A structural two-ring version of a tubular stack ofβ-rings in crystals of a cyclic D,L-hexapeptide

An X-ray analysis of single crystals (from MeOH) of cyclo(-D-Leu-L-MeLeu-D-Leu-L-MeLeu-D-Leu-L-MeLeu-) has been carried out. The analysis reveals that the molecules of the cyclopeptide occur in the crystals with two slightly different, almost hexagonal backbone conformations of the 520/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-type, and that pairs of molecules with the same conformation interact through their nonmethylated face, forming dimeric units (units A and B) with six interannular H-bonds. This kind of pairing reproduces well that expected for a two-ring element in a stack of antiparalle520/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-rings. The X-ray analysis has also revealed the presence in the A units of two water molecules, each at one of two equivalent sites located on the 3-fold axis of the units and equidistant from the center of gravity, and the presence in the B units of one water molecule at the center of the units. This provides experimental support for the idea that stacks of520/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-rings can serve as molecular channels.     

Igepal CO 520/CnH2n+1OH/H2O体系的相行为与结构特性

随直键醇CnH2n 1OH的链长增加,IgepalCO520/CnH2n 1OH/H2O体系三组分相图中层状液晶区域增加,使得W/O区域上移,W/O区域中最大水增溶量处所需摩尔比CnH2n 1OH/IgepalCO520降低,W/O区域中水的最大增溶量（摩尔分数）却不变．层状液晶中CnH2n 1OH的最佳链长为n＞8,W/O区域中CnH2n 1OH的最佳键长为n＞10,即能获得比较好的与IgepalCO520的配伍性．     

Influence of sulfate and nitrate uptake from aqueous solutions on surface exchange in Purolite A-520E resin

Removal of nitrate anions from aqueous solutions is the principal goal of many studies concerning water treatment. The ion exchanger capacity to remove nitrate from water was investigated using commercial type A-520E anion exchange resins. The nitrate and sulfate ion exchanger capacity, in static conditions, depends on the concentration of ions and on the pH value of the aqueous solution. The local structural environment exchanged on A-520E has been investigated by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and 2D and 3D atomic force microscopy (AFM). All images of the ion exchanger were obtained before and after contact with solutions that contain NO₃⁻ and SO₄²⁻ separately as well as mixtures of both.     

Development and application of Halanay-type theory: Evolutionary differential and difference equations with time lag

We consider extensions, developments and modifications of a result due to Halanay, and the application of “Halanay-type inequalities” in the analysis and numerics of retarded functional-differential equations, difference equations, and retarded functional-difference equations. Our emphasis is on the variety, structure and development, and future development, of Halanay-type results and their applications. We classify and present novel results of Halanay type (linear and non-linear, discrete, semi-discrete, and continuous) and establish their relevance to delay-differential equations, discretized analogues (we consider ?-methods), and difference equations. A rôle for such results in stability and contractivity analysis is made apparent.     

Synthesis of 13C4-labelled oxidized metabolites of the carcinogenic polycyclic aromatic hydrocarbon benzo[a]pyrene

Polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene (BaP), are ubiquitous environmental contaminants that are implicated in causing lung cancer. BaP is a component of tobacco smoke that is transformed enzymatically to active forms that interact with DNA. We reported previously development of a sensitive stable isotope dilution LC/MS method for analysis of BaP metabolites. We now report efficient syntheses of ¹³C₄-BaP and the complete set of its ¹³C₄-labelled oxidized metabolites needed as internal standards They include the metabolites not involved in carcinogenesis (Group A) and the metabolites implicated in initiation of cancer (Group B). The synthetic approach is novel, entailing use of Pd-catalyzed Suzuki, Sonogashira, and Hartwig cross-coupling reactions combined with PtCl₂-catalyzed cyclization of acetylenic compounds. This synthetic method requires fewer steps, employs milder conditions, and product isolation is simpler than conventional methods of PAH synthesis. The syntheses of ¹³C₄-BaP and ¹³C₄-BaP-8-ol each require only four steps, and the ¹³C-atoms are all introduced in a single step. ¹³C₄-BaP-8-ol serves as the synthetic precursor of all the oxidized metabolites of ¹³C-BaP implicated in initiation of cancer. The isotopic purities of the synthetic ¹³C₄-BaP metabolites were estimated to be ≥99.9%.