scholarly journals The structure of the Mn 4 Ca 2+ cluster of photosystem II and its protein environment as revealed by X-ray crystallography

2007 ◽  
Vol 363 (1494) ◽  
pp. 1129-1138 ◽  
Author(s):  
James Barber ◽  
James W Murray
Keyword(s):  
Fine Structure ◽  
Photosystem Ii ◽  
Water Splitting ◽  
Metal Ion ◽  
Spectroscopic Techniques ◽  
Precise Position ◽  
X Ray ◽  
X Ray Crystallography ◽  
X Ray Absorption ◽  
Protein Environment

The location, structure and protein environment of the Mn 4 Ca 2+ cluster, which catalyses the light-driven, water-splitting reaction of photosystem II, has been revealed by X-ray crystallography. However, owing to the low resolutions of the crystal structures reported to date, and the possibility of radiation damage at the catalytic centre, the precise position of each metal ion remains unknown. To some extent, these problems have been overcome by applying spectroscopic techniques like extended X-ray absorption fine structure. Taking into account the most recent results obtained with these two X-ray-based techniques, we have attempted to refine models of the structure of the Mn 4 Ca 2+ cluster and its protein environment.

Assessing nickel oxide electrocatalysts incorporating diamines and having improved oxygen evolution activity using operando UV/visible and X-ray absorption spectroscopy

2021 ◽  
Author(s):  
Takafumi Miura ◽  
Shun Tsunekawa ◽  
Sho Onishi ◽  
Toshiaki Ina ◽  
Kehsuan Wang ◽  
...  
Keyword(s):  
Nickel Oxide ◽  
Water Splitting ◽  
Oxygen Evolution ◽  
Absorption Spectroscopy ◽  
Spectroscopic Techniques ◽  
X Ray ◽  
Uv Visible ◽  
X Ray Absorption

The role of diamines incorporated in a nickel oxide electrocatalyst for water splitting was examined using operando UV/XAFS spectroscopic techniques.

Photosystem II: an enzyme of global significance

2006 ◽  
Vol 34 (5) ◽  
pp. 619-631 ◽  
Author(s):  
J. Barber
Keyword(s):  
Photosystem Ii ◽  
Water Splitting ◽  
Organic Molecules ◽  
Metal Cluster ◽  
Thylakoid Membranes ◽  
Chemical System ◽  
X Ray ◽  
X Ray Crystallography ◽  
Structural Details ◽  
Lipid Environment

Photosystem II (PSII) is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Powered by light, this enzyme catalyses the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases dioxygen into the atmosphere and provides the reducing equivalents required for the conversion of CO2 into the organic molecules of life. Recently, a fully refined structure of a 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography which gave organizational and structural details of the 19 subunits (16 intrinsic and three extrinsic) which make up each monomer and provided information about the position and protein environments of 57 different cofactors. The water-splitting site was revealed as a cluster of four Mn ions and a Ca2+ ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was modelled as a cubane-like structure composed of three Mn ions and the Ca2+ linked by oxo-bonds with the fourth Mn attached to the cubane via one of its oxygens. The overall structure of the catalytic site is providing a framework to develop a mechanistic scheme for the water-splitting process, knowledge which could have significant implications for mimicking the reaction in an artificial chemical system.

Solution X-ray Absorption Fine Structure Study of the Eu2+and Sr2+Ions:  Unexpected Solvent and Metal Ion Dependencies of the Solvation Numbers

2002 ◽  
Vol 106 (41) ◽  
pp. 9612-9622 ◽  
Author(s):  
Gilles Moreau ◽  
Rosario Scopelliti ◽  
Lothar Helm ◽  
Juris Purans ◽  
André E. Merbach
Keyword(s):  
Fine Structure ◽  
Metal Ion ◽  
Structure Study ◽  
X Ray ◽  
Absorption Fine ◽  
Solvation Numbers ◽  
X Ray Absorption

A comparative assessment of the zinc–protein coordination in 2Zn–insulin as determined by X-ray absorption fine structure (EXAFS) and X-ray crystallography

1983 ◽  
Vol 219 (1214) ◽  
pp. 21-39 ◽  
Keyword(s):  
Fine Structure ◽  
Metal Ion ◽  
Radial Distance ◽  
Exafs Spectrum ◽  
Ion Concentration ◽  
Model Compounds ◽  
X Ray ◽  
Absorption Fine ◽  
X Ray Absorption ◽  
Exafs Spectra

