Highly structure-sensitive EXAFS spectra from multinuclear Mn model compounds. Ab initio refinement of a diamond-structure of the S1-state Mn4-Ca-Cl cluster in photosystem II

Abstract

The structure of the manganese oxygen-evolving complex in photosystem II (PSII) cycles and its cyclic change have been investigated by an extended X-ray absorption fine structure (EXAFS) spectroscopy, based on a widely-accepted assumption that the Fourier transform of the k3-weighted EXAFS spectrum might be nearly a linear function of the radial distribution of scattering atoms. To check this assumption, we examined the relationship between the X-ray diffraction structure and the observed EXAFS radial distribution for some multinuclear Mn model compounds, by calculating a theoretical EXAFS spectrum from that structure using an ab initio self-consistent real space multiple-scattering code, FEFF8.10(WU). A surprising result emerged that the experimental and theoretical distributions are dramatically different to each other in most model compounds, indicating that the EXAFS spectrum is in general highly sensitive to the structure. This could occur as a result of significant (nonlinear) interference between the EXAFS spectra from several Mn ions in the cluster. This clearly confirms that it would be impossible to predict the local structures of any multinuclear clusters from the observed EXAFS spectra alone. As an alternative way to be taken, we have utilized the observed EXAFS spectra to refine the molecular structure, initiating from the rough X-ray diffraction structures in the case of authentic complexes, and from a "diamond-structure" model in the case of the S1-state Mn4-Ca-Cl cluster in PSII, which is being proposed by one of the authors (MK). The diamond-structure model could be readily refined in agreement with a 3.8¿ resolution X-ray structure (Witt-Saenger¿s group).