First-principles molecular-dynamics simulations of a hydrous silica melt: Structural properties and hydrogen diffusion mechanism

We use {\it ab initio} molecular dynamics simulations to study a sample of liquid silica containing 3.84 wt.% H$_2$O.We find that, for temperatures of 3000 K and 3500 K,water is almost exclusively dissolved as hydroxyl groups, the silica network is partially broken and static and dynamical properties of the silica network change considerably upon the addition of water.Water molecules or free O-H groups occur only at the highest temperature but are not stable and disintegrate rapidly.Structural properties of this system are compared to those of pure silica and sodium tetrasilicate melts at equivalent temperatures. These comparisons confirm the picture of a partially broken tetrahedral network in the hydrous liquid and suggest that the structure of the matrix is as much changed by the addition of water than it is by the addition of the same amount (in mole %) of sodium oxide. On larger length scales, correlations are qualitatively similar but seem to be more pronounced in the hydrous silica liquid. Finally, we study the diffusion mechanisms of the hydrogen atoms in the melt. It turns out that HOSi$_2$ triclusters and SiO dangling bonds play a decisive role as intermediate states for the hydrogen diffusion.Comment: 25 pages, 18 figures. submitte

Computational Modeling of Silicate Glasses: A Quantitative Structure-Property Relationship Perspective

This article reviews the present state of Quantitative Structure-Property Relationships (QSPR) in glass design and gives an outlook into future developments. First an overview is given of the statistical methodology, with particular emphasis to the integration of QSPR with molecular dynamics simulations to derive informative structural descriptors. Then, the potentiality of this approach as a tool for interpretative and predictive purposes is highlighted by a number of recent inspiring applications

Comparative classical and ab initio Molecular Dynamics study of molten and glassy germanium dioxide

A Molecular Dynamics (MD) study of static and dynamic properties of molten and glassy germanium dioxide is presented. The interactions between the atoms are modelled by the classical pair potential proposed by Oeffner and Elliott (OE) [Oeffner R D and Elliott S R 1998, Phys. Rev. B, 58, 14791]. We compare our results to experiments and previous simulations. In addition, an ab initio method, the so-called Car-Parrinello Molecular Dynamics (CPMD), is applied to check the accuracy of the structural properties, as obtained by the classical MD simulations with the OE potential. As in a similar study for SiO2, the structure predicted by CPMD is only slightly softer than that resulting from the classical MD. In contrast to earlier simulations, both the static structure and dynamic properties are in very good agreement with pertinent experimental data. MD simulations with the OE potential are also used to study the relaxation dynamics. As previously found for SiO2, for high temperatures the dynamics of molten GeO2 is compatible with a description in terms of mode coupling theory.Comment: 27 pages, 16 figure

A defect in cystathionine beta-lyase activity causes the severe phenotype of a Nicotiana plumbaginifolia methionine auxotroph

In plants and bacteria, methionine (Met) is synthesised through three consecutive reactions starting at the convergence point of one branch of the aspartate pathway and the sulphur reduction pathway. The substrates O-phosphohomoserine and cysteine converge to cystathionine. which is cleaved to homocysteine. Finally, homocysteine is methylated to Met. The second enzymatic step of Met synthesis, the cleavage of cystathionine to homocysteine, pyruvate and ammonia, is catalysed by cystathionine beta-lyase (CbL). Here, we report the functional complementation and phenotypical reversion of a Nicotiana plumbaginifolia mutant previously assumed to be defective in CbL activity using a heterologous bacterial protein targeted to the chloroplast. Molecular analysis revealed the stable integration and high expression rate of the chimeric gene in the complemented mutant. Up to 500-fold more CbL activity when compared to wild type was measured in partially purified extracts from the complemented mutant. Despite the high rate of overexpression and the strongly increased enzyme activity the content of Met was restored only to wild type levels. Furthermore, no change in free amino acid composition could be determined. These results are discussed with respect to regulation of the fluxes involved in Met biosynthesis. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved

Formation of ordered anodic alumina nanofibers during aluminum anodizing in oxalic acid at high voltage and electrical power

In this study, we modified the synthesis modes of aluminum anodizing in an aqueous solution of oxalic acid and suggested quick, one-step and convenient method for porous alumina surface modification to increase largely its surface area. Anodic alumina nanofibers consisting of pure aluminum oxide were formed by high field anodizing with evolving of Joule heating of aluminum foil of 60 and 100 μm thickness in 0.3 M aqueous solution of oxalic acid. It was shown that nanofibers formed on the surface of the films for an anodizing voltage of 90 V and a power range of 13.5–31.5 W cm−2. The thickness of the Al foil has proved to be responsible for the formation of the alumina nanofibers, which cannot be obtained in the same electrolyte and for similar current densities on 25 μm thick aluminum foil. We could show that during anodizing of 60 μm thick Al foil, regardless of anodic current density value, alumina nanofibers uniformly covered the entire surface of the films. However, during 100 μm Al foil anodizing, the pores were etched and the nanofibers partially dissolved and separated from the surface with increasing anodizing current density. Auger electron spectroscopy measurements showed that porous anodic alumina containing carbon impurities was obtained during anodizing of the 25 μm Al foil. Alternatively, the surface of the nanofibers formed on 60 and 100 μm Al foil consisted of pure alumina