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.     

Hybrid numerical methods in static and dynamic fracture mechanics

In this article, the concept of the hybrid numerical methods is clarified. On the basis of this concept, various hybrid numerical methods used in static and dynamic fracture mechanics are classified into five categories: (i) hybrid experimental–numerical methods, (ii) hybrid numerical–experimental methods, (iii) hybrid analytical–numerical methods, (iv) hybrid numerical–analytical methods, and (v) hybrid numerical–numerical methods. Features of each category of hybrid numerical method are presented with pertinent numerical results.     

Modification by H-termination in growth process of titanium silicide on Si(0 0 1)-2 × 1 observed with scanning tunneling microscopy

Formation processes of titanium silicide on hydrogen-terminated H/Si(0 0 1)-2 × 1 surface are studied at the atomic scale with a scanning tunneling microscopy (STM). Square-shaped nanoislands were observed on the Ti/H/Si(0 0 1) surface after annealed at 873-1073 K. These are the epitaxial nanoislands moderately grown due to the local orientation relationship between C49-TiSi₂ and Si(0 0 1), because passivation by surface hydrogen on Si(0 0 1) suppresses active and complex bond formation of Ti-Si.     

Photocatalytic decomposition of an alkylammonium cation in a Langmuir-Blodgett film of a titania nanosheet

The formation of inorganic-organic hybrid films of a titania nanosheet and an amphiphilic alkylammonium cation has been investigated, and the photocatalytic decomposition of the alkylammonium cation in the film has been pursued. When a solution of the amphiphilic alkylammonium salt (octadecylammonium chloride: ODAH+ Cl-) was spread on an interface between the air and a titania nanosheet suspension, the negatively charged nanosheets were adsorbed onto the floating monolayer of ODAH+ to form a hybrid monolayer. The hybridization was confirmed by pi-A isotherm measurements, atomic force microscopy, and X-ray photoelectron spectroscopy. Multilayered films were fabricated in a layer-by-layer way by transferring the hybrid monolayers onto glass plates. Areas per ODAH+ cation in the films were estimated from the infrared (IR) spectra of the films, but these areas were smaller than those estimated from the pi-A isotherm curves. The orientation of the alkyl chain of ODAH+ in the hybrid film was determined by means of polarized IR spectroscopy. The alkyl chains were tilted 41 +/- 1 degrees and 47 +/- 1 degrees from the surface normal for the films prepared from the 8 and 20 ppm (ppm = mg dm(-3)) suspensions, respectively. Together with X-ray diffraction data of the films, the structure of the hybrid film was discussed. When the films were illuminated with a UV light, the absorption intensities due to the alkyl chain of ODAH+ decreased exponentially, indicating the photocatalytic decomposition of ODAH+ by the titania nanosheets in the films. Deviation from the exponential trend in the decomposition rate was observed in the initial period for the hybrid films prepared from the suspensions at low concentrations. Interestingly, the layered structure of the hybrid film was disturbed significantly after the ODAH+ cations were decomposed.     

Melophlins P, Q, R, and S: four new tetramic acid derivatives, from two Palauan marine sponges of the genus Melophlus

Four new tetramic acid derivatives, named melophlins P, Q, R, and S (1-4), were isolated from two marine sponges of the genus Melophlus collected at Palau, together with seven known melophlins A, D, E, G, H, I, and O. The structures of the new compounds were elucidated on the basis of their spectral data. The absolute stereochemistries at the tetramic acid moieties of the new compounds were determined as 1 : 1 mixtures (racemic) by ESI-LC/MS analysis of derivatives obtained by oxidation and hydrolysis of the respective parent compounds. Melophlins P-S (1-4) showed cytotoxicity against the murine leukemia cell line L1210 with IC(50) values of 20.0, 10.5, 0.85, and 5.13 muM, respectively.     

Temporal formation of optical anisotropy and surface relief during polarization holographic recording in polymethylmethacrylate with azobenzene side groups

The formation of polarization holographic gratings with both optical anisotropy and surface relief (SR) deformation was studied for polymethylmethacrylate with azobenzene side groups. Temporal contributions of isotropic and anisotropic phase gratings were simultaneously determined by observing transitional intensity and polarization states of the diffraction beams and characterizing by means of Jones calculus. To clarify the mechanism of SR deformation, cross sections of SR were characterized based on the optical gradient force model; experimental observations were in good agreement with the theoretical expectation. We clarified that the anisotropic phase change originating in the reorientation of the azobenzene side groups was induced immediately at the beginning of the holographic recording, while the response time of the isotropic phase change originating in the molecular migration due to the optical gradient force was relatively slow.     

Conducting-AFM spectroscopy on ultrathin SiO2 films

We study the dielectric degradation phenomena of ultrathin SiO₂ films using conducting-AFM spectroscopy in a vacuum (1᎒^-5 Pa). In successive current-voltage characteristics, a change of the carrier transport (from Fowler-Nordheim tunneling to direct tunneling) and current instability (which is due to a change in conductance of leakage path by isolated traps) are observed before 'breakdown'. As an example of a possible application of conducting-AFM spectroscopy, we investigate a change in conductivity at a 'breakdown' area.     

Uniaxial elongational viscosity of PS/a small amount of UHMW-PS blends

A series of polystyrene (PS) and a small amount of ultra high molecular weight (UHMW) PS blends have been prepared by using tetrahydrofuran (THF). Matrix PS has an M_w of 423,000 (M_w/M_n= 2.36) and UHMW-PS has either an M_w of 3,220,000 (M_w/M_n= 1.05) or 15,400,000 (M_w/M_n=1.30) in the range of concentration from 0 wt% to 1.5 wt%. The influence of a small amount of UHMW on dynamic viscoelasticity was investigated. At the frequency lower than 0.001 rad/s, the enhancement of G′ was observed by the incorporation of a small amount of UHMW. And the degree of enhancement was in the order of M_w of UHMW and its concentration. The measurement of uniaxial elongational viscosity for the blends was performed and the effects of UHMW on strain-hardening properties were analyzed at equal strain-rate conditions. The concentration of UHMW where the strain-hardening becomes substantially stronger was determined. To get more insight into the cause of enhancement of strain-hardening at a certain concentration, the damping function from step-shear stress relaxation was measured. The influence of a small amount of UHMW on the damping function was found to be small. It was interpreted, from time- and strain-dependency points, that the enhancement of strain-hardening by a small amount of UHMW was governed by the long relaxation time. Received: 6 September 2000 Accepted: 11 January 2001