Synthesis of Nicotinamide Adenine Dinucleotide (NAD) Analogues with a Sugar Modified Nicotinamide Moiety

The synthesis of nicotinamide adenine dinucleotide (NAD) analogues in which the ribose unit of the nicotinamide moiety is replaced by a hexitol, altritol, and cyclohexenyl sugar mimic is described.     

Influence of molecular structure on secondary flow of polyolefin melts as investigated by laser-Doppler velocimetry

Laser-Doppler velocimetry (LDV) is applied to investigate velocity profiles in the entrance region of a slit die. Due to the high spatial resolution of the device and the accuracy of the velocity measurements the secondary flow patterns of different polyolefins have quantitatively been analyzed for the first time. A linear polyethylene is compared with two long-chain branched polyethylenes and a conventional linear polypropylene with a long-chain branched one. All materials are rheologically characterized with respect to their viscosity functions, elasticity, and elongational properties. For the two linear materials no indication of secondary flow is found, but the three long-chain branched polymers (two polyethylenes and one polypropylene) exhibit pronounced vortices. Neither viscosity nor elasticity seem to be decisive for the occurrence of secondary flow. The viscosity has an influence, however, on the size of the vortices and the velocities within them. All of the three long-chain branched polymers are strongly strain hardening which gives rise to the conclusion that this behavior may be a necessary condition for the formation of vortices. The linear polypropylene does not show any indication of strain hardening. The linear polyethylene, surprisingly, is significantly strain hardening, but it becomes less pronounced with higher strain rates. As most of the deformation in the entrance region takes place at elongational rates at which the strain hardening of the linear polyethylene is not significant, the findings on the linear polyethylene do not contradict the hypothesis that strain hardening and vortex formation in entrance flow may be related to each other. Received: 27 April 2000 Accepted: 30 November 2000     

The as-Grown SiC (0001) Surface as Observed by Reflection Electron Microscopy

Reflection electron microscopy (REM), capable of imaging surfaces in high resolution, reveals that the risers and treads in the terrace growth surfaces of vapor grown SiC(0001) bulk crystals are characterized on a microscopic scale by growth steps. At the risers microscopic growth occurs by the ‘step flow mode’. At the treads, growth occurs by island formation and by operation of growth spirals.     

A Convergent Strategy for the Modification of Peptide Nucleic Acids: Novel Mismatch-Specific PNA-Hybridization Probes

Coupling of nonnatural nucleobases to the orthogonally protected backbone 1 on the solid phase provided access to novel peptide nucleic acid (PNA) conjugates 2 , which are difficult to synthesize by standard routes. Hybridization probes containing the thiazolorange dye might allow DNA sequence analysis in real time. B−CH₂CO=modified nucleobase, fluorescent dye, etc; Boc, Fmoc=protecting groups.     

Rational Design of Azothiophenes—Substitution Effects on the Switching Properties

A series of substituted azothiophenes was prepared and investigated toward their isomerization behavior. Compared to azobenzene (AB), the presented compounds showed red-shifted absorption and almost quantitative photoisomerization to their (Z) states. Furthermore, it was found that electron-withdrawing substitution on the phenyl moiety increases, while electron-donating substitution decreases the thermal half-lives of the (Z)-isomers due to higher or lower stabilization by a lone pair–π interaction. Additionally, computational analysis of the isomerization revealed that a pure singlet state transition state is unlikely in azothiophenes. A pathway via intersystem crossing to a triplet energy surface of lower energy than the singlet surface provided a better fit with experimental data of the (Z)→(E) isomerization. The insights gained in this study provide the necessary guidelines to design effective thiophenylazo-photoswitches for applications in photopharmacology, material sciences, or solar energy harvesting applications.     

Tunable Porosities and Shapes of Fullerene‐Like Spheres

The formation of reversible switchable nanostructures monitored by solution and solid‐state methods is still a challenge in supramolecular chemistry. By a comprehensive solid state and solution study we demonstrate the potential of the fivefold symmetrical building block of pentaphosphaferrocene in combination with Cu^I halides to switch between spheres of different porosity and shape. With increasing amount of CuX, the structures of the formed supramolecules change from incomplete to complete spherically shaped fullerene‐like assemblies possessing an I_h‐C₈₀ topology at one side and to a tetrahedral‐structured aggregate at the other. In the solid state, the formed nano‐sized aggregates reach an outer diameter of 3.14 and 3.56 nm, respectively. This feature is used to reversibly encapsulate and release guest molecules in solution.     

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