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Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.


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Journal ArticleDOI
TL;DR: It is shown that two structurally distinct types of zinc sites are available in the enzyme, including the eight-zinc native enzyme and the enzyme with the vacant sites of (b) occupied by four lead ions.
Abstract: The zinc coordination in 5-aminolevulinate dehydratase was investigated by extended x-ray absorption fine structure (EXAFS) associated with the zinc K-edge. The enzyme binds 8 mol of zinc/mol of octameric protein, but only four zinc ions seem sufficient for full activity. The authors have undertaken a study on four forms of the enzyme: (a) the eight-zinc native enzyme; (b) the enzyme with only the four zinc sites necessary for full activation occupied; (c) the enzyme with the vacant sites of (b) occupied by four lead ions; (d) the product complex between (b) and porphobilinogen. They have shown that two structurally distinct types of zinc sites are available in the enzyme. The site necessary for activity has an average zinc environment best described by two/three histidines and one/zero oxygen from a group such as tyrosine or a solvent molecule at 2.06 {plus minus} 0.02 {angstrom}, one tyrosine or aspartate at 1.91 {plus minus} 0.03 {angstrom}, and one cysteine sulfur at 2.32 {plus minus} 0.03 {angstrom} with a total coordination of five ligands. The unoccupied site in (b) is dominated by a single contribution of four cysteinyl sulfur atoms at 2.28 {plus minus} 0.02 {angstrom}. Spectra from samples (c) and (d) show onlymore » small changes from that of (b), reflecting a slight rearrangement of the ligands around the zinc atom.« less

88 citations

Journal ArticleDOI
TL;DR: In this article, the local structure of iron monomers and dimers isolated in a rare gas Ar matrix using extended X-ray absorption fine structure measurements was investigated and compared with Mossbauer measurements on such systems.

87 citations

Journal ArticleDOI
TL;DR: In this article, an extensive investigation of the extended x-ray-absorption fine structure (EXAFS) at the L edges of the rare-earth atoms of aqueous ionic solutions was presented.
Abstract: An extensive investigation of the extended x-ray-absorption fine structure (EXAFS) at the L edges of the rare-earth atoms of aqueous ionic solutions of ${\mathrm{La}}^{3+}$, ${\mathrm{Ce}}^{3+}$, ${\mathrm{Ce}}^{4+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Pr}}^{3+}$, ${\mathrm{Eu}}^{3+}$, ${\mathrm{Dy}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ at concentrations of 50, 100, and 200 mM, is presented. The presence of anomalous peaks, appearing in the range from 5 to 7 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ and superimposed to the main single-frequency oscillatory signal, has been explained as due to double-electron transitions 2p4d\ensuremath{\rightarrow}5${\mathit{d}}^{2}$ in the case of ${\mathit{L}}_{3}$ and ${\mathit{L}}_{2}$ edges, and 2s4d\ensuremath{\rightarrow}6p5d for the ${\mathit{L}}_{1}$ spectra. The energy of the double-excitation absorption edge increases as the atomic number of the element of the rare-earth series is increased and is in fair agreement with previous theoretical bound-to-bound calculations. The intensity of the anomalous feature decreases for increasing Z numbers, as expected from theory, but the intensity values, calculated from comparison with the main single-electron absorption line, are lower than those calculated by other authors and the double-excitation peak disappears in the ${\mathrm{Tm}}^{3+}$ spectrum. A structural analysis of the EXAFS spectra was also carried out with the twofold aim of characterizing rare-earth water solutions and quantifying the errors introduced in the structural parameters by the mixing of single- and double-electron phenomena. The results show the rare-earth ions are always surrounded by 12 water molecules and the rare-earth--O distance decreases with Z number, varying from 2.56 \AA{} for ${\mathrm{La}}^{3+}$ down to 2.32 \AA{} for ${\mathrm{Tm}}^{3+}$. The presence of the anomalous peaks introduces small errors in the bond-length's determination, the effect being proportional to the magnitude of the double-excitation peak.

87 citations

Journal ArticleDOI
TL;DR: In this paper, the structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS.
Abstract: The structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS. In situ EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided (at 673 K) catalysts. Due to the high signal-to-noise ratio of the experimental data, detailed information about the structural parameters of the active phase could be obtained. The catalysts probably consist of small MoS2-like particles with an auerage local ordered structure of5-6 Mo atoms (Mo/C) and 7-8 Mo atoms (Co-Mo/C). Comparison with literature data shows that the MoS2 dispersion in these carbon-supported catalysts is probably as good as in their alumina-supported counterparts. A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms in both catalysts with a Mc-C coordination distance of 2.1 * 0.1 A. This short distance could imply an intimate interaction between the active phase and the carbon support, which may explain the high active-phase dispersion. From the value of 2.1 8, it can be inferred that the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur vacancy. The high-quality EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms in the Co-Mo/C catalyst, with a Mo-Co coordination distance of 2.8 f 0.1 A.

87 citations

Journal ArticleDOI
TL;DR: In this paper, an anharmonic oscillator model is applied to the EXAFS data of polycrystalline Zn to correct the values of R1 and σ12.
Abstract: Extended X-ray absorption fine structure (EXAFS) results are presented for solid and liquid Zn for the temperature range 273 to 782 K. At the higher temperatures the distribution of nearest neighbour atoms is asymmetric. Neglect of the asymmetry and use of EXAFS analysis methods developed for symmetric distribution functions gives an underestimate of the nearest neighbour distance R1 and the disorder parameter σ12. It is shown that a transform analysis can be useful in detecting asymmetric effects, and methods of analyzing the EXAFS of asymmetric distributions are presented. An anharmonic oscillator model is applied to the EXAFS data of polycrystalline Zn to correct the values of R1 and σ12. The corrected values of σ12 are found to be~20% higher than those predicted by a Debye model. An empirical, analytically simple asymmetric distribution function is fitted to the liquid data to obtain promising agreement with the nearest neighbour structure as deduced from X-ray diffraction studies. Finally it is shown...

87 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023110
2022264
2021156
2020164
2019164
2018151