Design,synthesis and structural analysis of mixed α/β-peptides that adopt stable cyclic hairpin-like conformations

The strategic replacement of four α-amino acid residues of a cyclo-(ααααα)₂ peptide by β-, β²- or β³-amino acids residues provided a series of novel 2:1 α/β-mixed peptides that were designed to adopt cyclic hairpin-like structures. It was shown that conformationally stable cyclo-(αβαβα)₂ isomers can be obtained using both enantiomers of the central two basic α-amino acid residues, a known α-amino acid turn sequence and several combinations of facing β-amino acid residues with no side chain or a hydrophobic side chain having specific regio- and stereochemistry. The X-ray analysis of two derivatives provides molecular details of the intra-molecular hydrogen bonding interaction, dihedral angles of the backbone and side chain positioning of the novel cyclic hairpin-like structures. One of these isomers forms an unprecedented hexagon-shaped nano-channel assembly in the crystal structure. Well-defined cyclic hairpin-like structures as described here and derivatives that can be readily designed based on this research can be used as scaffolds onto which functional groups can be grafted in a spatially controlled manner and as β-hairpin mimics with specific biological properties.     

A new approach to stochastic evolution equations with adapted drift

In this paper we develop a new approach to stochastic evolution equations with an unbounded drift A which is dependent on time and the underlying probability space in an adapted way. It is well-known that the semigroup approach to equations with random drift leads to adaptedness problems for the stochastic convolution term. In this paper we give a new representation formula for the stochastic convolution which avoids integration of non-adapted processes. Here we mainly consider the parabolic setting. We establish connections with other solution concepts such as weak solutions. The usual parabolic regularity properties are derived and we show that the new approach can be applied in the study of semilinear problems with random drift. At the end of the paper the results are illustrated with two examples of stochastic heat equations with random drift.     

Screening approach for chiral separation of pharmaceuticals. Part I. Normal-phase liquid chromatography

A strategy for rapid screening for the separation of chiral molecules of pharmaceutical interest by normal-phase liquid chromatography using three cellulose/amylose stationary phases is proposed. In a first step, the most important parameters for the separations were determined and studied for their effects by means of experimental designs. Results showed that the cellulose tris-(3,5-dimethylphenylcarbamate), the amylose tris-(3,5-dimethylphenylcarbamate) and the cellulose tris-(4-methylbenzoate) stationary phases have very broad and complementary enantiorecognition properties. The type of organic modifier used in the mobile phase appeared to have a dramatic influence on the quality of the separation. Based on the results of the preliminary study, a screening strategy was developed and successfully applied to a set of 36 diverse drugs. Enantiomeric separation was observed in 89% of cases and the analysis times were usually shorter than 20 min.     

The density dependence of hot-electron thermalization in liquid argon,krypton and xenon

The time required for thermalization of hot electrons in liquid argon, krypton and xenon increases with decreasing density to a minimum value of ≈ 7 ns at a density of ≈ 1.2 × 10²² cm^?3. Previous data for the solids are found to lie an extrapolations of the liquid-phase density dependence.     

High charge-carrier mobility in pi-deficient discotic mesogens: design and structure-property relationship

Hexaazatrinaphthylene (HATNA) derivatives with six alkylsulfanyl chains of different length (hexyl, octyl, decyl and dodecyl) have been designed to obtain new potential electron-carrier materials. The electron-deficient nature of these compounds has been demonstrated by cyclic voltammetry. Their thermotropic behaviour has been studied by means of differential scanning calorimetry and polarised optical microscopy. The supramolecular organisation of these discotic molecules has been explored by temperature-dependent X-ray diffraction on powders and oriented samples. In addition to various liquid crystalline columnar phases (Col(hd), Col(rd)), an anisotropic plastic crystal phase is demonstrated to exist. The charge-carrier mobilities have been measured with the pulse-radiolysis time-resolved microwave-conductivity technique. They are found to be higher in the crystalline than in the liquid crystalline phases, with maximum values of approximately 0.9 and 0.3 cm(2) V(-1) s(-1), respectively, for the decylsulfanyl derivative. Mobilities strongly depend on the nature of the side chains.     

Fatty acids add grease to exocytosis

In this issue of Chemistry & Biology, Rickman and Davletov suggest a novel mechanism for biological lipids to regulate synaptic transmission in the brain. Physiologically relevant concentrations of arachidonic acid help to dissociate a protein complex involved in exocytosis.     

Rational design of oligopeptide organizers for the formation of poly(ethylene oxide) nanofibers

Template pre-organized oligopeptides were conjugated to poly(ethylene oxide) chains yielding peptide-polymer-building blocks that self assemble into fiber-like nanostructures having a maximum length in the range of a micrometer.     

Tunable command layers for liquid crystal alignment

A simple method for the construction of a stable, tunable, self-assembled command layer for liquid crystal display purposes is described. A pyridine-functionalized oligosiloxane spontaneously forms an anisotropic, grooved surface on indium-tin-oxide, enabling it to align liquid crystalline molecules. The pyridine functions act as seeds for the epitaxial growth of stacks of highly ordered zinc phthalocyanines, the height of which can be controlled. These stacks increase the interaction between the surface and the liquid crystalline matrix by amplifying the surface ordering into the liquid crystal bulk. By varying the height of the stacks, direct control over the properties of the liquid crystal domains is achieved. These properties can be further tuned by adding to the liquid crystal, micro- and nanomolar concentrations of nitrogen-containing compounds, which are capable of interacting with and dissolving the stacks. The procedures we describe offer possibilities to use such tunable systems in LCD-based sensor devices as well as in solar-cell applications.     

New procedure for the preparation of highly sterically hindered alkenes using a hypervalent iodine reagent

New methodology is described to construct the olefinic bond in overcrowded alkenes using a hypervalent iodine reagent, and applied in the synthesis of molecular motors.     

Exploring the boundaries of a light-driven molecular motor design: new sterically overcrowded alkenes with preferred direction of rotation

Insight in the steric and electronic parameters governing isomerization processes in artificial molecular motors is essential in order to design more advanced motor systems. A subtle balance of steric parameters and the combination of helical and central chirality are key features of light-driven unidirectional rotary molecular motors constructed so far. In an approach to decrease the steric hindrance around the central olefinic bond (rotary axis) and thereby lowering the energy barrier for helix inversion resulting in an increased rotation rate, the boundaries of our molecular motor design are explored. In a new design of a light-driven molecular motor based on a sterically overcrowded alkene the methyl substituent adjacent to the stereogenic center, which is responsible for the control of the direction of rotation, is shifted one position away from the fjord region of the molecule compared to the second-generation motor systems. In contrast to previously developed light-driven molecular motors, there is a preference for the methyl substituent to adopt a pseudo-equatorial orientation. Nevertheless, this new type of motor is capable of functioning as a rotary molecular motor, albeit not with full unidirectionality. Under the combined influence of light and heat, there is a preferred clockwise rotation of one half of the molecule. Surprisingly, the effect of shifting the methyl substituent on the energy barrier for helix inversion is small and even a slight increase in the barrier is observed.