X-ray Absorption Near-edge Structure Signals Research Articles

Interactions of disulfide-constrained cyclic tetrapeptides with Cu(2+).

The purpose of this work is to characterize the interactions of two disulfide-constrained cyclic tetrapeptides [c(Ac-Cys-Pro-Phe-Cys-NH(2)), SS1; c(Ac-Cys-Pro-Gly-Cys-NH(2)), SS2] with Cu(2+) ions in order to facilitate the design of cyclic peptides as sensors for metal ions. The Cu(2+)-peptide complex cations at m/z 569.1315 for Cu(2+)-SS1 and m/z 479.0815 for Cu(2+)-SS2 were detected by mass spectrometry. The gas-phase fragmentation of the Cu(2+)-peptide complexes was studied by collision-induced dissociation and suggests the atoms involved in the coordination. Cu(2+) ion binds to a single SS1 or SS2 with K (d(app)) of 0.57 ± 0.02 and 0.55 ± 0.01 μM, respectively. Isothermal titration calorimetry data indicate both enthalpic and entropic contributions for the binding of Cu(2+) ion to SS1 and SS2. The characteristic wavenumber of 947 cm(-1) and the changes at 1,664 and 1,530 cm(-1) in the infrared spectrum suggest that the sulfydryl of cysteine, the carbonyl group, and amide II are involved in the coordination of Cu(2+). The X-ray absorption near-edge structure signal from the Cu(2+)-peptide complex corresponds to the four-coordination structure. The extended X-ray absorption fine structure and electron paramagnetic resonance results demonstrate the Cu(2+) ion is in an S/N/2O coordination environment, and is a distinct type II copper center. Theoretical calculations further demonstrate that Cu(2+) ion binds to SS1 or SS2 in a slightly distorted tetragonal geometry with an S/N/2O environment and the minimum potential energy.

Speciation of AsIII and AsV in hydrothermal fluids by in situ X-ray absorption spectroscopy

The speciation of arsenic (III) and (V) in aqueous acidic and basic solutions at ambient conditions, and of arsenic (V) in acidic solutions at 270 bars from 30 to 300 degreesC was investigated using in situ X-Ray Absorption Spectroscopy (XAS). Both Extended X-Ray Absorption Fine Structure (EXAFS) analysis and X-Ray Absorption Near Edge Structure (XANES) ab initio calculations were used to determine the structure of dominant arsenic species in solution. At ambient conditions, the deprotonation of AsIII(OH)3 and AsVO(OH)3 species with increasing pH is demonstrated by analysis of their XANES signals, in agreement with available arsenious and arsenic acid dissociation constants. In basic conditions, the dominant species is a deprotonated form of the As(OH)3 acid, with a very similar symmetry. The speciation of AsV in acidic hydrothermal fluids is dominated by the AsVO(OH)3 complex over a wide temperature and pressure range, to at least 300 degreesC and 300 bars. The first-shell As structure in the AsVO(OH)3 species and its deprotonated forms is found to be a distorted tetrahedron, similar to the base AsO4 unit of Na2HAsO4middle dot7H2O(s). This work demonstrates new capabilities of XAS coupled with molecular modelling to decipher deprotonation reactions that affect mineral solubilities and chemical elements mobility in aqueous crustal fluids.

Oxygen K-edge x-ray absorption near-edge structure in crystalline and amorphousmolybdenum trioxides

X-ray absorption near-edge structure (XANES) signals at the oxygen K edge in polycrystallineα-MoO3 andamorphous a-MoO3 thin film were analysed within the full-multiple-scattering (FMS) formalism. Significantly differentXANES signals were found for non-equivalent oxygen atoms in low-symmetry layered-typeα-MoO3 structure. The obtained results are in agreement with theexperimental data and allow us to interpret all XANES peaks forα-MoO3. Besides, the FMS XANES simulations, performed for several fragments ofα-MoO3 structure, allowed us to explain the O K-edge XANES in amorphousa-MoO3 thin film. We found that although the crystallographic structures ofα-MoO3 and a-MoO3 are strongly different, a cluster consisting of sixMoO6 octahedra joined by vertices produces the main contribution to both XANES signals.