Influence of Covalence on the Metal-Insulator Temperature in the CuIr 2 (S 1− x Se x ) 4 Spinels

Influence of the covalence on the metal-insulator (M-I) temperature in the CuIr 2 (S 1 m x Se x ) 4 spinel series is considered. For this purpose a vacancy model and the effective number of valence electrons per molecule are used. Calculations of the vacancy model parameters, i.e., the ion packing coefficients, the filling coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing selenium concentration x , leading to a decrease of the metal-insulator temperature. It was observed that the metal-insulator temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the metal-insulator transition. These effects are explained within the framework of the mixed valence of the iridium ions including structural defects.     

Elevation localization and head-related transfer function analysis at low frequencies

Monaural spectral features due to pinna diffraction are the primary cues for elevation. Because these features appear above 3 kHz where the wavelength becomes comparable to pinna size, it is generally believed that accurate elevation estimation requires wideband sources. However, psychoacoustic tests show that subjects can estimate elevation for low-frequency sources. In the experiments reported, random noise bursts low-pass filtered to 3 kHz were processed with individualized head-related transfer functions (HRTFs), and six subjects were asked to report the elevation angle around four cones of confusion. The accuracy in estimating elevation was degraded when compared to a baseline test with wideband stimuli. The reduction in performance was a function of azimuth and was highest in the median plane. However, when the source was located away from the median plane, subjects were able to estimate elevation, often with surprisingly good accuracy. Analysis of the HRTFs reveals the existence of elevation-dependent features at low frequencies. The physical origin of the low-frequency features is attributed primarily to head diffraction and torso reflections. It is shown that simple geometrical approximations and models of the head and torso explain these low-frequency features and the corresponding elevations cues.     

Facile access to monodisperse ultralarge rings

A facile access to monodisperse ultralarge rings counting 126, 174, and 294 ring atoms is described. It follows a reaction sequence that is well suited for the preparation of [2]catenanes but altered just in the sequence of the two steps cyclization and carbonate formation. The carbonate acts as a covalent template that is easily formed and later cleaved. The obtained monocyclic products are constitutional isomers of the catenanes.     

Enzymatic metabolism of ergosterol by cytochrome p450scc to biologically active 17alpha,24-dihydroxyergosterol

We demonstrate the metabolism of ergosterol by cytochrome P450scc in either a reconstituted system or isolated adrenal mitochondria. The major reaction product was identified as 17alpha,24-dihydroxyergosterol. Purified P450scc also generated hydroxyergosterol as a minor product, which is probably an intermediate in the synthesis of 17alpha,24-dihydroxyergosterol. In contrast to cholesterol and 7-dehydrocholesterol, cleavage of the ergosterol side chain was not observed. NMR analysis clearly located one hydroxyl group to C24, with evidence that the second hydroxyl group is at C17. 17alpha,24-Dihydroxyergosterol inhibited cell proliferation of HaCaT keratinocytes and melanoma cells. Thus, in comparison with cholesterol and 7-dehydrocholesterol, the 24-methyl group and the C22-C23 double bond of ergosterol prevent side chain cleavage by P450scc and change the enzyme's hydroxylase activity from C22 and C20, to C24 and C17, generating bioactive product.     

Effect of the Surface Conductance on Electrokinetic Potential Measured in Nonaqueous Media

The effect of the surface conductance on the -potential of dispersed particles determined from their electrophoretic mobility in nonaqueous electrolyte solutions was considered. The conductivity of dilute quartz suspensions and the electrophoretic mobility of quartz particles in NaBr solutions in butanol-1 and dimethyl sulfoxide as well as in LiBr solutions in acetone at the salt concentration C= 10^–5–10^–2M were determined by conductometry and microelectrophoresis. The dependences of surface conductivity and -potential on the electrolyte content were calculated by formulas of the Wagner and Henry theories. It was shown that, in the region of dilute solutions, (log C) curves thus obtained significantly differ from corresponding functions calculated by the Smoluchowski equation. At the same time, these dependences agreed closely with the (log C) dependences determined for the same systems by the streaming potential method with allowances for experimental values of the surface conductivity. Using aluminum oxide suspensions in NaBr and HBr ethanol solutions as examples, it was shown that, to obtain correct values of the -potential from electrophoretic mobility of porous particles impregnated with a solution, it is necessary to allow for the bulk conductance of the particles.     

Modeling of drying in films of colloidal particles

The process of film formation on a solid substrate from polymer colloid dispersion during solvent evaporation has been investigated by means of the Monte Carlo simulation method. Colloid particles are modeled as hard spheres. Time evolution of the colloid density distribution and coverage of the solid substrate are studied. Both density and structure of colloid film is shown to depend strongly on the evaporation rate. At a low evaporation rate, the coexistence of hexagonal and tetragonal domains of dried colloid monolayer has been observed. The results of monolayer structure are in good agreement with the confocal scanning laser microscopy observations of Dullens et al. (2004).