The oxidation of water to molecular oxygen is a characteristic reaction associated with photosynthesis in higher plants and cyanobacteria. Light quanta producing excitation of the photosystem results in the transfer of electrons one at a time until four electrons are removed from two water molecules to produce O2. Drs. Sauer and Klein are investing the chemical intermediates involved in this transfer of electrons and in the storing of the oxidizing equivalents needed to yield O2. Manganese is involved in the water-splitting enzyme, and they are using X-ray absorption spectroscopy to determine the oxidation states, ligation and local structure around the Mn. Together with studies using EPR spectroscopy, their results have verified the direct participation of Mn oxidation state changes in the water-splitting process. This proposal aims to characterize further the structure of the quartet of Mn atoms involved, the essential cofactors, inhibitors and protein ligands through the use of X-ray absorption spectroscopy, electron-nuclear and electron-electron double resonance spectroscopies. They hope to learn, most importantly, how water becomes incorporated into the water-splitting enzyme prior to the final formation and release of O2. Knowledge of this process will be most useful in development artificial systems whereby solar energy can be used to convert water into molecular oxygen and hydrogen fuel.
