Photosystem II (PSII) is a unique, specialized protein complex that uses light energy to oxidize water, resulting in the release of molecular oxygen into the atmosphere. PSII performs this reaction within a functional unit known as the water-oxidizing complex (WOC) or the oxygen-evolving complex (OEC). The WOC consists of an inorganic manganese-oxygen-calcium core (Mn4OxCa complex) surrounded by a functionally important ligand sphere. Understanding the structure of the WOC and its water-splitting mechanism is not only of academic interest, but is also important for the development of artificial water-splitting catalysts, for which the WOC often serves as a blueprint.
Although crystal structures are available for PSII, the detailed structure of the WOC cannot be discerned from them, because of specific radiation damage to the Mn4OxCa cluster. We are therefore trying to derive the structure of the water- splitting complex of photosystem II by a combination of biophysical techniques, such as magnetic resonance (EPR, NMR), X-ray spectroscopy, time-resolved mass spectrometry, electrochemistry and quantum mechanical calculations. These experiments are performed within a network of local and international collaborations. Special emphasis is given to the question of how substrate water is coordinated to the Mn4OxCa cluster. Comparative studies on Mn model complexes and genetically modified photosystem II complexes are carried out to guide data interpretation.
In a new second line of experiments, artificial water-splitting and hydrogen-producing catalysts are being studied under various experimental conditions with an electrochemical cell that is directly coupled to a membrane-inlet mass spectrometer. These activity studies will help our understanding of the water-splitting mechanisms and capacities of such artificial catalysts, which is crucial for their improvement. The ultimate goal is the construction of an ‘artificial leaf’ that uses sunlight to split water into O₂ and H₂
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