1,720,999 research outputs found
Nuovi sviluppi nell‘ analisi strutturalequantitativa di spettri XAS: applicazioni biologiche
No full textEXAFS and XANES simulations of Fe/Co hexacyanoferrate spectra by GNXAS and MXAN
No full textIn the present paper Full Multiple Scattering (FMS) theory has been applied to analyse the cobalt hexacyanoferrate XANES spectra. The use of the MXAN program has permitted to calculate the edge spectra and to perform a fitting procedure with the experimental data, obtaining a set of structural parameters. A previously reported EXAFS analysis (which includes terms up to four-body MS calculations) was performed via the GNXAS package, by a Multiple Edge approach of both Fe/Co K-edges and Fe/Co/Ni K-edges, and using the same structural parameters for all edges. The XANES data of Fe and Co K-edges are independently analysed here. An excellent reproduction of the XANES spectra, and a good agreement with the previous EXAFS results is obtained. The CN bond length using EXAFS has been determined with a statistical error of few thousandths of Å, whereas structural parameter using MXAN are probed within a 0.01-0.03 Å accuracy
Combined XANES and EXFAS analysis of Co2+, Ni2+ and Zn2+ aqueous solutions
No full textA combined x-ray-absorption near edge structure (XANES) and extended x-ray-absorption fine-structure (EXAFS) quantitative analysis of Co2+, Ni2+, and Zn2+ in water solutions based on fitting procedures of both low- and high-energy ranges has been carried out. The hydrogen contribution has been accounted for in both analyses and the effect of its inclusion on the structural parameters has been highlighted. The structural results obtained from the XANES and EXAFS analyses are in good agreement, confirming the validity of the application of the multiple-scattering theory in the low-energy range of the x-ray-absorption spectra. A systematic shortening of the ion-water first shell distance of about 0.03 Å is obtained from the XANES analyses, as compared to the EXAFS one. The origin of this systematic effect has been deeply investigated and it has been found to be due to the low-energy behavior of the real part of the Hedin-Lundqvist potential used in the calculations
Hemeproteins: Recent Advances in Quantitative XANES Analysis
No full textRecently, we have shown that multiple scattering (MS) theory, via the MXAN package, is able to reproduce the experimental X-ray absorption near edge structure (XANES) data of biological samples, in particular hemeproteins, from the rising edge up to similar to 150-200 eV above the edge. In the present work, we illustrate how XANES can be used either as an independent tool to provide bond-lengths and bond-angles for a metalloprotein active site in solution, or in combination with X-ray Diffraction for structural determinations of ligand binding geometry of the same diffracting protein crystal, providing atomic precision even for crystallographic structures solved at > 1.2 angstrom resolution. At low temperature, XANES can be applied to provide the Fe-heme structure of trapped intermediate conformations; of fight triggered processes, and some aspects of the ligand binding dynamics. Very recently, XANES difference spectra have been analyzed to provide the Fe-heme structure of multiple intermediates of carbonmonoxy-myoglobin, obtained under different photolysis protocols in solution state
A crystal-chemical investigation of Cr substitution in muscovite by XANES spectroscopy.
No full textThis work introduces a novel procedure to fit the scattering features of XANES spectra. The procedure was applied and validated on, a chromium-containing muscovite for which structural and crystal-chemical characterization is available from literature. The simulation, which involved clusters formed by up to 90 atoms, proved to be effective in representing the Cr local environment, even if the system is characterized by a very low metal content, as demonstrated by the extremely good correspondence with experimental data
Structure and dynamics of hemeproteins using X-ray Absorption Spectroscopy
No full textExperimental investigations on protein dynamics are usually carried out in solution; however, in most cases, the determination of protein structure is done by crystallography that relies on the diffraction properties of the protein arranged in a three-dimensional lattice. X-ray Absorption Spectroscopy (XAS) is an ideal tool to investigate the deep relationships between structure and dynamics, on a wide class of metal containing proteins, including hemeproteins. In fact, this technique can be used either as a structural refinement of X-ray diffraction on the same crystal, or as an independent tool to study structural dynamic properties both in crystal and solution. Different examples will be presented and discussed, with particular emphasis onto the original contribution due to the application of the MXAN software package to the analysis of XANES spectra. © 2009 IOP Publishing Ltd
Progresses in the MXAN Fitting Procedure
No full textRecently, a new method (MXAN) of extracting the local structural information available in the XANES spectra has been developed in the framework of the multiple scattering theory and successfully applied to the analysis of several system, both in solid and liquid state. In this paper we show the progress we have done in the MXAN procedure, in particular for what concerns the phenomenological broadening and the electronic charge fitting. We have also implemented parallelization on MXAN code by MPI library for both energy points and annealing for no-structure parameters. This approach reduce the computer time of a factor approximate to 10 (our cluster contains 14 nodes but the computer times scales with the number of nodes) and at the same time increase the accuracy of the method. The new version of the program will be discussed on the basis of several examples
Xanes and Exafs of Yba2cu3o-approximately-7 - Multiple-scattering Data-analysis and Evidence For A Structural Distortion At Tc
No full textThe polarized copper K-edge x-ray absorption near edge structure (XANES) of YBa2Cu3O7 has been calculated by using the full multiple scattering approach. The Cu K-edge EXAFS of YBa2Cu3O≈7 as a function of temperature is also analyzed. From this analysis we obtain evidence for a slight structural modification at Tc which can be associated with the superconductive transition
Detection of second hydration shells in ionic solutions by XANES: Computed spectra for Ni2+ in water based on molecular dynamics
No full textA general procedure to compute X-ray absorption near-edge structure (XANES) spectra within multiple-scattering theory starting from molecular dynamics (MD) structural data has been developed and applied to the study of a Ni2+ aqueous solution. This method allows one to perform a quantitative analysis of the XANES spectra of disordered systems using a proper description of the thermal and structural fluctuations. The XANES spectrum of Ni2+ in aqueous solution has been calculated using the structural information obtained from the MD simulations without carrying out any minimization in the structural parameter space. A very good reproduction of the experimental data was obtained including the second-shell water molecules in the calculation, thus showing that the second hydration shell provides a detectable contribution to the XANES spectra of ionic solutions. The analysis including the first-shell cluster only permitted us to quantitatively determine the effect of disorder on the amplitude of the XANES spectra for molecular complexes. These results simultaneously confirm the reliability of the procedure and the structural results obtained from MD simulations. The combination of MD and XANES is found to be very helpful to get important new insights into the quantitative estimation of structural properties of disordered systems
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