Detection of a flow induced magnetic field eigenmode in the riga dynamo facility

In a closed volume of molten sodium an intense single-vortex-like helical flow has been produced by an outside powered propeller. At a flow rate of 0.67 m(3)/s a slowly growing magnetic field eigenmode was detected. For a slightly lower flow, additional measurements showed a slow decay of this mode. The measured results correspond satisfactorily with numerical predictions for the growth rates and frequencies.     

Biocompatible copper(I) catalysts for in vivo imaging of glycans

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is the standard method for bioorthogonal conjugation. However, current Cu(I) catalyst formulations are toxic, hindering their use in living systems. Here we report that BTTES, a tris(triazolylmethyl)amine-based ligand for Cu(I), promotes the cycloaddition reaction rapidly in living systems without apparent toxicity. This catalyst allows, for the first time, noninvasive imaging of fucosylated glycans during zebrafish early embryogenesis. We microinjected embryos with alkyne-bearing GDP-fucose at the one-cell stage and detected the metabolically incorporated unnatural sugars using the biocompatible click chemistry. Labeled glycans could be imaged in the enveloping layer of zebrafish embryos between blastula and early larval stages. This new method paves the way for rapid, noninvasive imaging of biomolecules in living organisms.     

X-Ray Crystallographic Structure of the Cyclic Di-amino Acid Peptide: <Emphasis Type="Italic">N,N</Emphasis>′-Diacetyl-cyclo(Gly-Gly)

Abstract  

The cyclic di-amino acid peptide N,N′-diacetyl-cyclo(Gly-Gly), C₈H₁₀N₂O₄, crystallizes in the triclinic space group P[`1] P\bar{1} with unit cell parameters a = 9.4855(4) ?, b = 10.0250(3) ?, c = 10.0763(4) ?, α = 73.682(2)°, β = 82.816(2)°, γ = 81.733(2)°, V = 906.40(6) ?<sup>3</sup>, Z = 4 (2 molecules, A and B, per asymmetric unit), D<sub>c</sub> = 1.452 g cm<sup>−3</sup> and linear absorption coefficient 0.118 mm<sup>−1</sup>. The crystal structure determination was carried out with MoKα X-ray data measured at 120(2) K. In the final refinement cycle the data/restraints/parameter ratios were 4124/0/258 and goodness-of-fit on F<sup>2</sup> = 1.0008. Final R indices for [I > 2σ(I)] were R<sub>1</sub> = 0.0501, wR<sub>2</sub> = 0.1007 and R indices (all data) R<sub>1</sub> = 0.0864, wR<sub>2</sub> = 0.11180. The largest electron density difference peak and hole were 0.241 and −0.232 e ?<sup>−3</sup>, respectively. The DKP rings in both molecules A and B have boat conformations with pseudo mm2 (C<sub>2v</sub>) symmetry if the N atoms and CH<sub>2</sub> groups are considered identical. In each case, the prow and stern of the boat are the α-carbons C(3) and C(6). The overall molecular symmetry of molecules A and B is approximately C<sub>2</sub> with the twofold symmetry axis of the DKP boat being maintained through the centre of the DKP ring. Details of the molecular geometry are compared with that of the parent compound cyclo(Gly-Gly) in which the DKP ring is planar with exact symmetry [`1] \bar{1} (C_i).     

Structural Aspects of 5‐methylthio‐1‐oxa‐6,6a δIV‐dithia‐2‐azapentalenes

Alkylation of 5‐[1‐hydroxyiminoalkyl]‐1,2‐dithiole‐3‐thiones using methyl iodide in the presence of sodium hydroxide gives derivatives which may be considered as being 5‐methylthio‐1‐oxa‐6,6a δ^IV‐dithia‐2‐azapentalenes with quantitative yields. The structure of oxadithiazapentalene attributed to the alkylation products is founded on X‐ray analysis and NMR and IR data.     

