Structural characterization of the manganese cluster of the oxygen evolving complex of photosystem II using x-ray absorption spectroscopy.

X-ray absorption spectroscopy (XAS) has been used to characterize the native (S\sb1) state and chemically altered forms of the tetranuclear Mn cluster responsible for photosynthetic water oxidation in the Oxygen Evolving Complex (OEC) of Photosystem II. EXAFS (Extended X-ray Absorption Fine Structure) has been used to characterize three interactions in the OEC: 1.86A Mn-O/N nearest neighbor interactions, 2.72A Mn\sp\...Mn vectors, and a 3.3A Mn\sp\...X interaction (X = C, Ca, or Mn). The identity of X was addressed by substituting Sr, Dy or La for the native Ca cofactor. Fitting trends indicate that the scatterer at 3.3A is most likely Mn with some contribution from carbon. XANES (X-ray Absorption Near Edge Structure) has been used to determine oxidation state of several chemically reduced species of the Mn cluster. The S\sb1 state exists in an oxidation state $>$(III)\sb4 and $$3A) Mn\sp\...Metal interactions is dependent on the distribution of ligands about the Mn. Destructive interference from an extremely disordered first shell of ligands can obscure the presence of both these structural features. There is a small but reproducible change in the 2.7A Mn\sp\...Mn distance when Ca\sp{2+} is replaced by Sr\sp{2+}, Dy\sp{3+} or La\sp{3+}. These changes are the first direct evidence for changes in the Mn\sb2(\mu-O)\sb2 core structure following Ca\sp{2+} replacement.Ph.D.ChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104535/1/9527729.pdfDescription of 9527729.pdf : Restricted to UM users only