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Protonation of an { Mn1v(p2-O)}, core to form { (Mn~V)2(p2-O)(p2-OH)} leads t o perturbations of the physical properties of this unit including an increase in metal-metal separation (ca. 0.1 A) and loss of catalase reactivity.

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X-ray absorption spectroscopy has been used to characterize the Mn site in the photosynthetic oxygen evolving complex (OEC) reaction center core complex. Treatment in the dark with hydroxylamine or with dihydroquinone results in a shift of the XANES spectra to lower energy, consistent with reduction at the Mn site. Structural changes are seen in the EXAFS as well. In both cases, the structural changes are completely reversible on illumination.