Solid solubility of the oxisulfides of Y, La, and Gd

The solid solubility in the systems Y₂O₂S---La₂O₂S, Y₂O₂S---Gd₂O₂S, and Gd₂O₂S---La₂O₂S has been investigated. Solid solutions of all compounds, throughout the whole composition range, were readily obtained, using coprecipitated oxalates and a polysulfide flux.     

Dynamics of a triphenylene discotic molecule,HAT6, in the columnar and isotropic liquid phases

Discotic molecules have planar, disklike polyaromatic cores that can self-assemble into "molecular wires". Highly anisotropic charge transfer along the wires arises when there is sufficient intermolecular overlap of the pi-orbitals of the molecular cores. Discotic materials can be applied in molecular electronics, field-effect transistors, and-recently with record quantum efficiencies-photovoltaics (Schmidt-Mende, L.; Fechtenk?tter, A.; Müllen, K.; Moons, E.; Frien, R. H.; MacKenzie, J. D. Science 2001, 293, 1119). A combination of quasielastic neutron scattering (QENS) measurements with molecular dynamics simulations on the discotic molecule hexakis(n-hexyloxy)triphenylene (HAT6) shows that the dynamics of the cores and tails of discotic molecules are strongly correlated. Core and tail dynamics are not separated, the system being characterized by overall in-plane motion, on a time scale of 0.2 ps, and softer out-of-plane motions at 7 ps. Because charge transfer between the molecules is on similar time scales, these motions are relevant for the conducting properties of the materials. Both types of motion are dominated by van der Waals interactions. Small-amplitude in-plane motions in which the disks move over each other are almost entirely determined by tail/tail interactions, these also playing an important role in the out-of-plane motion. The QENS measurements reveal that these motions are little changed by passing from the columnar phase to the isotropic liquid phase, just above the clearing temperature. The model of four HAT6 molecules in a column reproduces the measured QENS spectrum of the liquid phase, suggesting that correlations persist within the liquid phase over about this number of disks.     

Syntheses and properties of related polyoxadiazoles and polytriazoles

New aromatic poly-1,2,4-triazoles and poly-1,3,4-oxadiazoles are studied as thermally stable membrane materials. Various groups were introduced onto the pendant phenyl groups of poly-1,2,4-triazoles. Glass transition temperature, degradation temperature, and cold crystallization behavior were studied as a function of these groups. Cold crystallization appeared to be highly sensitive to macromolecular regularity. The solubility of poly-1,3,4-oxadiazoles was highly improved upon incorporation of 5-t-butylisophthalic, 1,1,3-trimethyl-3-phenylindane, 4,4′-(2,2′-diphenyl) hexafluoro propane, and diphenyl ether groups into the polymeric main chain, whereas the high glass transition temperatures and degradation temperatures typical for aromatic poly-1,3,4-oxadiazoles were maintained. © 1994 John Wiley & Sons, Inc.     

Slow internal dynamics in proteins: application of NMR relaxation dispersion spectroscopy to methyl groups in a cavity mutant of T4 lysozyme

Recently developed carbon transverse relaxation dispersion experiments (Skrynnikov, N. R.; et al. J. Am. Chem. Soc. 2001, 123, 4556-4566) were applied to the study of millisecond to microsecond time scale motions in a cavity mutant of T4 lysozyme (L99A) using methyl groups as probes of dynamics. Protein expressed in E. coli cells with (13)CH(3)-pyruvate as the sole carbon source contained high levels of (13)C enrichment at a total of 80 Val gamma, Leu delta, Ile gamma (2), Ala beta, and Met epsilon methyl positions with little extraneous incorporation. Data for 72 methyl groups were available for analysis. Dispersion profiles with large amplitudes were measured for many of these residues and were well fit to a two-state exchange model. The interconversion rates and populations of the states, obtained from fitting relaxation dispersion profiles of each individual probe, were remarkably homogeneous and data for nearly all methyl groups in the protein could be collectively fit to a single cooperative conformational transition. The present study demonstrates the general applicability of methyl relaxation dispersion measurements for the investigation of millisecond time scale protein motions at a large number of side-chain positions. Potential artifacts associated with the experiments are described and methods to minimize their effects presented. These experiments should be particularly well suited for probing dynamics in high molecular weight systems due to the favorable NMR spectroscopic properties of methyl groups.     

