Spectroscopic and Electrochemical Study of the Interconversion and Decomplexation of Cobalt(II) Sandwich Polyoxometalates Based on a Dawson-Type Anion

The reaction of the trivacant Dawson polyoxometalate α-[P(2)W(15)O(56)](12-) and the divalent cations Co(2+) is known to form a symmetrically derived sandwich complex of formula ββ-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) [symbolized as ββ-Co(4)(P(2)W(15))(2)] at low pH (ca. pH 3). We have shown previously that, by a slight modification of the reaction conditions, trinuclear αβ-[(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)](17-) and dinuclear [(NaOH(2))(2)Co(2)(P(2)W(15)O(56))(2)](18-) complexes [symbolized as αβ-NaCo(3)(P(2)W(15))(2) and Na(2)Co(2)(P(2)W(15))(2), respectively] can be synthesized as aqueous-soluble sodium salts. αβ-NaCo(3)(P(2)W(15))(2) is a "lacunary" sandwich complex that can add a Co(2+) cation to form nearly quantitatively an unsymmetrical Dawson tetracobalt sandwich polyoxometalate, αβ-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) [symbolized as αβ-Co(4)(P(2)W(15))(2)]. Thus, for Co(4)(P(2)W(15))(2), the junctions between the trivacant {P(2)W(15)} subunits and the central tetrameric unit can be either both β type or β and α types. The interconversion between αβ-Co(4)(P(2)W(15))(2) and ββ-Co(4)(P(2)W(15))(2) and the decomplexation process at low pH, leading to the formation of αβ-NaCo(3)(P(2)W(15))(2) and/or Na(2)Co(2)(P(2)W(15))(2), have been followed in aqueous solution at various pH values by electrochemistry, UV-visible absorption spectroscopy, and (31)P NMR spectroscopy.     

Dispersion and Strichartz estimates for the Liouville equation

We consider the Liouville equation associated with a metric g of class C² and we prove dispersion and Strichartz estimates for the solution of this equation in terms of geodesics associated with g. We introduce the notion of focusing and dispersive metric to characterize metrics such that the same dispersion estimate as in the Euclidean case holds. To deal with the case of non-trapped long range perturbation of the Euclidean metric, we prove a global velocity moments effect on the solution. In particular, we obtain global in time Strichartz estimates for metrics such that the dispersion estimate is not satisfied.     

Hybrid Organic-Inorganic Materials Based on Nanobuilding Blocks Assembled through Electrostatic Interactions

Hybrid copolymers have been prepared by free radical polymerization, in toluene, of methylacrylate (MA) and varying amounts of an organotin oxo-cluster functionalized with two acrylamido groups, {(BuSn)₁₂O₁₄(OH)₆}(AMPS)₂. The tin based nanobuilding block is fully preserved in the final materials. These copolymers formed networks where the cross-linking is based on the electrostatic interactions that take place between the macrocation {(BuSn)₁₂O₁₄(OH)₆}^{2 +} and its two functional anions (AMPS^–). Accordingly, these copolymers can yield gels or solutions depending on the solvent used to swell them.     

Protein factors mediating selenoprotein synthesis

The amino acid selenocysteine represents the major biological form of selenium. Both the synthesis of selenocysteine and its co-translational incorporation into selenoproteins in response to an in-frame UGA codon, require a complex molecular machinery. To decode the UGA Sec codon in eubacteria, this machinery comprises the tRNASec, the specialized elongation factor SelB and the SECIS hairpin in the selenoprotein mRNAs. SelB conveys the Sec-tRNASec to the A site of the ribosome through binding to the SECIS mRNA hairpin adjacent to the UGA Sec codon. SelB is thus a bifunctional factor, carrying functional homology to elongation factor EF-Tu in its N-terminal domain and SECIS RNA binding activity via its C-terminal extension. In archaea and eukaryotes, selenocysteine incorporation exhibits a higher degree of complexity because the SECIS hairpin is localized in the 3' untranslated region of the mRNA. In the last couple of years, remarkable progress has been made toward understanding the underlying mechanism in mammals. Indeed, the discovery of the SECIS RNA binding protein SBP2, which is not a translation factor, paved the way for the subsequent isolation of mSelB/EFSec, the mammalian homolog of SelB. In contrast to the eubacterial SelB, the specialized elongation factor mSelB/EFSec the SECIS RNA binding function. The role is carried out by SBP2 that also forms a protein-protein complex with mSelB/EFSec. As a consequence, an important difference between the eubacterial and eukaryal selenoprotein synthesis machineries is that the functions of SelB are divided into two proteins in eukaryotes. Obviously, selenoprotein synthesis represents a higher degree of complexity than anticipated, and more needs to be discovered in eukaryotes. In this review, we will focus on the structural and functional aspects of the SelB and SBP2 factors in selenoprotein synthesis.     

