Photosynthetic water oxidation and O2 formation are catalyzed by a Mn4Ca complex bound to the proteins of photosystem II (PSII). The catalytic site, including the inorganic Mn4CaOnHx core and its protein environment, is denoted as oxygen-evolving complex (OEC). Earlier and recent progress in the endeavor to elucidate the structure of the OEC is reviewed, with focus on recent results obtained by (i) X-ray spectroscopy (specifically by EXAFS analyses), and (ii) X-ray diffraction (XRD, protein crystallography). Very recently, an impressive resolution of 1.9 Å has been achieved by XRD. Most likely however, all XRD data on the Mn4CaOnHx core of the OEC are affected by X-ray induced modifications (radiation damage). Therefore and to address (important) details of the geometric and electronic structure of the OEC, a combined analysis of XRD and XAS data has been approached by several research groups. These efforts are reviewed and extended using an especially comprehensive approach. Taking into account XRD results on the protein environment of the inorganic core of the Mn complex, 12 alternative OEC models are considered and evaluated by quantitative comparison to (i) extended-range EXAFS data, (ii) polarized EXAFS of partially oriented PSII membrane particles, and (iii) polarized EXAFS of PSII crystals. We conclude that there is a class of OEC models that is in good agreement with both the recent crystallographic models and the XAS data. On these grounds, mechanistic implications for the Osingle bondO bond formation chemistry are discussed. This article is part of a Special Issue entitled: Photosystem II.
