Direct Detection of Oxygen Ligation to the Mn4Ca Cluster of Photosystem II by X-ray Emission Spectroscopy

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Ligands play critical roles during the catalytic reactions in metalloproteins through bond formation/breaking, protonation/deprotonation, and electron/spin delocalization. While there are well-defined element-specific spectroscopic handles, such as X-ray spectroscopy and EPR, to follow the chemistry of metal catalytic sites in a large protein matrix, directly probing particular ligand atoms like C, N, and O is challenging due to their abundance in the protein. FTIR/Raman and ligand-sensitive EPR techniques such as ENDOR and ESEEM have been applied to study metal-ligand interactions. X-ray absorption spectroscopy (XAS) can also indirectly probe the ligand environment; its element-specificity allows us to focus only on the catalytic ... continued below

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Pushkar, Yulia; Long, Xi; Glatzel, Pieter; Brudvig, Gary W.; Dismukes, G. Charles; Collins, Terrence J. et al. June 16, 2009.

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Ligands play critical roles during the catalytic reactions in metalloproteins through bond formation/breaking, protonation/deprotonation, and electron/spin delocalization. While there are well-defined element-specific spectroscopic handles, such as X-ray spectroscopy and EPR, to follow the chemistry of metal catalytic sites in a large protein matrix, directly probing particular ligand atoms like C, N, and O is challenging due to their abundance in the protein. FTIR/Raman and ligand-sensitive EPR techniques such as ENDOR and ESEEM have been applied to study metal-ligand interactions. X-ray absorption spectroscopy (XAS) can also indirectly probe the ligand environment; its element-specificity allows us to focus only on the catalytic metal site, and EXAFS and XANES provide metal-ligand distances, coordination numbers, and symmetry of ligand environments. However, the information is limited, since one cannot distinguish among ligand elements with similar atomic number (i.e. C, N. and O). As an alternative and a more direct method to probe the specific metal-ligand chemistry in the protein matrix, we investigated the application of X-ray emission spectroscopy (XES). Using this technique we have identified the oxo-bridging ligands of the Mn{sub 4}Ca complex of photosystem II (PS II), a multisubunit membrane protein, that catalyzes the water oxidizing reaction. The catalytic mechanism has been studied intensively by Mn XAS. The fundamental question of this reaction, however, is how the water molecules are ligated to the Mn{sub 4}Ca cluster and how the O-O bond formation occurs before the evolution of O{sub 2}. This implies that it is necessary to follow the chemistry of the oxygen ligands in order to understand the mechanism.

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  • Journal Name: Journal of the American Chemical Society

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  • Report No.: LBNL-2328E
  • Grant Number: DE-AC02-05CH11231
  • Office of Scientific & Technical Information Report Number: 971504
  • Archival Resource Key: ark:/67531/metadc930137

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  • June 16, 2009

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  • Nov. 13, 2016, 7:26 p.m.

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  • Nov. 18, 2016, 2:34 p.m.

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Pushkar, Yulia; Long, Xi; Glatzel, Pieter; Brudvig, Gary W.; Dismukes, G. Charles; Collins, Terrence J. et al. Direct Detection of Oxygen Ligation to the Mn4Ca Cluster of Photosystem II by X-ray Emission Spectroscopy, article, June 16, 2009; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc930137/: accessed June 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.