Chirality and numbering of substituted tropane alkaloids

The strict application of IUPAC rules for the numbering of tropane alkaloids is not always applied by authors and there is hence a lot of confusion in the literature. In most cases, the notation of 3, 6/7-disubstituted derivatives has been chosen arbitrarily, based on NMR and MS data, without taking into account the absolute configuration of these two carbons. This paper discusses the problem and the relevance of CD and NMR to determine molecular configurations. We report on the use of (1)H-NMR anisochrony (Δδ) induced by the Mosher's chiral auxiliary reagents (R)-(-)- and (S)-(+)-α-methoxy-α-trifluoromethyl-phenylacetyl chlorides (MTPA-Cl), to determine the absolute configuration of (3R,6R)-3α-hydroxy-6β-senecioyloxytropane, a disubstituted tropane alkaloid isolated from the aerial parts of Schizanthus grahamii (Solanaceae). These analytical tools should help future works in correctly assigning the configuration of additional 3, 6/7 disubstituted tropane derivatives.     

Vortex dynamics in YBaCuO single crystals with point- and line-like defects-flux creep studies

Studies of vortex dynamics and pinning were conducted in irradiated Y₁Ba₂Cu₃O₇ single crystals by observing the decay of supercurrents with time. The materials contained either point-like defects from proton irradiation or line-like defects from irradiation with energetic heavy ions. Analysis of the first case gives clear support for collective creep theory. With linear defects, the experiments give evidence for thermal smearing of the vortex pinning potential at higher temperatures, as predicted theoretically.     

Withanolides from Withania adpressa

From the leaves of Withania adpressa, a plant endemic to Sahara of Morocco and Algeria, the novel steroidal lactone (22R)‐14α,15α,17β,20β‐tetrahydroxy‐1‐oxowitha‐2,5,24‐trien‐26,22‐olide (= (15S,17S)‐14,15,17,20‐tetrahydroxy‐22,26‐epoxyergosta‐2,5,24‐triene‐1,26‐dione; 1 ), was isolated, along with three known compounds, withanolides F ( 2 ), J ( 3 ), and oleanolic acid. Their structures were mainly solved by in‐depth 1D‐ and 2D‐NMR (including ADEQUATE) experiments, as well as by HR‐MS analyses and chemical evidence.     

Injection, intersubband relaxation and recombination in GaAs multiple quantum wells

Results of continuous excitation, time-delayed and time-resolved cathodoluminescence experiments on undoped and Be-doped GaAs multiple quantum wells of thickness 5–11 nm are reported for temperatures 5–300 K and excitation intensities 1–10³ W/cm². Comparison is made with similar investigations of bulk epitaxial GaAs layers. Increasing structural localisation of the carriers is identified to lead to a qualitative change of the type of recombination. Increasingly faster radiative excitonic recombination leads to a bypass of impurity and trap capture processes in undoped as well as in doped material, at low temperatures as well as at room temperature. Despite the exponential character of excitonic decay, the luminescence transients are found to be very complex due to an interplay of intersubband scattering and recombination processes and time-dependent carrier temperature. Transients are analysed in detail, excitonic lifetimes and intersubband scattering times are derived. It is argued that both the lifetime reduction in quantum wells and the novel process of recombination heating lead to a strongly increased quasiequilibrium temperature of excited carriers in the wells as compared to bulk material. Injection of carriers from 18 nm GaAlAs barriers to GaAs wells is found to occur in less than 10^-12 s without loss.     

Über den Einfluss nichtpolarer Lösungsmittel auf Lage und Intensität von Absorptionsbanden in den Elektronenspektren apolarer Molekeln: II. Polyacetylene

The dispersive interaction of nonpolar solutes with nonpolar solvents is investigated in model systems consisting of solutions of dimethyl-tetraacetylene, di-t-butyl-tetraacetylene, dimethyl-pentaacetylene or di-t-butyl-pentaacetylene in binary mixtures of cyclohexane/carbondisulfide or n-pentan/carbondisulfide. For this purpose the ¹Σ → ¹Σ transition in the electronic spectrum of the solutions has been recorded. With increasing concentration of the more strongly interacting solvent component (i.e. carbondisulfide) bathochromic shifts δν of the transition frequency accompanied by a decrease in oscillator strength f_M are observed, which are consistent with calculated values based on a previously proposed exciton model [1].