1,720,985 research outputs found
A quasi-solid state electrochemical cell for in situ EXAFS measurements on biological samples
No full textSaccharose solid matrix enbedded proteins: a new method fro XAS sample preparation
No full texti.f. 1,950; biophysic
In situ X-ray absorption studies of the electrochemical reduction of hydroxoeobalamin
No full textXAS fluorescence is combined with electrochemical technique to study the reduction of hydroxocobalamin ( OH-Co(III)Cbl ) to cob(II)alamin at the cobalt K-edge. The electrochemical control permits to select a well-defined cobalt oxidation states. The experimental set-up used permits in situ reactions allowing reliable comparison among XANES spectra. The absorption edge shifts to lower energy with the progressive reduction of cobalt. The size of the peak at about 7726 eV (A peak) decrease in Co(H)cbl spectrum. Experimental XANES of OH-Co(III)cbl and Co(II)cbl were simulated by multiple scattering theory. Theoretical and experimental data are in agreement, peak intensity being correlated to the coordination number confirming that cobalt is five coordinate in Co(II)Cbl sample
X-ray absorption spectroscopy and electrochemistry on biological samples
No full textIn order to study metalloproteins, which change the metal oxidation states during the catalytic cycle, we have developed an electrochemical cell for in situ XAS measurement on biological samples. To be able to use proteins and mutants that are usually available in small quantities the cell was designed to minimise: a) cavity of RVC working electrode and b) cavities for electric contact between RVC working electrode and the other electrodes (counter and reference). The sample volume of 0.4 ml is sufficient for measurements at several applied potentials. We have investigated the reduction of (a) the hydroxocobalamin (from Co(III) to Co(I)) and (b) microperoxidase (from Fe(III) to Fe(II)). We have then determined the correct energy shift of XANES in the two systems. In the case of hydroxocobalamin, reduction from Co(III) to Co(II) produces the most significant structural changes (Giorgetti et al. 1997) The reduction from Co(II) to Co(I) produces mainly electronic effects with no apparent change of the coordination number. Microperoxidase XANES spectrum shifts by 1 eV ±0.5 eV upon oxidation
X-ray absorption spectroscopic study of "costa type" organocobalt coenzyme B12 models
No full textThis paper presents the results of an X-ray absorption spectroscopy (XAS) study of six "Costa type" organocobalt B12 model compounds [LCo(DO)(DOH)pnR]ClO4, where L = H2O and (DO)(DOH)pn = N2,N2′-propanediylbis(2,3-butanedione 2-imine 3-oxime) and the R groups are different axial alkyl ligands. The dibromo and dichloro complexes [Co(DO)(DOH)pnX2] have been studied as well. The structural data obtained from the EXAFS spectra, namely the Co-C bond lengths, have been correlated with the pKa and Taft σ* of the alkyl group
The dinuclear copper site structure of Agaricus bisporus tyrosinase in solution probed by X-ray absorption spectroscopy
No full textWe have measured the x-ray absorption near edge structure (XANES) spectra of the enzyme tyrosinase from the mushroom Agaricus bisporus in solution in the oxy and deoxy forms, The spectra, obtained under the same conditions as the analogous forms of mollusc hemocyanin (Hc), show that the oxidation state of copper changes from Cu(II) (oxy form) to Cu(I) (deoxy form), and the copper active site(s) of A. bisporus tyrosinase in solution undergoes the same main conformational changes as Hc. We have applied the multiple scattering theory to simulate the XANES spectra of various alternative geometries of the copper site, accounting for the residual differences between Hc and tyrosinase. While oxy-Hc is reasonably fitted only by the pseudo-square-pyramidal geometry reported by its crystallographic data, oxytyrosinase can be fitted, starting from the Hc coordinates, either by distortions toward a pseudo-tetrahedral geometry, with inequivalent copper sites, or by an apically distorted square-pyramidal geometry (with an elongation of the apical distance of no more than 0.2 Angstrom)
Structural studies on polygalacturonate gels: an EXAFS investigation combined with molecular modelling
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Comparison of the X-ray absorption properties of the binuclear active site of molluscan and arthropodan hemocyanins
No full textThe structural characteristics of oxy- and deoxy-hemocyanins have been investigated using X-ray absorption spectroscopy both in the near-edge (XANES) and for the first shell contribution in the EXAFS region. Several arthropodan and molluscan hemocyanins have been studied in order to trace the inter- and intra-phyla differences. The XANES spectra of oxy-hemocyanins of the different species are remarkably similar, consistent with a very strongly conserved co-ordination geometry of the copper active site. In contrast, small but significant differences are observed between the deoxy-forms of arthropodan and molluscan proteins. In particular, the XANES spectra of deoxy-arthropodan hemocyanins (with the exception of L. polyphemus Hc) show a more intense edge feature at approximately 8983 eV. This difference is tentatively assigned to a more planar geometry of the copper-ligands system in the arthropodan rather than in the molluscan proteins. The first shell analysis of the EXAFS modulation is consistent with the presence of n = 3N∈2 imidazole nitrogens at an average distance of 1.92±0.03 Å from copper in all the deoxy-hemocyanins investigated. Binding of dioxygen results for all hemocyanins in the increase of the number of first shell back-scattering atoms to n = 5 with average distances of 1.93 Å. Alternatively, by separating the contribution of N∈2 imidazole nitrogens and of peroxide O-atoms, n = 3 ligands at 1.98 ± 0.03 Å and n = 2 ligands at 1.87 ± 0.03 Å are found
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