The Mechanism of Water Splitting and Oxygen Evolution in Photosynthesis

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The oxidation of water to molecular oxygen is a characteristic reaction associated with photosynthesis in higher plants. Light quanta producing excitation of the photosystem result in the transfer of electrons one at a time until four electrons are removed from two water molecules to produce oxygen. The chemical intermediates involved in transferring electrons at high reduction potentials and in storing the oxiding equivalents needed to yield oxygen are being investigated. Manganese is involved in the water-splitting enzyme, and X-ray spectroscopy at the Stanford Synchrotron Radiation Laboratory is being used to determine the oxidation state, ligation and local structure around the Mn. Together with studies using EPR spectroscopy, results have verified the direct participation of Mn oxidation state changes in the water-splitting process. This proposal aims to characterize the nature of the manganese binding site, first by identifying the proteins involved and then in determining the specific amino acids involved in providing ligands to the manganese., In the near future it is hoped to characterize the molecular species involved, through the use of electron-nuclear and electron-electron double resonance, and X-ray spectroscopy as well the nature of the way in which water becomes incorporated into the water-splitting enzyme prior to the final formation and release of oxygen. Knowledge of this process will be most useful in developing artificial systems whereby solar energy can be used to convert water into molecular oxygen and hydrogen fuel.***