Molar heat capacities and volumes of transfer of cytosine,thymine, caffeine and 1,3-diethylthymine to aqueous solutions of glycyl-glycine and L-α-alanyl-L-α-alanine at 25°C

Densities and specific heat capacities of ternary aqueous systems containing dipeptides (glycyl-glycine or L--alanyl-L--alanine) and nucleic acid bases (cytosine or thymine) or their alkyl derivatives (1,3-diethylthymine or caffeine) were determined at 25°C by flow calorimetry and flow densimetry. The partial molar volumes and heat capacities of transfer at infinite dilution of the different nucleic acid bases from water to water+dipeptide solutions were obtained therefrom. Except for the case of the transfer of cytosine to aqueous glycyl-glycine solutions where a small positive dependence of the transfer quantities was observed with the dipeptide concentration, the values of the heat capacities of transfer were in general low, positive or negative, depending on the compensation of hydrophobic-hydrophilic interactions between the dipeptide and the base. The volumes of transfer of most of the bases are very small, within the limit of the experimental error.     

The BKK root count in

The root count developed by Bernshtein, Kushnirenko and Khovanskii only counts the number of isolated zeros of a polynomial system in the algebraic torus . In this paper, we modify this bound slightly so that it counts the number of isolated zeros in . Our bound is, apparently, significantly sharper than the recent root counts found by Rojas and in many cases easier to compute. As a consequence of our result, the Huber-Sturmfels homotopy for finding all the isolated zeros of a polynomial system in can be slightly modified to obtain all the isolated zeros in .

     

Correlations between Van der Waals' volume and retention index general equation applicable to different homologous series

Summary Correlations between retention index (R_I) and Van der Waals' volume (V_W) in homologous series of n-alcohols, n-aldehydes and esters are studied on the stationary phases Carbowax 1540 and squalane. Influence of functional groups is quantified and a method for obtaining one general equation, valid for an indeterminate number of homologous series, is proposed.     

Synthesis and performance of HPTLC plates modified with cyanopropyl- and octadecyl-trichlorosilane

Chemical modification of commercial high performance thin-layer chromatography plates with various mixtures of cyano-propyltrichlorosilane and octadecyltrichlorosilane is described. Surface coverage by different treatments is demonstrated as well as the variations in chromatographic performance. With regard to the development in aqueous media the utility of CN/ ODS plates compared to ODS plates is also shown.     

Characterization of OV-1/FFAP-mixture coated glass capillary columns used for the separation of free fatty acids

As an alternative to acid pretreated UCON and FFAP capillaries for the analysis of wide-boiling free fatty acid mixtures, OV-1 : FFAP phase mixtures were used on high-temperature silylated inert glass capillary columns. The HETP-carrier gas velocity curves, peak asymmetry factors, coating efficiency, gas phase and stationary phase contributions to the mass transfer resistance were determined for various OV-1 : FFAP ratios. Mixed-phase capillaries showed optimum performance at a 2 : 1 OV-1 : FFAP ratio. The thermal and long-term stability of OV-1-stabilized FFAP columns surpassed those of the UCON and pure FFAP reference column used.     

Volumetric studies and thermodynamics of viscous flow of hydroxamic acids in acetone + water solvent at temperatures 303.15 and 313.15 K

Densities ρ and viscosities η of two hydroxamic acids, N-phenyl-2-chlorobenzo- and N-o-tolyl-4-chlorobenzo-, have been determined as a function of their concentration in aqueous acetone solution at temperatures 303.15 and 313.15 K. Apparent molar volumes, standard-state partial molar volumes and relative viscosities have been calculated. The viscosity data have been analyzed using Jones-Dole equation. The activation thermodynamic parameters of viscous flow have been evaluated using Feakins equation. These were obtained to throw light on the mechanism of viscous flow. Thermodynamic interactions in solutions have been studied in terms of a number of excess functions calculated from the experimental data. The effect of hydroxamic acid concentration and temperature on these parameters has been discussed. The results were interpreted in the light of solute-solvent interactions in aquo-organic media.     

Effective atom volumes for implicit solvent models: comparison between Voronoi volumes and minimum fluctuation volumes

An essential element of implicit solvent models, such as the generalized Born method, is a knowledge of the volume associated with the individual atoms of the solute. Two approaches for determining atomic volumes for the generalized Born model are described; one is based on Voronoi polyhedra and the other, on minimizing the fluctuations in the overall volume of the solute. Volumes to be used with various parameter sets for protein and nucleic acids in the CHARMM force field are determined from a large set of known structures. The volumes resulting from the two different approaches are compared with respect to various parameters, including the size and solvent accessibility of the structures from which they are determined. The question of whether to include hydrogens in the atomic representation of the solute volume is examined. Copyright 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1857-1879, 2001     

Large volume sample application by a simplified “closed” on-column injector in capillary gas chromatography

Summary A new simplified version of a closed on-column injector is introduced. Because of its design isobaric injection conditions do not have to be followed and a wide range of injection temperatures above the boiling point of the sample solvent can be chosen for on-column injections in capillary gas chromatography. Also, when following certain basic injection rules, injections of large sample volumes (20 l or more) give accurate and reproducible results without further problems.Presented at the 17th International Symposium on Chromatography, September 25–30, 1988, Vienna, Austria.     

药用缓释辅料——羟丙基甲基纤维素的合成及其缓释性能的研究

本文研究了药物缓释材料HPMC的合成方法,探讨了碱纤维素制备,醚化剂浓度,混合醚化剂配经以及产品后处理对高粘度HPMC的影响。本文还初步对KCl,氨茶碱,硝酸异山梨醇酯等药物采用合成的HPMC作辅料,进行了缓释制剂的研究,结果表明合成的HPMC具有满意的缓释性能。     

Kinetics of the thermal dehydrations and decompositions of some mixed metal oxalates

Both isothermal and programmed temperature experiments have been used to obtain kinetic parameters for the dehydrations and the decompositions in nitrogen of the mixed metal oxalates: FeCu(ox)₂·3H₂O, CoCu(ox)₂·3H₂O and NiCu(ox)₂·3.5H₂O, [ox=C₂O₄]. Results are compared with those reported for the thermal decompositions of the individual metal oxalates, Cuox, Coox·2H₂O, Niox·2H₂O and Feox·2H₂O. X-ray photoelectron spectroscopy (XPS) was also used to examinee the individual and the mixed oxalates. Dehydrations of the mixed oxalates were mainly deceleratory processes with activation energies (80 to 90 kJ·mol⁻¹), similar to those reported for the individual hydrated oxalates. Temperature ranges for dehydration were broadly similar for all the hydrates studied here (130 to 180°C). Decompositions of the mixed oxalates were all complex endothermic processes with no obvious resemblance to the exothermic reaction of Cuox, or the reactions of physical mixtures of the corresponding individual oxalates. The order of decreasing stability, as indicated by the temperature ranges giving comparable decomposition rates, was NiCu(ox)₂>CoCu(ox)₂>FeCu(ox)₂, which also corresponds to the order of increasing covalency of the Cu−O bonds as shown by XPS. In celebration of the 60^th birthday of Dr. Andrew K. Galwey