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X-ray absorption spectroscopy is used for studying a water oxidation catalyst generated by electrodeposition from aqueous solutions containing phosphate and [Co.sup.2+] (Co-Pi). A model is suggested wherein the Co oxo/hydroxo clusters of Co-Pi are composed of edge-sharing Co[O.sub.6] octahedra, the structural motif found in cobaltates, whereas cobaltates have extended planes of Co[O.sub.6] octahedra, the Co-Pi clusters are of molecular dimensions.

The synthesis and properties of the first strontium-manganese molecular complex are described. Magnetization versus dc field sweeps at various temperatures and scan rates have shown hysteresis loops, confirming the complex to be a new single-molecule magnet.

X-ray absorption and emission spectroscopy and extended X-ray absorption fine structure studies are used for examining the electronic and geometric structures of [Mn.sub.4]Ca cluster and the mechanism of the water oxidation reaction. The geometric and electronic structures of the [Mn.sub.4]Ca cluster in the S stated have helped in understanding the mechanism of O-O bond formation during the photosynthetic water-splitting process.

The article presents a study on the cluster of manganese-calcium (Mn4Ca) complex in photosynthesis. It mentions the production of oxygen through photosynthetic oxidation of water in cyanobacteria, algae and plants. It also highlights the changes of Extended X-ray absorption fine structure (EXAFS) distance and potential protonation states at the terminal of water molecules.

In the 1.9 Å structural model of photosystem II (PDB: 3ARC), the amino acid residue Glu333 of the D1 polypeptide coordinates to the oxygen-evolving Mn4CaO5 cluster. This residue appears to be highly significant in that it bridges the two Mn ions (MnB3 and the "dangling" MnA4) that are also bridged by the oxygen atom O5. This oxygen atom has been proposed to be derived from one of two substrate water molecules and to become incorporated into the product dioxygen molecule during the final step in the catalytic cycle. In addition, the backbone nitrogen of D1-Glu333 interacts directly with a nearby Cl- atom. To further explore the influence of this structurally unique residue on the properties of the Mn4CaO5 cluster, the D1-E333Q, mutant of the cyano bacterium Synechocystis sp. PCC 6803 was characterized with a variety of biophysical and spectroscopic methods, including polarography, EPR, X-ray absorption, and FTIR difference spectroscopy. The kinetics of oxygen release in the mutant were essentially unchanged from those in wild-type. In addition, the oxygen flash yields exhibited normal period-four oscillations having normal S state parameters, although the yields were lower, indicative of the mutant's lower steady-state dioxygen evolution rate of approximately 30% compared to that of the wild-type. The S1 state Mn-XANES and Mn-EXAFS and S2 state multiline EPR signals of purified D1-E333Q, PSU core complexes closely resembled those of wild-type, aside from having lower amplitudes. The Sn+1,-minus-Sn FTIR difference spectra showed only minor alterations to the carbonyl, amide, and carboxylate stretching regions. However, the mutation eliminated a negative peak at 3663 cm-1 in the weakly H-bonding O-H stretching region of the S2-minus-S1 FTIR difference spectrum and caused an approximately 9 cm-1 downshift of the negative feature in this region of the S1-minus-S0 FTIR difference spectrum. We conclude that fully functional Mn4CaO5 clusters assemble in the presence of the D1-E333Q mutarion but that the mutation decreases the yield of assembled clusters and alters the H-bonding properties of one or more water molecules or hydroxide groups that are located on or near the Mn4CaO5 cluster and that either deprotonate or farm stronger hydrogen bonds during the S0 to S1 and S1 to S2 transitions. [ABSTRACT FROM AUTHOR]

In the current X-ray crystallographic structural models of photosystem II. Glu354 of the CP43 polypeptide is the only amino acid ligand of the oxygen-evolving Mn4Ca cluster that is not provided by the D1 polypeptide. To further explore the influence of this structurally unique residue on the properties of the Mn4Ca cluster, the CP43-E354Q mutant of the cyanobacterium Synechocvstis sp. PCC 6803 was characterized with a variety of biophysical and spectroscopic methods, including polarography. EPR. X-ray absorption, FTIR. and mass spectrometry. The kinetics of oxygen release in the mutant were essentially unchanged from those in wild type. In addition, the oxygen flash yields exhibited normal period four oscillations having normal S state parameters, although the yields were lower, correlating with the mutant's lower steady-state rate (approximately 20% compared to wild type). Experiments conducted with H218O showed that the fast and slow phases of substrate water exchange in CP43-E354Q thylakoid membranes were accelerated 8.5- and 1.8-fold, respectively, in the S3 state compared to wild type. Purified oxygen-evolving CP43-E354Q PSII core complexes exhibited a slightly altered S1 state Mn-EXAFS spectrum, a slightly altered S2 state multiline EPR signal, a substantially altered S2-minus-S1 FTIR difference spectrum, and an unusually long lifetime for the S2 state (> 10 h) in a substantial fraction of reaction centers. In contrast, the S1 state Mn-EXAFS spectrum was nearly indistinguishable from that of wild type. The S2-minus-S1 FTIR difference spectrum showed alterations throughout the amide and carboxylate stretching regions. Global labeling with 15N and specific labeling with L-[1-13C]alanine revealed that the mutation perturbs both amide II and carboxylate stretching modes and shifts the symmetric carboxylate stretching modes of the α-COO group of D1-A1a344 (the C-terminus of the D1 polypeptide) to higher frequencies by 3-4 cm-1 in both the S1 and S1 states. The EPR and FTIR data implied that 76-82% of CP43-E354Q PSII centers can achieve the S2 state and that most of these can achieve the S3 state, but no evidence for advancement beyond the S2 state was observed in the FTIR data, at least not in a majority of PSII centers. Although the X-ray absorption and EPR data showed that the CP43-E354Q mutation only subtly perturbs the structure and spin state of the Mn4Ca cluster in the S4 state, the FTIR and H218O exchange data show that the mutation strongly influences other properties of the Mn4Ca cluster, altering the response of numerous carboxylate and amide groups to the increased positive charge that develops on the cluster during the S1 to S2transition and weakening the binding of both substrate water molecules (or water-derived ligands), especially the one that exchanges rapidly in the S3 state. The FTIR data provide evidence that CP43-G1u354 coordinates to the Mn4Ca cluster in the S1 state as a bridging ligand between two metal ions but provide no compelling evidence that this residue changes its coordination mode during the S1 to S2 transition. The H218O exchange data provide evidence that CP43-Glu354 interacts with the Mn ion that ligates the substrate water molecule (or water-derived ligand) that is in rapid exchange in the S3 state. [ABSTRACT FROM AUTHOR]

High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six- coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3dz²4pz mixing mechanism. In the square pyramidal Mn- (V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3dxz,yz transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed. [ABSTRACT FROM AUTHOR]