31P magic angle spinning NMR spectroscopy of paramagnetic rare-earth-substituted Keggin and Wells-Dawson solids

Paramagnetic rare-earth elements have been examined as NMR structural probes in polyoxoanionic solids, which have a variety of applications as luminescent materials that are usually disordered and therefore intractable by traditional structural methods. Thirteen Keggin and Wells-Dawson polyoxotungstates containing substitutions with lanthanides of different effective magnetic moments have been examined by 31P magic angle spinning NMR spectroscopy. The electron-nuclear dipolar interaction dominating the spinning sideband envelopes is determined by the lanthanide's magnetic moment and was found to be a sensitive probe of the nature of the polyoxoanion, of the positional isomerism, and of the ion stoichiometry. Electron-nuclear dipolar anisotropies computed based on the point-dipole approximation are generally in good agreement with the experimental results. The choice of a specific lanthanide as a structural probe can be tailored to the desired distance range between the phosphorus atoms and the paramagnetic centers to be probed. This approach is expected to be particularly useful in the paramagnetic polyoxoanionic materials lacking long-range order.     

Fourier-transform Carr-Purcell-Meiboom-Gill NMR experiments on polymers in colloidal dispersions: how many polymer molecules per particle?

Fourier transform relaxation NMR has been used to study how the mobility of poly(ethylene oxide) is affected by its adsorption onto colloidal silica particles of various sizes. Novel results have been obtained which illustrate the unexploited potential of this method for the study of interfacial species in complex systems. The results quantify how polymer mobility varies along an adsorption isotherm. When the particles are in excess, the polymer is strongly adsorbed and hence has a large spin-spin magnetic relaxation rate constant, R(2). The value of R(2) in this region increases with particle size, because the associated reduction in particle surface curvature results in a reduction in the mobility of the adsorbed polymer. This is accompanied by a reduction in the signal intensity, as a higher fraction of the polymer is adsorbed in the form of train segments too immobile to detect using the Carr-Purcell-Meiboom-Gill pulse sequence. When the polymer concentration reaches approximately 0.5 mg m(-2), the initial region of high affinity adsorption ends and so the polymer solution concentration increases. This is accompanied by a reduction in R(2), which then approaches the value for a simple polymer solution in the absence of particles. The results are corroborated by comparison with rheological measurements and molecular dynamics simulations of an analogous particle-polymer system.     

Effects of surfactants and electrolytes on adsorbed layers and particle stability

Adsorbed polymer and polyelectrolyte layers on colloidal silica nanoparticles have been studied in the presence of various salts and surfactants using photon correlation spectroscopy and solvent relaxation NMR. Poly(ethylene oxide) (PEO; molar mass 103.6 kg mol (-1)) adsorbed with a relatively high affinity and gave a layer thickness of 4.2 +/- 0.2 nm. While the nonionic surfactant used only increased this thickness slightly, anionic surfactants had a much greater effect, mainly due to repulsions between adsorbed aggregates, leading to expansion of the layer. A nonionic/anionic surfactant mixture was also tested and resulted in a larger increase in layer thickness than any of the individual surfactants. The dominant factor on addition of salt was generally the reduced solvency of PEO, which resulted in a further increase in the layer thickness but in some cases caused flocculation. This was not the case when the surfactant was sodium dodecylbenzenesulfonate; instead screening of the intermicellar repulsions possibly combined with surfactant-cation binding resulted in a reduction in the layer thickness. In comparison the affinity between silica and sodium polystyrenesulfonate was very weak. Anionic surfactants and salts did not noticeably increase the strength of adsorption, but instead encouraged flocculation. The situation was different with a nonionic surfactant, which was able to adsorb to silica itself and apparently facilitated a degree of polyelectrolyte adsorption as well.     

An efficient route to unsymmetrical bis(azolium) salts: CCC-NHC pincer ligand complex precursors

The coupling of N-heterocyclic azoles (imidazoles, benzimidazoles, triazoles) to bromobenzenes (1,2-; 1,3-; or 1,4) in a step-wise, sequential manner was accomplished by manipulation of reaction time and stoichiometry, which provided straight-forward access to unsymmetrical bis(azolium) salts in only three isolation steps from commercially-available starting materials. Eight mono(azole) substituted bromobenzenes, four mono(azolium)bromobenzene salts, twelve unsymmetrical bis(azole)benzenes, and fourteen unsymmetrical bis(azolium) salts, which are precursors for pincer ligand complexes, are reported.     

A Convenient Route to Tetraalkylammonium Perfluoroalkoxides from Hydrofluoroethers

Hydrofluoroethers are shown to alkylate tertiary amines readily under solvent‐free conditions, affording valuable tetraalkylammonium perfluoroalkoxides bearing α‐fluorines. The reaction of R_FCF₂? OCH₃ (R_F=CF₂CF₃, CF₂CF₂CF₃, and CF(CF₃)₂) with NR¹R²R³ produces twenty new α‐perfluoroalkoxides, [(CH₃)NR¹R²R³][R_FCF₂O] under mild conditions. These α‐perfluoroalkoxides are easy to handle, thermally stable, and can be used for the perfluoroalkoxylation of benzyl bromides.     

