Full miscibility of disk- and rod-shaped mesogens in the nematic phase

Mesogens containing rod-shaped moieties as well as one disk-shaped group exhibit nematic phase behavior and are entirely miscible with rod- and disk-shaped mesogens in the nematic phase. In the phase diagram, an extra nematic phase appears at nearly equimolar disk-rod ratios, where the nematic biaxial phase has been predicted.     

Stereochemistry of the Inhibition of δ-Chymotrypsin with Optically Active Bicyclic Organophosphates: 31P-NMR studies

The inhibition of δ-chymotrypsin with optically active, axially and equatorially substituted trans-3-(2,4-dini-trophenoxy)-2,4-dioxa-3λ⁵-phospbubicyclo[4.4.0]decan-3-ones ( = hexahydro-4H-1,3,2-benzodioxaphosphorin 3-oxides) was investigated. Their inhibitory power was determined by kinetic measurements, and the stereochemical course of the reaction of stoichiometric amounts of the enzyme and inhibitor was monitored with ³¹P-NMR spectroscopy at pH 7.8. The irreversible inhibitors show significant enantioselectivity (the (S_p)-enantiomer reacting faster) and yield diastereoisomeric, covalently phosphorylated derivatives of δ-chymotrypsin. ³¹P-NMR Spectroscopic studies of the inhibition by the axially substituted inhibitor revealed for the racemic (±) -2a first a resonance at –4.4 ppm and later, while inhibition proceeded, a second one at –4.5 ppm. The reaction with optically active (+) -2a showed only one signal at –4.4 ppm and its enantiomer (–) -2a only one signal at –4.5 ppm. Using the equatorially substituted racemic epimer (±) -2b , we observed the main resonance at –5.3 ppm and two minor ones at –4.4 and –4.5 ppm. The optically active compound (+) -2b showed two peaks at –4.5 and –5.3 ppm, whereas its antipode (–) -2b revealed two signals at –4.4 and –5.3 ppm. Comparing the ³¹P chemical shifts of the corresponding covalent phosphoserine derivatives 4a (-5.7 ppm, axial) and 4b (-4.5 ppm, equatorial) shows the inhibition with the axial compounds 2a to proceed via neat inversion of the configuration at the P-atom, whereas the equatorial epimers 2b with a higher conformational flexibility seem to follow a different stereochemical pathway which results in both inversion and retention.     

Recent synchrotron radiation microdiffraction experiments on polymer and biopolymer fibers

The status of synchrotron radiation (SR) microdiffraction techniques developed at the ID13 beamline of the European Synchrotron Radiation Facility (ESRF) is reviewed for polymer and biopolymer fiber applications. Beam sizes in the micrometer-range have been used to study the local structure of whole fibers such as viscose-rayon or poly(p-phenylene terephthalamide). The possibilities for in situ studies during stretching, extrusion, or indentation will be discussed.     

Decay of correlation functions in hard-sphere mixtures: structural crossover

We investigate the decay of pair correlation functions in a homogeneous (bulk) binary mixture of hard spheres. At a given state point the asymptotic decay r-->infinity of all three correlation functions is governed by a common exponential decay length and a common wavelength of oscillations. Provided the mixture is sufficiently asymmetric, size ratios q less than or approximately 0.7, we find that the common wavelength reflects either the size of the small or that of the big spheres. By analyzing the (complex) poles of the partial structure factors we find a sharp structural crossover line in the phase diagram. On one side of this line the common wavelength is approximately the diameter of the smaller sized spheres whereas on the other side it is approximately the diameter of the bigger ones; the wavelength of the longest ranged oscillations changes discontinuously at the structural crossover line. Using density functional theory and Monte Carlo simulations we show that structural crossover also manifests itself in the intermediate range behavior of the pair correlation functions and we comment on the relevance of this observation for real (colloidal) mixtures. In highly asymmetric mixtures, q< or =0.1, where there is metastable fluid-fluid transition, we find a Fisher-Widom line with two branches. This line separates a region of the phase diagram where the decay of pair correlations is oscillatory from one in which it is monotonic.     

The Beginnings of Organic Photochemistry

Although sunlight induced photochemistry must have occurred on the planet Earth for billions of years, the chemical changes caused by light have attracted systematic scientific scrutiny only relatively recently. How did scientists first conceive the idea that the interaction of materials with light could not only cause physical phenomena, but could also alter their chemical nature? When sunlight began to be employed as a heat source for distillation, the eventual discovery of photochemical reactions was assured. One can envision three types of changes that would have aroused the curiosity of laboratory chemists: color changes; the evolution of gas bubbles (oxygen in photosynthesis); and the precipitation of a photoproduct less soluble than its precursor. Less predictable was the observation that sunlight caused crystalline santonin to burst because it is converted into a product with a different crystal lattice. In the course of the eighteenth and nineteenth centuries a variety of photochemical reactions, some observed by chance, others uncovered in carefully planned studies, ultimately led to a major systematic investigation that established photochemistry as a viable branch of chemistry.     

N-methyl-N-trimethylsilyltrifluoroacetamide-promoted synthesis and mass spectrometric characterization of deuterated ephedrines

Synthesis and mass spectrometric characterization of drugs or metabolites labeled by stable isotopes has been of great interest in fields of clinical, forensic and doping control analysis. Deuterated ephedrine and p-hydroxypseudoephedrine were prepared from corresponding amines by a novel procedure utilizing N- methyl-N-trimethylsilyltrifluoroacetamide and deuterated iodomethane. The mechanism of methylation was studied by mass spectrometry using phenylethylamine as a model compound, and a rearrangement based on an intermediate six-membered ring structure with a trimethylsilyl-enol-ether is proposed giving rise to a leaving group of trimethyliodosilane and the desired monomethylated product. Deuterated analogues to frequently quantitated ephedrines were readily synthesized with purities >90%, and mass spectra recorded under different ionization and dissociation conditions presented distinct fragmentation processes including eliminations of water and methylamine as well as the generation of a benzyl cation.     

Synthesis, modeling, and anti-tubulin activity of a D-seco paclitaxel analogue

[reaction: see text] We have previously described a model of paclitaxel-microtubule binding that led to the prediction that analogues of paclitaxel lacking any D ring could stabilize microtubules as well as paclitaxel if the substituent present at C4 did not have unfavorable steric interactions with the binding pocket. We report the synthesis of a 4-methyl paclitaxel analogue, compound 1, which bears this prediction out. Compound 1 is as potent as paclitaxel at microtubule stabilization in vitro; however, it has only about one-four-hundredth the cytotoxicity of paclitaxel.     

1,6- and 2,6-di-t-butyl-cyclopent [f] azulenide; double bond localization in a planar 14 π-perimeter

The stable diatropic 1,6- and 2,6-di-t-butyl-cyclo-pent[f]azulenides $\text{9a}$ and $\text{b}$ are prepared and shown to be dominated by polymethide conjugation rather than by 14π-perimeter conjugation.     

Preparation and characterization of hollow spheres of rutile

Hollow spherical particles of rutile were obtained by coating colloidal polystyrene beads with a titanium oxide hydrate layer and subsequently calcining at elevated temperatures under an oxygen atmosphere. In order to investigate the optimum conditions for the preparation of these hollow beads the maximum temperature and heating rate were systematically varied. The dimensions of the voids and the shell thickness of the hollow beads can be tailored by the size of the polystyrene beads and the thickness of the inorganic layer, respectively.     

Über die Alkalienbestimmung in Mineralien mit Hilfe des Interferometers

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