Cowaniin, a C-glucosidic ellagitannin dimer linked through catechin from Cowania mexicana

A new complex tannin, cowaniin (1) was isolated from the leaves and stems of Cowania mexicana (Rosaceae), and its structure was characterized as novel C-glucosidic tannin dimer linked through (+)-catechin on the basis of spectral and chemical evidence. The inhibitory effect on activation of the Epstein-Barr virus early antigen was assessed for cowaniin. Six known polyphenols and related compounds, including a nitrile glucoside, purshianin, were also characterized.     

Effects of bulk impurity concentration on the reactivity of metal surface: sticking of hydrogen molecules and atoms to polycrystalline Nb containing oxygen

Nonmetallic impurities segregated onto metal surfaces are able to drastically decrease the chemical reactivity of metals. In the present paper, effects of bulk impurities on the reactivity of metallic surfaces were investigated in a wide temperature range on an example of the sticking of hydrogen molecules and atoms to Nb [polycrystalline, with mainly (100)] containing solute oxygen. At all the investigated surface temperatures, T(S) (300-1400 K), we found the bulk oxygen concentration C(O) to have a strong effect on the integral probability, alpha(H(2) ), of dissociative sticking of H(2) molecules followed by hydrogen solution in the metal lattice: alpha(H(2) ) monotonically decreased by orders of magnitude with increasing C(O) from 0.03 to 1.5 at. %. The sticking coefficient alpha(H(2) ) was found to depend on T(S) but not on the gas temperature. The effect of C(O) on alpha(H(2) ) is explained by the presence of oxygen-free sites (holes in coverage) serving as active centers of the surface reaction in the oxygen monolayer upon Nb. In contrast to H(2) molecules, H atoms were found to stick to, and be dissolved in, oxygen-covered Nb with a probability comparable to 1, depending neither on C(O) nor on T(S). This proves that, unlike H(2) molecules, H atoms do stick to be dissolved mainly through regular surface sites covered by oxygen and not through the holes in coverage.     

Membrane potential across reverse osmosis membranes under pressure gradient

Membrane potential measurement has been widely used for the characterization of ionic membranes such as ion-exchange membranes without solvent permeability. However, there have been few studies on membrane potentials across pressure-driven processes such as reverse osmosis (RO) membranes with solvent permeability. In the present study, the membrane potential across RO membranes in NaCl and MgCl2 under the pressure gradient, DeltaP=0-0.3 MPa, was measured. The experimental results were analyzed by the theoretical model based on the Donnan equilibrium and the extended Nernst-Planck flux equation considering the pressure effect. The theoretical values agreed well with the experimental ones. This indicates that membrane potential is useful for characterizing the effective charge density of the active layer of RO membranes under pressure gradient.     

Teaching-Learning of Evaluative Criteria for Mathematical Arguments Through Classroom Discourse: A Cross-National Study

Videotaped lessons of 5th graders on equivalent fractions from 7 American and 6 Japanese classrooms were analyzed in terms of a recurrent pattern in public discourse among a teacher and students. This pattern—called inquiry, response, feedback—occurs when a teacher initiates discourse (mostly with an inquiry), a student or students respond (often with an answer to the teacher inquiry), and the teacher provides feedback to the student's response. We found2 approaches to the teaching-learning of the criteria for evaluating mathematical arguments. In the Japanese classroom, students were encouraged to offer their own argument to the whole class and evaluate arguments proposed by other students. They seldom were given direct evaluation by their teacher. In contrast, American teachers often gave individual elaboration as well as direct evaluation to the student's responses, and some of the teachers offered their own opinions about mathematics, about valid ways of argumentation, or about both. The Japanese approach would help students acquire evaluative criteria indirectly through participating in mathematical discourse, whereas the American approach would help students learn modes of arguments through direct instruction.     

In-plane magnetoresistance of electron-doped superconducting thin films Pr0.9LaCe0.1CuO4

We measured the in-plane magnetoresistance of Pr_0.9LaCe_0.1CuO₄ (PLCCO) epitaxial thin films under various magnetic fields H applied parallel to the tetragonal c-axis. The measurements were performed at the superconducting state as well as the normal state. As the magnetic field is between the low critical field H_c1 and upper critical field H_c2, a critical scaling behavior of electrical resistivity is found. We analyze the electrical transport properties and show the magnetic field H dependence of glass transition temperature T_g and the characteristic temperature T^* for the PLCCO film, which may shed some light on vortex behavior in electron-doped superconductors.     

