A.F. Collings and C. Critchley eds. ,WILEY-VCH , Weinheim, 2005, Chapt. 2
    [2] S. Petrie, R. Stranger, P. Gatt and R. Pace, European J. Chem. (2007) 13, 5082-5089