Direct Detection of Oxygen Ligation to the Mn4Ca Cluster of Photosystem II by X-ray Emission Spectroscopy
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.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Physical Biosciences Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 971504
- Report Number(s):
- LBNL-2328E; JACSAT; TRN: US201004%%183
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
Similar Records
Chloride ligation in inorganic manganese model compounds relevantto photosystem II studied using X-ray absorption spectroscopy
The O 2 -Evolving Complex of Photosystem II: Recent Insights from Quantum Mechanics/Molecular Mechanics (QM/MM), Extended X-ray Absorption Fine Structure (EXAFS), and Femtosecond X-ray Crystallography Data