Liquid crystalline paracyclophane derivatives

Various paracyclophane derivatives incorporating 4,4'-biphenyl, 2,5-diphenyl-1,3,4-thiadiazole, phenyl benzoate and 2,6-disubstituted naphthyl rigid cores were synthesized and their mesomorphic behaviour was studied using polarizing microscopy, DSC and X-ray diffraction. Most of these macrocyclic compounds possess liquid crystalline properties with unexpectedly high clearing temperatures compared to those of conventional calamitic mesogens. In this way, the coupling of two appropriate rigid units using flexible chains to form a macrocycle constitutes a new and powerful approach towards mesophase induction and stabilization. The types of mesophase formed by these macrocycles do not depend only on the nature of the bridging chains, but also strongly on the structure of the rigid aromatic system. The smectic A phase and the E phase are formed by polyetherbiphenylophanes. Poly-ethercyclophanes incorporating the 2,5-diphenylthiadiazole rigid core form nematic and smectic C phases. The nematic phase is the only mesophase when the rigid core is the phenyl benzoate unit. No mesomorphic properties could be detected for macrocycles which featured either the benzyl phenyl ether moiety or the 2,6-disubstituted naphthalene unit in their constitution.     

Biased random walks on directed trees

Summary. We define directed rooted labeled and unlabeled trees and find measures on the space of directed rooted unlabeled trees which are invariant with respect to transition probabilities corresponding to a biased random walk on a directed rooted labeled tree. We use these to calculate the speed of a biased random walk on directed rooted labeled trees. The results are mainly applied to directed trees with recurrent subtrees, where the random walker cannot escape. Received: 12 March 1997/ In revised form: 11 December 1997     

Ensembles de classe (0, 1, q,q + 1) de PG (d,q)

We determine all sets Q of points of any finite dimensional protective space P such that each line intersecting Q in more than one point, either is contained in Q or contains exactly one point not on Q. If P is a finite protective space of order q, these sets are the so called sets of class (0, 1, q, q + 1).     

Nouveaux complexes de structure η4-trimethylenemethane obtenus a partir du tetracarbonylferrate de sodium par insertion d'allene dans une liaison metalcarbone

The mechanism of formation of α,β-unsaturated ketones in the reaction of disodium tetracarbonylferrate, with an alkyl bromide and allene, is investigated; new intermediate complexes are isolated. In the anion resulting from insertion of allene into an ironacyl bond the negative charge is delocalised over the metallic center and the organic ligand and confirms an η⁴-trimethylenemethane type behaviour. Reactions with (CH₃)₃SiCl and proton yield neutral η⁴-trimethylenemethane complex. In the case of proton a subsequent sigmatropic shift yields a η⁴-heterodiene complexe. The anion is isolated as its ionic cristalline PPN⁺ salt.     

Reactivite des gem-dibromocyclopropanes—IV : Ouverture acido-catalysee de gem-dihalogenocyclopropylcarbinols

By acid catalysis, gem-dihalogenocyclopropylcarbinols are converted into homoallylic α-dihalogenated cations which in general eliminate the hydrogen halide and lead to the 3-halogenopentadienyl cations from which cyclopentenones can be formed by cyclisation. For bicyclo[4.1.0]heptanes, aromatisation of the 3-halogenopentadienyl cation in benzyl bromide is observed.     

Cover Picture

The cover picture shows an array of several hundred synthetically produced variants of the 44 amino acids comprising the hyAP-WW protein domain. The array was produced by a stepwise SPOT synthesis on a cellulose membrane. At each synthesis site (spot) a WW domain is bound to the membrane through a C-terminal ester bond. The secondary structure of the WW domain is shown in green as a ribbon. The domains of the single spots differ only by a single amino acid. All the domains were tested simultaneously for their ability to bind to a peptide motif (red) common to many proteins. The binding was evident when visualized by chemoluminescence, with the Spots having various intensities. The systematic analysis undertaken here enabled molecular biology to be carried out that would have otherwise have required great effort, or not been done at all. This chemical technique also allows the construction of many non-genetically coded building blocks. Through the use of modern synthetic techniques for the coupling of peptides this method should also allow the synthesis of proteins. The combination of molecular biological and chemical methods opens up opportunities for the preparation of protein chips for diagnostics and drug discovery. More about this can be found in the article by Schneider-Mergener et al. on p. 897ff.     

Silver- and Coomassie-staining protocols: detection limits and compatibility with ESI MS

Staining protocols for PAGE have to be sensitive and should not impair further MS analysis of selected samples. In this study, the MS compatibility of different silver- and Coomassie-staining protocols with a nano-LC-MS/MS system was systematically elucidated. Altogether, 13 different silver-staining, 1 imidazole-staining and 2 Coomassie-staining protocols were used and compared to each other for their achieved sequence coverage and their detection sensitivity. Three proteins were used as model proteins (bovine serum albumin, rabbit L-lactate dehydrogenase, bovine milk beta-lactoglobulin) in decreasing concentration (12 pmol down to 30 fmol) and different staining protocols were applied. The conclusion of this study is that two silver-staining protocols (Blum, H. et al.,. Electrophoresis 1987, 8, 93-99 and Shevchenko, A. et al.,. Anal. Chem. 1996, 68, 850-858) combine good sequence coverage and good sensitivity and are recommended for nano-LC-MS/MS analysis.