Preconditioned conjugate gradients for solving the transient Boussinesq equations in three-dimensional geometries

In this paper we present a new version of the ‘modified finite element method’ (MFEM) presented by Gresho, Chan, Lee and Upson.¹ The main modification of the original algorithm is the introduction of a cost-effective and memory-saving iterative solver for the discretized Poisson equation for the pressure. The vectorization of the preconditioner has been especially considered. For low Prandtl number problems we also split the advection-diffusion operator of the energy equation into explicit and implicit parts. In that sense the present approach is related to the recent implicitization of the diffusive terms introduced by Gresho and Chan² and by Gresho.³ The algorithm is applied to the study of buoyancy-driven flow oscillations occuring in a horizontal crucible of molten metal under the action of a horizontal temperature gradient.     

Finite element simulation of viscoelastic fluids of the integral type

In the present paper, we develop an algorithm for calculating the steady flow of viscoelastic fluids of the integral type. The calculation is based upon a simple integration of the strains along the streamlines, and the method remains valid for arbitrary elements. The technique is applied to the flow of a Maxwell fluid through a wedge and to the calculation of the hole-pressure error for the Doi-Edwards fluid.     

Electrochemical properties of a new cytostatic agent: 2-Formylpyridine thiosemicarbazone

Summary The electrochemical characteristics of 2-formylpyridine thiosemicarbazone (2-FPTS) have been studied by direct current, alternating current, and differential pulse polarography and cyclic voltammetry at a mercury electrode. This new cytostatic agent is reduced in a two-electron irreversible step at the nitrogen double bond of the side chain, in the pH range 0.2–12. A double anodic wave very close to the oxidation wave of mercury, corresponding to the formation of an organo-mercury compound, develops in neutral and alkaline media. These different electrochemical reactions are complicated by adsorption. In alkaline media, the carbon-nitrogen double bond reduction current decreases progressively with increasing pH due to formation of an electrochemically inactive species. Finally, a catalytic reduction wave of protons is developed in neutral solutions. Quantitative determination of the compound was possible in the range 1×10^–7 to 1×10^–3 M, using the diffusion controlled reduction wave at pH 7. The qualitative detection limit is 2×10^–8 M (4 ng/ml).

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Elektrochemische Eigenschaften einer neuen cytostatischen Substanz: 2-Formylpyridin-thiosemicarbazon

Zusammenfassung Die elektrochemischen Eigenschaften von 2-Formylpyridin-thiosemicarbazon wurden mit Hilfe der Gleichstrom-, Wechselstrom- und Differentialpuls-Polarographie sowie der cyclischen Voltammetrie an der Hg-Elektrode untersucht. Die neue cytostatische Substanz wird in einer irreversiblen Zweielektronen-Stufe an der Stickstoff-Kohlenstoff-Doppelbindung der Seitenkette reduziert (pH-Bereich 0,212). In neutralen und alkalischem Medium entwickelt sich eine anodische Doppelstufe dicht bei der Oxidationsstufe des Quecksilbers, entsprechend der Bildung einer quecksilberorganischen Verbindung. Diese elektrochemischen Reaktionen werden durch Adsorptionsvorgange verkompliziert. In alkalischem Medium nimmt der Reduktionsstrom der Kohlenstoff-Stickstoff-Doppelbindung mit zunehmendem pH ab, was der Bildung einer elektrochemisch inaktiven Substanz zugeschrieben wird. In neutraler Lösung wird eine katalytische protonische Reduktionsstufe gebildet. Die quantitative Bestimmung war im Bereich von 1·10^–7 bis 1·10^–3 M möglich, wobei die diffusionskontrollierte Reduktionsstufe bei pH 7 benutzt wurde. Die qualitative Nachweisgrenze liegt bei 2·10^–8 M (4 ng/ml).

     

Catalysis and specificity in enzymatic glycoside hydrolysis: a 2,5B conformation for the glycosyl-enzyme intermediate revealed by the structure of the Bacillus agaradhaerens family 11 xylanase.

BACKGROUND: The enzymatic hydrolysis of glycosides involves the formation and subsequent breakdown of a covalent glycosyl-enzyme intermediate via oxocarbenium-ion-like transition states. The covalent intermediate may be trapped on-enzyme using 2-fluoro-substituted glycosides, which provide details of the intermediate conformation and noncovalent interactions between enzyme and oligosaccharide. Xylanases are important in industrial applications - in the pulp and paper industry, pretreating wood with xylanases decreases the amount of chlorine-containing chemicals used. Xylanases are structurally similar to cellulases but differ in their specificity for xylose-based, versus glucose-based, substrates. RESULTS: The structure of the family 11 xylanase, Xyl11, from Bacillus agaradhaerens has been solved using X-ray crystallography in both native and xylobiosyl-enzyme intermediate forms at 1.78 A and 2.0 A resolution, respectively. The covalent glycosyl-enzyme intermediate has been trapped using a 2-fluoro-2-deoxy substrate with a good leaving group. Unlike covalent intermediate structures for glycoside hydrolases from other families, the covalent glycosyl-enzyme intermediate in family 11 adopts an unusual 2,5B conformation. CONCLUSIONS: The 2,5B conformation found for the alpha-linked xylobiosyl-enzyme intermediate of Xyl11, unlike the 4C1 chair conformation observed for other systems, is consistent with the stereochemical constraints required of the oxocarbenium-ion-like transition state. Comparison of the Xyl11 covalent glycosyl-enzyme intermediate with the equivalent structure for the related family 12 endoglucanase, CelB, from Streptomyces lividans reveals the likely determinants for substrate specificity in this clan of glycoside hydrolases.     

Improving the accuracy of carbon-to-hydrogen ratio determination for P,N, S,O, Cl,and Br-containing organic compounds using atomic emission detection

The objective of this work was to investigate the dependence of atomic emission detector C and H response on microwave-induced plasma conditions and to improve the accuracy of carbon-to-hydrogen ratio determination for trialkylphosphates, herbicides, chlorophenols, and sulfur-containing organic compounds. Compounds which differed structurally from the analytes were used as reference compounds. It was found that when the oxygen concentration in the helium was the maximum for the instrument (9%) relative errors in carbon-to-hydrogen ratio determination were 3–8%, irrespective of analyte and reference compound structure, whereas when working in the mode of operation recommended by the manufacturer of the instrument (1.5% oxygen in helium) the respective errors were 10–20% or higher. This improvement in the accuracy of carbon-to-hydrogen ratio determination was accompanied by a factor of ten decrease in sensitivity.