Separated and aligned molecular fibres in solid state self-assemblies of cyclodextrin [2]rotaxanes

The conformations of two [2]rotaxanes, each comprising alpha-cyclodextrin as the rotor, a stilbene as the axle and 2,4,6-trinitrophenyl substituents as the capping groups, have been examined in solution and in the solid state, using (1)H NMR spectroscopy and X-ray crystallography, respectively. In solution, introducing substituents onto the stilbene prevents the cyclodextrin from being localized over one end of the axle. Instead the cyclodextrin moves back and forth along the substituted stilbene. In the solid state, the axles of the rotaxanes form extended molecular fibres that are separated from each other and aligned along a single axis. The molecular fibres are strikingly similar to those formed by the axle component of one of the rotaxanes in the absence of the cyclodextrin, but in the latter case they are neither separated nor all aligned.     

Transformations in the medium-range order of fused silica under high pressure

Molecular dynamics simulations of fused silica at shock pressures reproduce the experimental equation of state of this material and explain its characteristic shape. We demonstrate that shock waves modify the medium-range order of this amorphous system, producing changes that are only clearly revealed by its ring size distribution. The ring size distribution remains practically unchanged during elastic compression but varies continuously after the transition to the plastic regime.     

Lamellar phase of stacked two-dimensional rafts of actin filaments

We examined liquid crystalline phases of the cytoskeletal polyelectrolyte filamentous (F-)actin in the presence of multivalent counterions. As a function of increasing ion concentration, the F-actin rods in either an isotropic or a nematic phase will transform into a new and unexpected lamellar phase of cross-linked rafts (L(XR) phase), before condensing into a bundled phase of parallel, close-packed rods. This behavior is generic for alkali earth divalent ions Mg2+, Ca2+, Sr2+, and Ba2+, and the structural transitions are achieved without any architecture-specific actin-binding linker proteins.     

Vibrationally averaged spin densities on muons and protons in the hydroxyl and ethyl radicals

The observations of the residual isotope effect for muonated radicals are discussed using calculations on the hydroxyl and ethyl radicals. The origins are partly due to the non-linear increase of the hyperfine coupling constants with increase in bond length, although a significant contribution comes from the anharmonicity of the molecular vibrations.     

Hydrolysis of α-amino acid esters in mixed ligand complexes with bis(piperidine)palladium(II)

Summary -Amino acid esters, NH₂CH(R)CO₂R, interact withcis- [Pd(pip)₂(OH₂)2]²⁺ (pip = piperidine) in aqueous solution according to the equilibrium (1). The kinetics of hydrolysis of the ester group in the complexes [Pd(pip)₂(NH₂CH(R)CO₂R°)]²⁺ have been studied by pH-stat methods and rate constants for the kinetic processes (2) and (3) determined for methyl glycinate, ethyl glycinate, ethyl L--alaninate, methyl L--phenylalaninate, ethyl picolinate, methyl L-cysteinate and methyl L-histidinate. For the first five esters, substantial rate enhancements are observed for base hydrolysis (factors of 2.8 × 10⁵ fold for methyl glycinate to 4.9 × 10⁷-fold for ethyl picolinate). The effects with methyl L-cysteinate and methyl L-histidinate are much less marked as the mixed ligand complexes do not involve alkoxycarbonyl donors. Thus with methyl cysteinate donation occursvia the primary amino group and the thiolate group. Activation parameters have been determined for reactions (2) and (3) with methyl glycinate. Possible mechanisms for these hydrolytic reactions are considered. Nucleophilic attack by water or hydroxide ion on a chelate ester species satisfactorily accounts for the experimental observations.     

The effect of additives on the polymerization of methyl methacrylate. tris(1,2-dibromo-propyl)phosphate

The effect of the additive, Tris(1,2-dibromopropyl)phosphate, on the kinetics of free radical polymerization of methyl methacrylate is studied. No marked differences were observed in the homopolymerization despite the reported effect of this additive on the mechanical properties of the resulting polymer. It was concluded that the additive was a multifunctional transfer agent producing linear and branched polymer molecules. Several model transfer agents have been studied to confirm these conclusions. H-NMR spectra of the polymer confirm that there is no change in the stereoregularity of the polymer prepared in the presence of this additive, and no copolymerization of the phosphate unit.     

Generalized selection rules: An augmentation procedure for the determination of symmetry invariants

A group Augmentation procedure is presented for the analytic determination of tensional invariants of arbitrary order for all point groups. The results are an ideal pratical adjunct to the generalized selection rule approach, which focuses predominantly on the concept of symmetry invariant operator products, and reduces in the non-quantum limit to the definition of classical tensorial invariants. The relationship to rotational averages is discussed, and some applications to spectroscopic processes are considered.     

Über die Schmelzcharakteristik von Oligourethanen

Ohne Zusammenfassung     

Glyco- and peptidomimetics from three-component Joullié-Ugi coupling show selective antiviral activity

Chlorination-elimination chemistry coupled with three-component Joullié-Ugi reaction and facile deprotection allowed efficient access to an array of polyhydroxylated pyrrolidines through parallel synthesis that may be considered to be a library of imino (aza) sugars (glycomimetics) and/or dihydroxyprolyl peptides (peptidomimetics). The utility of generating such a library was illustrated by screening against 15 different targets that revealed potent and selective inhibition of the Gaucher's disease glycosyltransferase enzyme glucosylceramide synthase and of primary pathogen model for human hepatitis C virus (HCV) and bovine diarrhoeal virus (BVDV). An observed selectivity for this HCV model over hepatitis B virus and remarkably low toxicity suggest a novel mode of action.     

Palladium-catalyzed synthesis of 2,1'-disubstituted tetrahydrofurans from gamma-hydroxy internal alkenes. Evidence for alkene insertion into a Pd-O bond and stereochemical scrambling via beta-hydride elimination

Palladium-catalyzed reactions of gamma-hydroxy internal acyclic alkenes with aryl bromides afford 2,1'-disubstituted tetrahydrofurans in good yields with diastereoselectivities of 3-5:1. The analogous transformations of substrates bearing internal cyclic alkenes afford fused bicyclic and spirocyclic tetrahydrofuran derivatives in good yields with excellent diastereoselectivities (>20:1). A series of deuterium labeling experiments indicate that the origin of the modest diastereoselectivity in reactions of acyclic internal alkene substrates likely derives from a series of reversible beta-hydride elimination and sigma-bond rotation processes that occur following a rare intramolecular alkene syn-insertion into an intermediate Pd(Ar)(OR) complex. In addition, these studies shed light on the chemoselectivity of insertion, suggesting that the alkene inserts into the Pd-O bond in preference to the Pd-C bond.