High-Resolution Mn EXAFS of the Oxygen-Evolving Complex in Photosystem II:  Structural Implications for the Mn4Ca Cluster

The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2−3.5 Å, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn−Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn−Mn distances between 2.7 and 2.8 Å and, hence, the presence of three di-μ-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures.