92 research outputs found
XAS STUDY OF SOLUBILIZATION LOCI OF BROMINATED MOLECULES IN AQUEOUS MICELLAR SOLUTIONS
No full textX-ray absorption measurements performed on various systems containing brominated hydrocarbons showed
remarkable differences in the bromine K-edge spectra recorded in polar or nonpolar media. For this reason,
the brominated hydrocarbons can be used to monitor the polarity of the medium in which they are buried. By
determining the coordination of the bromine atom, information on the interactions between the probe molecule
and several systems such as micelles, macromolecules, membranes, solvent molecules, and host molecules in
inclusion compounds can be obtained. Brominated hydrocarbons in aqueous micellar solutions of sodium and
rubidium deoxycholate and sodium dodecyl sulfate have been investigated by means of the XAS technique.
Experimental and calculated EXAFS spectra and Fourier transform functions are presented. The results are
supported by the XANES experimental spectra. As already found for 2-bromopropane, the polarity of the
solubilization loci of bromoethane in sodium and rubidium deoxycholate micellar solutions decreases by increasing
the solute concentration. On the contrary, in micellar solutions of sodium dodecyl sulfate, these molecules are
embedded in a more apolar environment. The polarity of the spectra of 2-bromopropane changes with the probe
concentration, while bromoethane presents a marked apolar coordination which seems to be independent from
the probehost ratio. The bromine intermolecular coordination in 1-bromobutane has been found to be apolar
in both classes of surfactants. Functionalized surfactants with -ene, -yne, or bromo as the terminal group of
the alkyl chain were used by different researchers to investigate micellar structures. In fact, for a micellar
aggregate with a hydrocarbon core and an outer region containing the polar heads, the chain terminal group
may be buried in the micelle core or placed in the head group region of the micelle, in contact with water. In
the latter case, the alkyl chain flexibility may give rise to a chain folded conformation which increases the
probability of finding the terminal group at the micelle-water interface. EXAFS data analysis of aqueous
micellar solutions of sodium and rubidium 12-bromo dodecyl sulfate and sodium 1 I-bromoundecanoate at the
Br K-edge has been accomplished. The chemical constitution of the locus of solubilization of the terminal
bromine has been determined, and it has been found to be apolar for the dodecyl sulfate salts and polar in the
case of sodium 1 1-bromoundecanoate
Da Origene a Gerolamo e ritorno: primi appunti su Maria «nomomathes»
No full textThe article focuses on the use of the adjective «nomomathes» in reference to Mary, the mother of Jesus. After a preliminary contextualization of the term both in Greek inscriptions from the Jewish diaspora in the West (2nd - 4th centuries CE) and in contemporary Christian literature, I analyze its occurrences and interpretation in Origen’s Homilies on Luke as well as in their Latin translation by Jerome and in the latter’s Homily on the Nativity of the Lord. The evolution of the characterization of Mary as a reader, interpreter, and exegete of the Scriptures is thus highlighted
X-ray Absorption Spectroscopy Study on the Electrochemical Reduction of Co((DO)DOH)pn)Br2
No full textThe reduction process of Co-III((DO)(DOH)(pn))Br-2 has been studied by X-ray absorption spectroscopy. A structural rearrangement takes place during the first reduction step, from Co-III to Co-II. It involves the cleavage of both axial bromide ligands. However, the reduction from Co-II to Co-I proceeds with retaining of the coordination geometry. These findings have also been confirmed by an 'ad hoc' electrochemical experiment. (C) 2000 Elsevier Science Ltd. All rights reserved
A quasi-solid state electrochemical cell for in situ EXAFS measurements on biological samples
No full textIn situ XAS studies of the elctrochemical reduction of hydroxocolamin
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(II)cbl spectrum. Experimental XANES of OH-Co(ITI)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
In-situ X-ray absorption spectroelectrochemical study of hydroxocobalamin
No full textAn in situ X-ray absorption spectroscopy (XAS) spectroelectrochemical study of aquocobalamin (system B(12a)-B(12r)-B(12s)) has been carried out in aqueous solutions buffered at different pH values. To the best of our knowledge, this is the first structural study of aquocobalamin at room temperature under controlled oxidation conditions. Most of the previous work was in fact performed using frozen samples chemically treated to produce the species. The spectroelectrochemical approach offers several advantages: (1) the reduction products may be studied without poisoning the system with chemical reductive reagents and (2) any possible variation of the oxidation state owing to the electrons produced by the incident beam is avoided as the electrode, under potentiostatic control, acts as a scavenger. The spectroelectrochemical approach, together with more careful data analysis, has led to an improved interpretation of the XAS data. These conditions were not met in previous works where the oxidation state was not controlled and multiple scattering contributions were not taken into account. The general shape of the XAS spectra of the different species is not greatly affected by pH. A signature for the base-off square-planar coordination has been evidenced for the Co(II) compound at basic pH. A new signature for Co(I), indicating square-planar coordination, has been identified on the experimental spectra and simulated in theoretical X-ray absorption near-edge structure (XANES) studies. The flexibility of the electrochemical approach, that permits to unambiguously establish the formal oxidation state, has led to very reliable values for energy shift and peak intensity variations. The experimental XANES and extended X-ray absorption fine structure (EXAFS) spectra with a very good signal-to-noise ratio have been processed using the GNXAS package that takes into account multiple scattering contributions. EXAFS and XANES independent analysis result in the same structural model. The reduction from Co(III) to Co(II) produces the most significant structural changes: the cobalt coordination number decreases from six to five, and the edge position shifts by 2.4 ± 0.3 eV. In addition, the XANES spectra are strongly modified. The reduction from Co(II) to Co(I) produces mainly electronic effects with no apparent change of the coordination number. A discussion of the limits and potentialities of EXAFS in this type of study has also been included
Saccharose 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
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