Chloride ligation in inorganic manganese model compounds relevant to Photosystem II studied using X-ray absorption spectroscopy |
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| Authors: | Shelly A. Pizarro Hendrik Visser Roehl M. Cinco John H. Robblee Samudranil Pal Sumitra Mukhopadhyay Henry J. Mok Kenneth Sauer Karl Wieghardt William H. Armstrong Vittal K. Yachandra |
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| Affiliation: | (1) Melvin Calvin Laboratory, Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-5230, USA;(2) Department of Chemistry, University of California, Berkeley, CA 94720-5230, USA;(3) Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02467-3860, USA;(4) Max Planck Institut für Bioorganische Chemie, 45413 Mülheim an der Ruhr, Germany;(5) Present address: Sandia National Labs, P.O. Box 969, Livermore, CA 94551, USA;(6) Present address: School of Chemistry, University of Hyderabad, 500 046 Hyderabad , India;(7) Present address: Chemistry Department, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;(8) Present address: CIS-US Inc., 10 DeAngelo Drive, Bedford, MA 01730, USA |
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| Abstract: | Chloride ions are essential for proper function of the photosynthetic oxygen-evolving complex (OEC) of Photosystem II (PS II). Although proposed to be directly ligated to the Mn cluster of the OEC, the specific structural and mechanistic roles of chloride remain unresolved. This study utilizes X-ray absorption spectroscopy (XAS) to characterize the Mn–Cl interaction in inorganic compounds that contain structural motifs similar to those proposed for the OEC. Three sets of model compounds are examined; they possess core structures MnIV3O4X (X=Cl, F, or OH) that contain a di- -oxo and two mono--oxo bridges or MnIV2O2X (X=Cl, F, OH, OAc) that contain a di--oxo bridge. Each set of compounds is examined for changes in the XAS spectra that are attributable to the replacement of a terminal OH or F ligand, or bridging OAc ligand, by a terminal Cl ligand. The X-ray absorption near edge structure (XANES) shows changes in the spectra on replacement of OH, OAc, or F by Cl ligands that are indicative of the overall charge of the metal atom and are consistent with the electronegativity of the ligand atom. Fourier transforms (FTs) of the extended X-ray absorption fine structure (EXAFS) spectra reveal a feature that is present only in compounds where chloride is directly ligated to Mn. These FT features were simulated using various calculated Mn–X interactions (X=O, N, Cl, F), and the best fits were found when a Mn–Cl interaction at a 2.2–2.3 Å bond distance was included. There are very few high-valent Mn halide complexes that have been synthesized, and it is important to make such a comparative study of the XANES and EXAFS spectra because they have the potential for providing information about the possible presence or absence of halide ligation to the Mn cluster in PS II.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00775-003-0520-1Abbreviations bpea N,N-bis(2-pyridylmethyl)ethylamine - EXAFS extended X-ray absorption fine structure - FT Fourier transform - IPE inflection point energy - OEC oxygen evolving complex - PS II Photosystem II - tacn 1,4,7-triazacyclononane - XANES X-ray absorption near edge structure - XAS X-ray absorption spectroscopy - XRD X-ray diffraction |
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| Keywords: | Chloride cofactor K-edge absorption Manganese EXAFS Photosystem II |
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