Local relaxation in lanthanum silicate oxyapatites by Raman scattering and MAS‐NMR

To characterize the local relaxation in the structure of lanthanum silicate oxyapatite materials, six compositions with different cation and oxygen stoichiometries (La₈Ba₂Si₆O₂₆, La₉BaSi₆O_26.5, La₁₀Si_5.5Mg_0.5O_26.5, La_9.33SiO₂₆, La_9.67SiO_26.5 and La_9.83Si_5.5Al_0.5O_26.5) were investigated by combining Raman scattering and ²⁹Si and ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS‐NMR) spectroscopies. Only [SiO₄]⁴⁻ species were evidenced and the hypotheses of [Si₂O₇]⁶⁻ and [Si₂O₉]⁸⁻ entities were ruled out. Both oxygen excess and cation vacancies induce local distortions in the structure, which leads to nonequivalent [SiO₄]⁴⁻ species, characterized by different ²⁹Si MAS‐NMR signals and by splitting of Raman signals. Copyright © 2011 John Wiley & Sons, Ltd.     

Concerted and stepwise proton-coupled electron transfers in aquo/hydroxo complex couples in water: oxidative electrochemistry of [Os(II)(bpy)(2)(py)(OH(2))](2+)

Successive oxidation of transition metal(II) aqua complexes (M(II)OH(2) to M(III)OH) is a domain in which proton-coupled electron transfer reactions are extremely common. The mechanism of these PCET reactions-concerted or stepwise-is an important issue in the understanding and design of natural or artificial systems catalyzing the formation of dioxygen by four-electron oxidation of water. Concerted proton-coupled electron transfer from an aqua metal(II) to a hydroxo metal(III) complex requires the close proximity of a proton-accepting group with a pK value between those of the aqua complexes. Otherwise, stepwise electron-proton or proton-electron pathways involving high-energy intermediates are followed. Concerted proton-electron pathways involving water as proton-acceptor or proton-donor group are inefficient. Cyclic voltammetry of the title complex in buffered aqueous solution and re-examination of previous results for the same complex attached to an electrode surface are used to establish these conclusions, which provide a starting point on the route to higher degrees of oxidation, such as those involved in the catalysis of water oxidation.     

An EPR study of the C-O bond activation of 1,2-epoxybutane by ground-state Al atoms

Group 13 metal atoms react with ethers under matrix isolation conditions to give a number of interesting products. This work has been extended to include the reaction of Al atoms with 1,2-epoxybutane (CH(3)CH(2)H(2)) and its isotopomers, 1,2-epoxybutane-1,1-d(2) (CH(3)CH(2)D(2)) and 1,2-epoxybutane-2-d(1) (CH(3)CH(2)H(2)). The paramagnetic species generated in the reaction have been studied by electron paramagnetic resonance (EPR) spectroscopy. Two divalent Al insertion products were spontaneously formed. Species A, with the magnetic parameters a(Al) = 855 MHz, a(H)(1) = 28.8 MHz, a(H)(2) = 13.6 MHz, and g = 2.0014, is the C(1)-O insertion radical CH(3)CH(2). Species B, thought to result from the insertion of Al atoms into the C(2)-O bond, CH(3)CH(2), has the magnetic parameters g = 2.0003, a(Al) = 739 MHz, a(H)(1) = 15.1 MHz, a(H)(2) = 18.5 MHz, and a(H)(1) = 37.8 MHz. Support for these assignments was obtained by comparing the experimental values of the Al and H hyperfine interaction (hfi) with those calculated using a DFT method. At temperatures < 150 K, there is evidence for the formation of the alkyl radical CH(3)CH(2)CH(O(-))CH(2)* due to ring opening at the C(1)-O bond, while at higher temperatures a radical with magnetic parameters similar to those reported for 1-methallyl was detected.     

Structural characterization of heparins from different commercial sources

Seven commercial heparin active pharmaceutical ingredients and one commercial low molecular weight from different manufacturers were characterized with a view profiling their physicochemical properties. All heparins had similar molecular weight properties as determined by polyacrylamide gel electrophoresis (M _N, 10–11 kDa; M _W, 13–14 kDa; polydispersity (PD), 1.3–1.4) and by size exclusion chromatography (M _N, 14–16 kDa; M _W, 21–25 kDa; PD, 1.4–1.6). one-dimensional ¹H- and ¹³C-nuclear magnetic resonance (NMR) evaluation of the heparin samples was performed, and peaks were fully assigned using two-dimensional NMR. The percentage of glucosamine residues with 3-O-sulfo groups and the percentage of N-sulfo groups and N-acetyl groups ranged from 5.8–7.9%, 78–82%, to 13–14%, respectively. There was substantial variability observed in the disaccharide composition, as determined by high performance liquid chromatography (HPLC)-mass spectral analysis of heparin lyase I–III digested heparins. Heparin oligosaccharide mapping was performed using HPLC following separate treatments with heparin lyase I, II, and III. These maps were useful in qualitatively and quantitatively identifying structural differences between these heparins. The binding affinities of these heparins to antithrombin III and thrombin were evaluated by using a surface plasmon resonance competitive binding assay. This study provides the physicochemical and activity characterization necessary for the appropriate design and synthesis of a generic bioengineered heparin.     

The numerical integration of relative equilibrium solutions. The nonlinear Schrodinger equation

We analyse different error propagation mechanisms for conservativeand nonconservative time-integrators of nonlinear Schrödingerequations. We use a geometric approach based on interpretingwaves as relative equilibria.