On the use of relative deceleration values for the determination of silicon by PIGE

By spiking the samples with a compound containing both a non-analyte and an analyte element, to which a relative deceleration property for 5 Me V protons has been ascribed, relative deceleration values for the samples could be obtained by measuring the prompt -yields induced in the non-analyte element. These values are used to correct for matrix effects in elemental analysis using PIGE techniques. In addition to this, the analytical results thus obtained can also be compared to those obtained by the simultaneous application of the analyte spiking technique to the collected yield data. These non-analyte spiking approaches were used to analyze silicon in polymer materials. These methods were tested by analyzing the two reference standards BCS 308 and Standard Chrome Ore XXXI. Li₂SiO₃, containing both the non-analyte as well as the analyte element, was selected as comparator and spike. Results obtained for the polymer samples were also compared to those obtained by PIXE.     

A theoretical investigation of the enantioselective hydrogenation mechanism of α-ketoesters

The enantioselective hydrogenation of α-ketoesters to α-hydroxyesters over Pt/Al₂O₃ catalysts modified by cinchona alkaloids is an interesting model reaction for the investigation of heterogeneous catalysis capable of producing optically active products. The aim of the present theoretical study is to rationalize the interaction between protonated cinchona alkaloids (modifiers) and methyl pyruvate (substrate) by investigating the possible weak complexes formed by these two species. For this purpose we use molecular mechanics and the AM1 semiempirical method. The optimization leads to two stable forms of the complexes, where the substrate is bound to the modifier via hydrogen bonding between the oxygen of the α-carbonyl of pyruvate and the quinuclidine nitrogen of the alkaloid. In such complexes the methyl pyruvate is transformed into a half-hydrogenated species which can be adsorbed on the platinum surface and, after hydrogenation, leads to methyl lactate product. The results show that adsorption of the complex leading to (R)-methyl lactate is more favorable than that of the corresponding system yielding (S)-methyl lactate, which may be the key for the enantio-differentiation.     

Fabrication of "hairy" colloidosomes with shells of polymeric microrods

Novel colloidosome microcapsules with aqueous gel cores and integral shells of rodlike polymeric particles have been prepared and characterized. The synthesis is based on templating water-in-oil emulsions stabilized by rodlike particles followed by gelling of the aqueous phase, dissolution of the oil phase in ethanol, and redispersion of the obtained colloidosome capsules in water. Such capsules may find applications as vehicles for delivery and controlled release of drugs, cosmetics, and food supplements.     

Conformational analysis of a stereochemically complete set of cis-enediol peptide analogues

A conformational analysis of a stereochemically complete set of peptide analogues based on a cis-enediol unit is presented. The cis-enediol unit, which can replace a two or a three amino acid segment of a peptide, contains two "side chains", four asymmetrical carbon atoms, and six free dihedral angles. To determine the accessible conformational space, the molecules are divided into three fragments, each containing two free dihedral angles. The energy surfaces are computed for all dihedral angle values, and the possible conformations of the cis-enediol unit analogues are built using all combinations of the surface minima. Such a "build-up" procedure, which is very fast, is able to reproduce 75% of the minima obtained from a full dihedral angle exploration of the conformational space. The cis-enediol unit minima are compared with the corresponding di- and tripeptide minima; all peptide minima can be closely matched by a cis-enediol unit minimum of low energy (less than 2.2 kcal/mol above the lowest energy conformer). However, there are low energy minima of the cis-enediol unit that have no corresponding minima in peptides. The results are shown to depend strongly on the chirality of the analogues. The ability of each of the stereoisomers to mimic natural peptides, evaluated by the present approach, is correlated with its experimental activity in a renin inhibition assay.     

Effect of Groundwater Composition on Arsenic Detection by Bacterial Biosensors

A luminescent bacterial biosensor was used to quantify bioavailable arsenic in artificial groundwater. Its light production above the background emission was proportional to the arsenite concentration in the toxicologically relevant range of 0 to 0.5 μM. Effects of the inorganic solutes phosphate, Fe(II) and silicate on the biosensor signal were studied. Phosphate at a concentration of 0.25 g L⁻¹ phosphate slightly stimulated the light emission, but much less than toxicologically relevant concentrations of the much stronger inducer arsenite. No effect of phosphate was oberved in the presence of arsenite. Freshly prepared sodium silicate solution at a concentration of 10 mg L⁻¹ Si reduced the arsenite-induced light production by roughly 37%, which can be explained by transient polymerization leading to sequestration of some arsenic. After three days of incubation, silicate did not have this effect anymore, probably because depolymerization occurred. In the presence of 0.4 mg L⁻¹ Fe(II), the arsenite-induced light emission was reduced by up to 90%, probably due to iron oxidation followed by arsenite adsorption on the less soluble Fe(III) possibly along with some oxidation to the stronger adsorbing As(V). Addition of 100 μM EDTA was capable of releasing all arsenic from the precipitate and to transform it into the biologically measurable, dissolved state. The biosensor also proved valuable for monitoring the effectiveness of an arsenic removal procedure based on water filtration through a mixture of sand and iron granules.     

