Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 13, Issue 18, Pages 5082-5089Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200700003
Keywords
density functional calculations; metalloproteins; mixed-valent compounds; photosystem II; structure elucidation; X-ray diffraction
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Density functional theory (DFT) calculations, at the Becke-Perdew/TZP level of theory, were used to investigate a set of CaMn4-containing clusters that model the active site of the water-oxidizing complex (WOC) of photosystem II (PSII). Metal-atom positions for three representative isomeric clusters of the formula [CaMn4C9N2O16H10]+center dot 4H(2)O are in good agreement with the disparate Mn-4 geometries of the three most recent Xray crystal structures. Remarkably, interconversion between these three isomeric clusters is found to be facile, resulting from subtle changes in the coordination environment around the CaMn4 centre. Ibis result provides a clear rationalisation of the marked differences in reported crystal structures. Recent concerns have been raised regarding the opportunity for X-ray-damage-induced distortion of the metal-containing active centre during crystallographic analysis. Our calculations suggest. that an even greater problem may be presented by the apparent fluxionality of the CaMn4 skeleton within the active centre. Structural rearrangement may well precede crystallographic analysis, for example by the preferential freezing-out of one of several near-isoenergetic structures during the workup for crystallisation. This prospect, which our calculations cannot exclude, highlights the difficulties that will continue to be faced by experimentalists seeking unambiguous structural information on the WOC's active site.
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