Oxygenic photosynthesis is one of the major life supporting processes on earth. The oxygen evolving complex (OEC) of Photosystem II (PS II) in higher plants, algae and cyanobacteria contains 4 Mn ions. Despite much progress in recent years, the mechanism of water oxidation is still unclear. One fundamental question in that regard that remains to be clarified is: What are the redox states of the four Mn ions in the different states (S0-S4) that the OEC transverses through during the process of water oxidation? Different conclusions were drawn based on EPR, Mn K-edge XANES (X-ray absorption near edge structure) or UV absorption measurements and information from other techniques is required to solve this problem. We propose to use the technique of high resolution Mn K_ spectroscopy (HRXFS) to gain information on the redox states of the Mn ions in the states S0-S3. This technique is very sensitive to redox changes of the metal centers and unlike Mn K-edge XANES only marginally affected by changes of ligand geometry. A highly sensitive HRXFS detector has been built in the group of Prof. Steve Cramer and measurements on Mn model compounds and the first results with PS II preparations have shown the feasibility of these experiments.
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