Zinc(II) as a versatile template for the design of dipolar and octupolar NLO-phores

Thermally stable dipolar and octupolar (D2d, D3) NLO-phores are readily accessible by combining one, two, or three 4,4'-bis(dialkylaminostyryl)-[2,2']-bipyridyl ligands with zinc(II) salts. The off-resonant beta0 values point out the superiority of octupoles versus dipoles in terms of nonlinearity/transparency tradeoff. The octahedral tris(bipyridyl)zinc(II) complex exhibits a very large beta0 value (241 x 10-30 esu), which is the largest ever reported for octupolar molecules.     

Synthesis of modified Weinreb amides: N-tert-butoxy-N-methylamides as effective acylating agents

An efficient preparation of N-methyl-O-tert-butylhydroxylamine hydrochloride has been settled, which allowed the synthesis of modified Weinreb amides. Nucleophilic addition of organolithium and Grignard reagents on these N-tert-butoxy-N-methylamides afforded efficiently the corresponding ketones and reduction with DIBAL furnished the corresponding aldehydes in good yields up to 97%.     

General access to iminosugar C-glycoside building blocks by means of cross-metathesis: a gateway to glycoconjugate mimetics

[reaction: see text] Cross-metathesis reactions of alpha-1-C-allyl-1-deoxynojirimycin derivatives 7a,b and various functionalized alkenes mediated by Grubbs's catalyst 3 are reported. The reactions showed reasonable to very good yields and excellent E/Z selectivity. This methodology allows the efficient and convergent synthesis of iminosugar C-glycosides with a great degree of structural diversity in the aglycone, opening the way to a variety of new glycoconjugate mimetics of biological interest.     

Bestimmung von Metallspuren in Polymeren mit Hilfe der Atom-Absorptionsspektrophotometrie

Zusammenfassung Die Atom-Absorptionsspektrophotometrie erlaubt die schnelle und genaue Bestimmung von Metallen in Polymeren, und zwar sowohl im ppm- als auch im Prozentbereich. Bei löslichen Polymeren werden vorzugsweise 2%ige Lösungen untersucht. Polymere, die in keinem geeigneten Lösungsmittel löslich sind, werden mit H₂SO₄/H₂O₂ aufgeschlossen. für flüchtige Metallverbindungen wird eine Umfällmethode mit anschließender Anreicherung der Metallspuren beschrieben. Untersucht werden Polystyrol, Celluloseester, Polyacrylnitril, ABS-Polymere, Polycarbonat, Polyvinylchlorid und seine Copolymeren, Polyamid, Polyäther, Wolle und Baumwolle. Nachweisgrenzen und günstigste Konzentrationsbereiche für die Elemente Li, Na, K, Mg, Ca, B, Al, Si, Sn, Pb, Sb, Cu, Ag, Au, Zn, Cd, Hg, Ti, Fe und Ni werden angegeben.

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Determination of trace metals in polymers by atomic absorption spectrophotometry

Atomic absorption spectrophotometry allows quick and exact determination of both ppm and percent quantities of metals in polymers. Preferably 2% solutions of soluble polymers are analyzed. Polymers insoluble in a suitable solvent are ashed in H₂SO₄/H₂O₂. For volatile metal compounds a double precipitating method followed by enrichment of trace metals is described. The following polymers are studied: polystyrene, cellulose esters, polyacrylonitrile, acrylonitrile-butadiene-styrene-terpolymers, polycarbonate, Polyvinylchloride and its copolymers, polyamide, polyether, wool, and cotton. Detection limits and most favourable concentration ranges are reported for the elements Li, Na, K, Mg, Ca, B, Al, Si, Sn, Pb, Sb, Cu, Ag, Au, Zn, Cd, Hg, Ti, Fe, and Ni.

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Herrn Prof. Dr. K. Hansen zum 60. Geburtstag gewidmet.

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An dieser Stelle sei Herrn Dr. A. Reichle, dem Leiter des Wissenschaftlichen Laboratoriums der Farbenfabriken Bayer AG, Werk Dormagen, für seine Unterstützung gedankt.Den Herren K. Schumann und W. Kremer danke ich für ihre sorgfältige experimentelle Mitarbeit.     

Selective functionalization at the small rim of calix[6]arene. Synthesis of novel non-symmetrical N3, N4 and N3ArO biomimetic ligands

Eight novel calix[6]arene-based biomimetic ligands for transition metal ions have been synthesized. They display a non-symmetrical N₃, N₄ or N₃ArO binding core that mimics enzyme active sites presenting histidine and tyrosine residues. The key step for their synthesis is the mono-alkylation at the small rim of the C_3v symmetrical trimethyl ether derivative of tBu-calix[6]arene with N-Boc-2-chloroethylamine to yield a novel calix[6]arene synthon. Its combined O-alkylation with a chloromethyl aromatic amine and N-deprotection or alkylation or reductive alkylation with a salicylaldehyde derivative yielded the calix[6]arene-based ligands with mixed N/O donors.