Chemical modification of horseradish peroxidase with several methoxypolyethylene glycols

The chemical modification of ε-NH₂ lysine residues of horseradish peroxidase (E.C. 1.11.1.7) with several mPEG was carried out. The modified enzymes were studied through Chromatographic and electrophoretic methods; the extent of mPEG linking was determined using¹H-NMR. Peroxidase, modified with mPEG ranging from 350 to 5000, activated with 4-nitrophenylchloroformate (mPEGpn), showed a better thermal stability than the native, but there was no correlation between the length of the polymer adduct and the improvement. The enzyme was modified with mPEG (5000) activated by cyanuric chloride (mPEGcc). The number of modified lysine increased, but the thermal behavior of mPEGcc peroxidase was similar to those of mPEGpn enzymes. In all cases, the modification did not markedly change the stability in organic solvents.     

A Boron‐Fluorinated Tris(pyrazolyl)borate Ligand (FTp*) and Its Mono‐ and Dinuclear Copper Complexes [Cu(FTp*)2] and [Cu2(FTp*)2]: Synthesis,Structures, and DFT Calculations

Reaction of [Si(3,5‐Me₂pz)₄] ( 1 ) with [Cu(MeCN)₄][BF₄] ( 2 ) gave the mono‐ and dinuclear copper complexes [Cu₂(^FTp*)₂] ( 3 ) and [Cu(^FTp*)₂] ( 4 ). Both complexes contain the so‐far unprecedented boron‐fluorinated ^FTp* ligand ([FB(3,5‐Me₂pz)₃]^? with pz=pyrazolyl) originating from 1 , acting as a pyrazolyl transfer reagent, and the [BF₄]^? counter anion of 2 , serving as the source of the {BF} entity. The solid‐state structures as well as the NMR and EPR spectroscopic characteristics of the complexes were elaborated. Pulsed gradient spin echo (PGSE) experiments revealed that 3 retains (almost entirely) its dimeric structure in benzene, whereas dimer cleavage and formation of acetonitrile adducts, presumably [Cu(^FTp*)(MeCN)], is observed in acetonitrile. The short Cu???Cu distance of 269.16 pm in the solid‐state is predicted by DFT calculations to be dictated by dispersion interactions between all atoms in the complex (the Cu?Cu dispersion contribution itself is only very small). As revealed by cyclic voltammetry studies, 3 shows an irreversible (almost quasi‐reversible at higher scan rates) oxidation process centred at E^pa=?0.23 V (E⁰_1/2=?0.27 V) (vs. Fc/Fc⁺). Oxidation reactions on a preparative scale with one equivalent of the ferrocenium salt [Fc][BF₄] (very slow reaction) or air (fast reaction) furnished blue crystals of the mononuclear copper(II) complex [Cu(^FTp*)₂] ( 4 ). As expected for a Jahn–Teller‐active system, the coordination sphere around copper(II) is strongly distorted towards a stretched octahedron, in accordance with EPR spectroscopic findings.     

Demodulation of Spatial Carrier Images: Performance Analysis of Several Algorithms Using a Single Image

Optical full-field techniques have a great importance in modern experimental mechanics. Even if they are reasonably spread among the university laboratories, their diffusion in industrial companies remains very narrow for several reasons, especially a lack of metrological performance assessment. A full-field measurement can be characterized by its resolution, bias, measuring range, and by a specific quantity, the spatial resolution. The present paper proposes an original procedure to estimate in one single step the resolution, bias and spatial resolution for a given operator (decoding algorithms such as image correlation, low-pass filters, derivation tools …). This procedure is based on the construction of a particular multi-frequential field, and a Bode diagram representation of the results. This analysis is applied to various phase demodulating algorithms suited to estimate in-plane displacements.     

An adaptive Lagrangian boundary element approach for three‐dimensional transient free‐surface Stokes flow as applied to extrusion,thermoforming, and rheometry

An adaptive (Lagrangian) boundary element approach is proposed for the general three‐dimensional simulation of confined free‐surface Stokes flow. The method is stable as it includes remeshing capabilities of the deforming free surface and thus can handle large deformations. A simple algorithm is developed for mesh refinement of the deforming free‐surface mesh. Smooth transition between large and small elements is achieved without significant degradation of the aspect ratio of the elements in the mesh. Several flow problems are presented to illustrate the utility of the approach, particularly as encountered in polymer processing and rheology. These problems illustrate the transient nature of the flow during the processes of extrusion and thermoforming, the elongation of a fluid sample in an extensional rheometer, and the coating of a sphere. Surface tension effects are also explored. Copyright © 2001 John Wiley & Sons, Ltd.