Highly efficient synthesis of functionalized tertiary alcohols catalyzed by potassium alkoxide-crown ether complexes

A highly efficient Mukaiyama aldol reaction between ketones and trimethylsilyl enolates in the presence of potassium alkoxide-crown ether complexes as Lewis base catalysts (0.3-5 mol %), which minimized the competing retro-aldol reaction, was developed. These catalysts promoted other addition reactions of trimethylsilyl reagents to ketones and aldimines, such as silyltrifluoromethylation, silylcyanation, and silylphosphonylation. A direct hydrophosphonylation of ketones also proceeded when the catalysts were used as a Brønsted base under mild reaction conditions.     

Solution structure of S-DNA formed by covalent base pairing involving a disulfide bond

Here, we present the solution structure of a DNA duplex containing a disulfide base pair (S-DNA). The unnatural nucleoside "S" possessing a thiophenyl group as base was incorporated into a self-complementary singled-stranded oligonucleotide. Crosslinking of the disulfide base pair was analyzed by non-denaturing polyacrylamide gel electrophoresis. Under oxidizing conditions a high molecular weight band as 18 mer, corresponding to the double-stranded molecule (5'-GCGASTCGC: 3'-CGCTSAGCG), was found, whereas single-stranded self-complementary 9 mer oligonucleotide GCGASTCGC was detected in the presence of a reducing agent. These results suggest that the oligonucleotide is covalently linked by disulfide bonding under oxidizing conditions, which can be reversibly reduced to two thiol groups under reducing conditions. CD spectrum of S-DNA (CGASTCG) under oxidizing conditions suggested that the duplex had a right-handed double-stranded structure similar to that of natural DNA (B-form, CGATCG). NMR studies confirmed that this CGASTCG resembled natural B-DNA and that the two phenyl rings derived from the disulfide base pairing intercalated into the duplex. However, these two phenyl rings were not positioned in the same plane as the other base pairs. Specifically, NOEs suggest that although CGASTCG adopts a structure similar to B-type DNA, the S-DNA duplex is bent at the point of disulfide base pairing to face the major groove.     

Ultra-large lateral grain growth by double laser recrystallization of a-Si films

A new double laser recrystallization technique that can produce ultra-large direction- and location-controlled lateral grains is presented. An excimer laser is combined with a pulse-modulated Ar⁺ laser to yield grains of tens of micrometers in size. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer-laser fluence, the Ar⁺-laser power and the pulse duration. The process has a wide process window and is insensitive to both the excimer-laser fluence and the Ar⁺-laser power fluctuations. Preheating of the a-Si film with the Ar⁺ laser before firing the excimer laser is necessary for inducing lateral grain growth. The transient excimer-laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The surface roughness of the recrystallized poly-Si is measured by atomic force microscopy. Received: 14 September 2000 / Accepted: 24 February 2001 / Published online: 27 June 2001     

First preparation and structural determination of 1,1-disubstituted dibenzo[bc,fg][1,4]silathiapentalene derivatives by x-ray crystallographic analysis and MO calculations

1,1-Dimethyldibenzo[bc,fg][1,4]silathiapentalene ( 1a ) was prepared by treatment of 1,9-bis(methyl-sulfinyl)dibenzothiophene with EtMgBr or of dibenzothiophene with n-butyllithium, and then with dimethyl dichlorosilane. The structure of 4,4-dimethyl-dibenzo[bc,fg][1,4]silathiapentalene 1-oxide ( 2 ), obtained by oxidation of compound 1a with mCPBA, was determined by X-ray crystallographic analysis. The structure of compound 2 determined experimentally was compared to the structure obtained by semiempirical molecular orbital calculations (AM1). The MO calculations of compound 1a and its phenyl analog 1b were also performed by AM1 to evaluate their structures. © 1996 John Wiley & Sons, Inc.