Structural analysis of models for AsxS1−x glasses

The structural models of chalcogenide glasses As_xS_1?x for x = 0.2 and 0.5 have been constructed by the three-step procedure and the characteristic dependence of atomic scale structure on the As content has been analysed. The AsS bonds are most favorable and the distributions of bond lengths and bond angles are very narrow around their minimum energy configurations. The glass of x = 0.2 consists of cross-linked linear chains and the number of molecular units increases drastically in the range of x = 0.4 ≈ 0.5. Furthermore, the two-dimensional network cannot be seen for x = 0.2 and 0.5, which is the essential difference from the glass with x = 0.4.     

Elastic modulus of the crystalline regions of polyimide derived from poly(amic acid)–biphtalic dianhydride and p-phenylene diamine

The elastic modulus E_l of the crystalline regions in the direction parallel to the chain axis was measured by X-ray diffraction for polyimide derived from polyamic acid (biphthalic dianhydride and p-phenylene diamine). These specimens were cured by two different routes: curing at 200 °C, and at 400 °C for 1 h, respectively (2STEPS), and curing from 80 °C to 400 °C stepwise (nine steps) for 1 h at each step (STEPWISE). The E_l values of 54–169 GPa were obtained for the STEPWISE specimen and 80–178 GPa for the 2STEPS specimen, depending on the meridional reflection employed for measurement of the E_l value. A linear relationship between the E_l value and the fiber identity period was obtained from each meridional diffraction, such that the E_l value increased with an increase in the fiber identity period. The E_l value of the fully extended structure was estimated to be 210 GPa. These are considered to be due to the coexistence of polymorphs with different skeletal structures. The crystalline regions of the 2STEPS specimen seems to consist of a more extended skeleton than those of the STEPWISE specimen. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3294–3301, 1999     

The role of the membrane-assisted hydrophobic interaction between di-, tri-, or tetrapeptide catalysts and amino acid esters for the enhancement of stereoselective hydrolysis reactions

The peptide catalyst having the amino acid sequence (Z-L -Leu (or Phe)-L -His) was found to be the most stereoselective among the L -histidyl group-containing di-, tri-, or tetrapeptide catalysts in the hydrolysis of enantiomeric amino acid substrates in vesicular membranes. The role of the membrane-assisted hydrophobic interaction between the peptide catalyst and enantiomeric substrates for the improvement of the hydrolysis stereoselectivity was demonstrated by means of direct measurements of nuclear Overhauser effect spectroscopy (NOESY) spectra of the interaction system and by the extremely high stereoselectivity itself obtained by intensifying the membrane-promoted hydrophobic interaction between the catalyst and the substrates.     

On voids in InxGa1−xSb crystals grown by an ultrasonic- vibration-introduced Czochralski method

The shape and size of voids, and the grown region surrounding a large void were investigated in In_xGa_1–xSb crystals, which were pulled under the introduction of ultrasonic vibrations into the source melt by using a modified Czochralski apparatus. The presence of voids in crystals resulted from (1) atmosphere gas was confined beneath the growth interface of a seed crystal during the seeding procedure, and (2) many bubbles–generated by the cavitation effect due to ultrasonic vibrations–were caught in the growing crystal. Voids were circular, deformed trapezoid, and complex in shape, and were in a range of 20 μm to 3 mm in size. Even in case of a void with the diameter as large as 2 mm, the grown region surrounding it was in the single crystalline state. In this interesting region, the microscopic variation of In concentration and the abrupt change of growth rate were observed.     

Quantum-dot structures measuring Hamming distance for associative memories

Two types of quantum-dot circuits measuring a Hamming distance using the Coulomb repulsion effect are proposed and analysed. They have structures where a quantum-dot array is arranged on a gate electrode of an ultrasmall MOSFET. The device parameters for successful operation are clarified from Monte Carlo simulation.     

Preparation of SiO2-TiO2 Aerogels Using Supercritical Impregnation

The preparation of SiO₂-TiO₂ aerogels by supercritical impregnation of titanium alkoxides into silica alcogels was investigated. A mixture of CO₂ and 2-propanol with dissolved titanium tetraisopropoxide modified with acetylacetone was used as the impregnation medium. Prior to the experiments, the supercritical behaviour of the impregnation solution was investigated. The microstructure and properties of aerogels prepared by the supercritical impregnation method were almost identical to those generated by the liquid impregnation. However, the time for impregnation was substantially decreased and the homogeneity of the impregnated titanium distribution on the aerogel increased.     

Crown ether strategy toward chemical activation of biological protein functions

The chemical activation of biological proteins is outlined, in which small molecules are used to alter the chemical and physical properties of biological proteins through direct or indirect interactions. Crown ethers have the potential to modulate the protein functions by supramolecular complexations, because they bind alkylammonium and other ionic residues of the proteins as well as ionic components in their systems. Two interesting examples are described in which crown ether derivatives improved the protein functions: (1) enhancement of reactivity and enantioselectivity in lipase‐catalyzed asymmetric reactions; and (2) generation of catalytic activity in the oxidation with cytochrome c. This chemical activation based on crown ether chemistry can be viewed as a complementary method to biological mutation in modifying the biological protein functions.