Damping of mesh-induced errors in Free-Lagrange simulations of Richtmyer-Meshkov instability

Mesh-induced errors at material interfaces are identified as a source of unphysical behaviour in Lagrangian numerical simulations of Richtmyer-Meshkov instability. The mesh geometry introduces interface perturbations with wavelengths of the same order as the mesh resolution. When a shock propagates through the interface, these perturbations can grow, severely contaminating the predicted interface development. Here an algorithm is presented which damps small-scale interface perturbations. A body force is applied at the interface which depends upon the disturbance amplitude and growth rate, and which resembles surface tension. Using this technique, qualitative improvements are obtained in Free-Lagrange simulations of single-mode Richtmyer-Meshkov instability. Growth rate behaviour and the evolution of the instability are seen to agree well with previously published results. Received 9 March 2000 / Accepted 30 May 2000     

158 A Ge V p-Pb Pb-Pb碰撞中的单奇异和多奇异粒子产生

用描写相对论性核 核碰撞的LUCIAE模型及相应的MonteCarlo事例产生器分析了WA97最近发表的 1 5 8AGeVp Pb和Pb Pb碰撞中单奇异粒子 (Λ ,Λ)和多奇异粒子 (Ξ-,Ξ-,Ω-,Ω-)的多重数和横质量分布 .由于LUCIAE模型包含了弦碎裂微观过程中奇异夸克压低因子随碰撞体系的能量、中心度和质量的增大而增大 ,以及由它导致的相对论性核 核碰撞宏观过程中奇异粒子产额随上述三因素之增而增的物理机制 ;LUCIAE模型计算结果能较好描写WA97实验所揭示的相对论性核 核碰撞中奇异粒子产额随中心度之增而增和奇异粒子增强随奇异夸克数之增而增的实验事实 ,以及单奇异和多奇异粒子的横质量分布 .     

Noncovalent complexes of water-soluble polymers and organoplatinum compounds as potential time-release antitumor agents

The complexation of appropriate organoplatinum compounds by water-soluble polymers has the potential to provide a time-release formulation which might afford a substantial reduction in the severity of side effects associated with use of organoplatinum antitumor agents. Both poly-(N-vinylpyrrolidone)(PVP) and poly(N-vinyl-5-methyl-2-oxazolidone)(PVOM) form stable, biologically active, molecular complexes with organoplatinum compounds containing polar aromatic ligands. Platinum compounds suitable for complexation by these polymers include cis-dichloro(4-substituted o-phenylenediamine)platinum(II), 4-substitutedcatecholato(1,2-diaminocyclohexane)platinum(II), and 4-substitutedphthallato(1,2-diaminocyclohexane)-platinum(II) compounds.     

Design strategies for libration point missions: theory to application

Spacecraft missions to the libration points in Sun-planet and planet-moon systems are of increasing interest. A number of such missions have already been launched. As the theoretical understanding of the dynamical structure in these regions of space expands, however, trajectory design to support actual mission requirements is more complex. Baseline trajectories for some recent mission scenarios are presented. Example approaches for transition of the designs to higher fidelity models is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defining protease specificity with proteomics: a protease with a dibasic amino acid recognition motif is regulated by a two-component signal transduction system in Salmonella.

Microbial proteases play diverse and important roles in bacterial virulence but their detection and characterisation is often hampered by their limited abundance or lack of expression in the absence of suitable environmental signals. We describe here a sensitive proteomic approach to detect proteases that are under the control of a virulence regulator and to characterise their recognition motifs. Using MG++-depleted growth media or a mutant strain of Salmonella in which the PhoP-PhoQ virulence regulatory system is constitutively active, truncated forms of DnaK, elongation factor G, elongation factor Tu and ribosomal protein S1 proteins were detected. Two other global regulatory mutants and cells exposed to acid or to oxidative stress failed to produce the truncated proteins, indicating specific control of the protease activity by the PhoP-PhoQ system. Our results suggest that at least two proteases are induced. To define the proteolytic cleavage sites of one of the proteases, peptides from each of the truncated proteins were identified by tryptic mass fingerprinting/nanoelectrospray mass spectrometry and mapped onto the sequence of the intact protein. Alignment of the regions around the cut site indicates that the protease recognises a dibasic amino acid motif characteristic of the omptin protease family. The induction of such proteases in bacteria depleted of Mg++ ions may contribute to the PhoPQ-mediated resistance of Salmonella to cationic antimicrobial peptides. Additionally, our results suggest it would be prudent to keep the concentration of this ion above micromolar levels during bacterial sample preparation for proteomic analyses.