STRUCTURAL CHARACTERIZATION OF THE MANGANESE SITES IN THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX USING X-RAY ABSORPTION-SPECTROSCOPY

Manganese X-ray absorption spectra (XAS) are reported for the S, state of highly purified, highly concentratedpreparations of the oxygen-evolving complex (OEC) from photosystem II (PSII). Improvements in concentration (ca. 1.5mM Mn) and detection efficiency (13-element solid-state detector array) have permitted a substantial improvement in dataquality relative to previous solution XAS studies of PSII. Principal findings are that there is no need to include a shell of oxygens at ca. 1.75 Å in order to account for the Mn EXAFS, that there are 2-3 Mn-Mn distances of ca. 2.7 Å, and that there are one and possibly two shells of additional scatterers at longer distance (ca. 3.3 and 4.2 Å) from the Mn. Even with this higher quality data, it is not possible to use EXAFS to determine whether chloride is coordinated to the Mn. The structuralconsequences of these results are discussed in the context of proposed structural models. It is concluded that neither a cubanenor previously prepared butterfly type clusters can account for the observed features. Although EXAFS alone cannot uniquelydetermine the structure of the OEC, the present data together with other physical observations suggest an alternate model for the OEC consisting of a trinuclear Mn cluster together with a mononuclear Mn. A new approach for interpreting X-rayabsorption near edge structure (XANES) spectra by using edge shape rather than only the energy of the first inflection pointis presented. This method correctly determines the Mn(II) concentration in crystallographically characterized mixed-valenceMn trimers. The OEC XANES spectrum is unusually broad, and the edge fitting method suggests that one interpretation of this broad edge is the presence of mixed-valence Mn(II/III/III/IV) in the St state. The XANES and EXAFS data takentogether point to a manganese environment which is more heterogeneous than previously appreciated. © 1990, American Chemical Society. All rights reserved.