A copper-catalyzed C-N bond formation involving sp-hybridized carbons. A direct entry to chiral ynamides via N-alkynylation of amides

A copper-catalyzed new C-N bond formation involving a sp-hybridized carbon is described here leading to a facile entry for syntheses of chiral ynamides. This direct N-alkynylation of amides should have a significant impact on the future development of synthetic methodologies employing ynamides.     

Hydrogen cycling of niobium and vanadium catalyzed nanostructured magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.     

Nitrofuran antibiotic residues in pork: The FoodBRAND retail survey

Use of nitrofuran drugs in food-producing animals has been prohibited within the EU because they may represent a public health risk. Monitoring compliance with the ban has focused on the detection of protein-bound nitrofuran metabolites which, in contrast to the parent compounds, are stable and persist in animal tissues. As part of the “FoodBRAND” project, an extensive survey of pork was undertaken across 15 European countries. Samples (n = 1500) purchased at retail outlets were analysed for the nitrofuran metabolites AOZ, AMOZ, AHD and SEM using LC–MS/MS determination of nitrobenzaldehyde derivatives. Limits of quantification for the method were 0.1 μg/kg (AOZ, AMOZ), 0.2 μg/kg (SEM) and 0.5 μg/kg (AHD). Of the 1500 samples tested, measurable residues of nitrofuran metabolites were confirmed in 12 samples (0.8% incidence overall) of which 10 samples were purchased in Portugal (AOZ, 0.3 μg/kg; AMOZ, 0.2–0.6 μg/kg) and one sample each in Italy (AMOZ, 1.0 μg/kg) and Greece (AOZ, 3.0 μg/kg).     

Cyclo dehydration reaction of polyhydrazides. II. Kinetic parameters obtained from isothermal thermogravimetry

The kinetics of the thermal conversion reaction of poly-(1,3-phenyl-1,4-phenyl)-hydrazide into poly-(1,3-phenyl-1,4-phenyl)-1,3,4-oxadiazole have been studied with isothermal thermogravimetry in continuation of a study with nonisothermal thermogravimetry described in a previous paper. Although the isothermal measurements are much more time-consuming, they provide some new information and insight about the cyclo dehydration reaction of the polyhydrazide. The physical state of the sample, rubbery or glassy, seems to influence the kinetics considerably. The kinetic parameters determined with the isothermal method for the polymer in its glassy state agree well with the parameters derived from the previously reported nonisothermal measurements, while the kinetic parameters for the expected rubbery state differ considerably. The morphological state or the history of the polymer has also a considerable influence on the kinetics of the isothermal conversion process. The powder form of the polymer has a much lower isothermal conversion rate than the film form.     

Assembly of a supramolecular capsule on a molecular printboard

A molecular capsule based on ionic interactions between two oppositely charged calix[4]arenes, 1 and 2, was assembled both in solution and on a surface. In solution, the formation of the equimolar assembly 1.2 was studied by (1)H NMR, ESI-MS, and isothermal titration calorimetry, giving an association constant (K(a)) of 7.5 x 10(5) M(-1). A beta-cyclodextrin self-assembled monolayer (beta-CD SAM) on gold was used as a molecular printboard to anchor the tetraguanidinium calix[4]arene (2). The binding of tetrasulfonate calix[4]arene 1 was monitored by surface plasmon resonance spectroscopy. Rinsing of the surface with a high ionic strength aqueous solution allows the removal of the tetrasulfonate calix[4]arene (1), while by rinsing with 2-propanol it is possible to achieve the complete desorption of the tetraguanidinium calix[4]arene (2) from the beta-CD SAM. The K(a) for the capsule formation on a surface is 3.5 x 10(6) M(-1), thus comparing well with the K(a) determined in solution.