Accurate ( ca .0.1Å) knowledge of the metal environment in metalloproteins is essential to understanding their function. Single crystal X-ray analysis has provided detailed descriptions of metal environments in a number of crystallizable proteins but their accuracy has often been limited by their restricted diffraction patterns. The technique of X-ray absorption fine structure (EXAFS) is not limited to crystals and can provide very accurate radial distances between metal ions and their ligands. It has therefore great potential for the study of biochemical metal-containing systems in solution. The method depends on the analysis of the oscillations in the absorption or fluorescence spectrum extending over several hundreds of electron volts above the metal absorption edge. The very intense synchrotron X-ray sources make EXAFS applicable to biological systems where the metal ion concentration is low, typically in the millimolar range. We have determined EXAFS spectra for 2Zn–insulin and a variety of Zn–ligand model compounds of known crystal structures in both the absorption and the fluorescence modes. The X-ray crystallographic refinement of 2Zn-insulin with 1.5 Å data provides estimated standard deviations for well defined atoms ranging from 0.03 to 0.06 Å which thus determine the zinc-ligand distances sufficiently accurately for comparison with those derived from EXAFS. The experimental procedures for obtaining the spectra with the use of the storage ring doris at the Deutsche Elektrononen Synchrotron (DESY) are described. The shapes of the exafs spectra of 2Zn-insulin and the zinc complexes are remarkably similar. These results emphasize one of the major weaknesses of the technique: the difficulty in distinguishing between atoms of similar atomic mass such as oxygen and nitrogen in the present instance. The small but real effect of solvent on the spectrum of one complex has important implications; it reveals the ability of EXAFS to provide evidence for structural changes in the metal coordination and that other structures possibly more relevant to function may exist. For the detailed analysis we used ab initio calculations on the model compounds. The method used is outlined and the programs are cited; the theoretical basis for these calculations is to be found in Lee & Pendry ( Phys . Rev . B 11, 2795-2811 (1975)). Its application to the 2Zn-insulin EXAFS spectrum showed that there is good agreement in both the exafs and the X-ray crystallographic methods for the CE1 and NE2 to Zn radial distance; significant discrepancies, however, exist for the other atoms in the coordinating structure. This failure stems essentially from the smaller contribution of outer atoms to the EXAFS spectrum. To resolve these correctly the amplitudes in the EXAFS spectrum need to be more accurate, and there needs to be a more adequate theory to deal with multiple scattering effects. Until these have been achieved it is probably more profitable to make use of our exact knowledge of the bonding behaviour and geometry of such coordinating groups as imidazole rings.

scholarly journals Structure and Photocatalytic Activity of PdCrOx Cocatalyst on SrTiO3 for Overall Water Splitting

Catalysts ◽  
2019 ◽  
Vol 9 (1) ◽  
pp. 59 ◽  
Author(s):  
Tomoki Kanazawa ◽  
Shunsuke Nozawa ◽  
Daling Lu ◽  
Kazuhiko Maeda
Keyword(s):  
Fine Structure ◽  
Water Splitting ◽  
Photoelectron Spectroscopy ◽  
Uv Light ◽  
Irradiation Time ◽  
X Ray ◽  
Overall Water Splitting ◽  
Uv Light Irradiation ◽  
Transmission Electron ◽  
X Ray Absorption

The mechanism of PdCrOx multi-component cocatalyst formation on SrTiO3 was investigated using transmission electron microscopy, X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy. The PdCrOx/SrTiO3 samples were synthesized by a photodeposition method under UV light irradiation (λ > 300 nm) for various time periods (0–5 h). The fine structure and valence state of the Pd species of PdCrOx nanoparticles were varied from Pd oxide to a mixture of metallic Pd and oxidized Pd species with an increase in the irradiation time. The overall water-splitting activity of PdCrOx was strongly dependent on the photoirradiation time during deposition. Although longer photoirradiation time during preparation did not influence the H2 evolution activity of PdCrOx/SrTiO3 from aqueous methanol solution, it was effective in suppressing the O2 photoreduction activity, which is one of the backward reactions during overall water splitting.

Extended Core Edge Fine Structure in the E.M.

1980 ◽  
Vol 38 ◽  
pp. 120-121
Author(s):  
R.D. Leapman
Keyword(s):  
Fine Structure ◽  
Electron Scattering ◽  
Inelastic Electron ◽  
High Resolution Imaging ◽  
X Ray ◽  
Analytical Electron ◽  
Analytical Electron Microscope ◽  
Core Electrons ◽  
Resolution Imaging ◽  
X Ray Absorption

Extended X-ray Absorption Fine Structure (EXAFS) analysis makes use of synchrotron radiaion to measure modulations in the absorption coefficient above core edges and hence to obtain information about local atomic environments. EXAFS arises when ejected core electrons are backscattered by surrounding atoms and interfere with the outgoing waves. Recently, interest has also been shown in using inelastic electron scattering1-4. Some advantages of Extended X-ray-edge Energy Loss Fine Structure (EXELFS) are: a) small probes formed by the analytical electron microscope give spectra from μm to nm sized areas, compared with mm diameter areas for the X-ray technique, b) EXELFS can be combined with other techniques such as electron diffraction or high resolution imaging, and c) EXELFS is sensitive to low Z elements with K edges from ˜200 eV to ˜ 3000 eV (B to Cl).

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