Modification of HPTLC-plates by in situ chemical bonding with non-polar and polar organosilanes

A method has been developed for the preparation of modified silica plates for high performance thin-layer chromatography (HPTLC). Some typical organosilanes were thus allowed to react in situ with the silica of Merck HPTLC-plates. This method was found to be highly reproducible, simple and cheap. Non-polar plates were prepared and compared with commercial plates from Merck, Whatman and Macherey-Nagel. Modification with cyanodecyltrichlorosilane resulted in plates that showed good coverage, efficiency and low residual silica activity. Silica modified with a multifunctional silane has different properties in different organic solvents. It will appear to be non-polar in a polar solvent and vice versa. New advantageous separation systems are thus made feasible by the presence of cyano groups on the plate. The utility of modified thin-layer plates is demonstrated by the separation of some homologues of p-hydroxybenzoic acid esters and of some polycyclic aromatic hydrocarbons.     

Polychloroprene rubbers used for adhesives: a small angle X-ray scattering preliminary investigation under stretching

Summary Three types of polychloroprene rubbers (Du Pont de Nemours Co. Neoprenes AD, WHV and W) used for adhesives and having different crystallization rates have been investigated.The mechanical properties of the Neoprenes samples were determined and the films were examined under stretching by SAXS for a preliminary investigation on their different superreticular order.

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Zusammenfassung Drei Typen von Polychloropren-Kautschuk mit verschiedener Kristallisationsgeschwindigkeit (Neoprene AD, WHV und W von Du Pont de Nemours), die als Klebemittel dienen, wurden als gedehnte Filme mechanisch und röntgenographisch (SAXS) untersucht. Sie zeigen Unterschiede in der Überstruktur.

     

EFFECT OF BASE SEQUENCE ON THE ULTRAVIOLET IRRADIATION PRODUCTS OF DOUBLE-STRANDED POLYNUCLEOTIDES CONTAINING BROMOURACIL AND ADENINE

Abstract —The effects of ultraviolet irradiation of double-stranded synthetic polynucleotides containing BrU and A have been investigated. Homopolymer pairs and alternating copolymers composed of either ribo- or deoxyribo-nucleotides were prepared and were irradiated with either 313 nm or ˜ 285 nm light. Strand separation and a modest amount of strand breakage followed irradiation of the homopolymer pairs. Changes in the ultraviolet absorption spectra of the polymers during irradiation reflected the sum of hyperchromic increases caused by progressive strand separation and loss of absorbance caused by photoproduct formation. Extensive debromination occurred. An RNase digest of irradiated poly(rA)–poly¹⁴C(rBrU), analysed by column chromatography, showed components similar to those found previously upon irradiation of single-stranded poly(rBrU). Little photoproduct was released by RNase digestion as mononucleotides. The major photoproduct was in the dinucleotide fraction, and may be 5,5'-diuracil. Base sequence had a profound effect on the sensitivity of the polynucleotides. Irradiation of alternating copolymers with doses of light comparable to those that produced major photochemical changes in the homopolymer pairs brought about little if any change in the copolymers of alternating base sequence.     

Supramolecular structures of coronene and alkane acids at the Au(111)-solution interface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.     

Direct observation of cations and polynucleotides explains polyion adsorption to like-charged surfaces

We show an experimental approach for directly observing the condensation of polynucleotides and their electrolyte counterions at a liquid/solid interface. X-ray standing waves (XSW) generated by Bragg diffraction from a d = 20 nm Si/Mo multilayer substrate are used to measure the distinct distribution profiles of the polyanions and simple cations along the surface normal direction with subnanometer resolution. The 1D spatial sensitivity of this approach is enhanced by observing the XSW induced fluorescence modulations over multiple orders of Bragg peaks. We study the interesting divalent cation driven adsorption of anionic polynucleotides to anionic surfaces by exposing a hydroxyl-terminated silica surface to an aqueous solution with ZnCl2 and mercurated poly-uridylic acid (a synthetic RNA molecule). The in situ long-period XSW measurements are used to follow the evolution of both the Zn and Hg distribution profiles during the adsorption process. The conditions and physical mechanisms that govern the observed divalent cation adsorption and subsequent polynucleotide adsorption to an anionic surface are explained by a thermodynamic model that incorporates nonlinear electrostatic effects.     

Styrene copolymers as pollutant adsorbents : Safe sampling volume determination

The determination of safe sampling volumes of benzene and acetone on air samplers filled with Porapak Q, Chromosorb 101, 102 and 103 is described. Two direct methods are employed; one method is based on frontal chromatography, while the second one is a concentration method followed by thermal desorption. The results are compared to those determined by the indirect pulse chromatographic method. The safe sampling volumes determined by the direct methods were in accordance. The indirect method gives slightly different values; however, this procedure is carried out at "infinite dilution". The effect of the pollutant concentration on the safe sampling volumes was also examined. The values decrease with increasing pollutant concentration.     

Analysis and interpretation of excess molar properties of amphiphile + water systems : Part 3. Excess molar volumes of isopropoxyethanol + water and isobutoxyethanol + water

Excess molar volumes at 25°C, are reported for the systems isoPrOEtOH + water and isoBuOEtOH + water across their entire composition ranges. These data have been compared to those for their normal alkyl analogs in order to assess the effects of both - and β-chain branching.

The results are discussed in terms of the relative abilities of the amphiphiles to form structured aggregates.     

Quantum chemical calculations and mutational analysis suggest heat shock protein 90 catalyzes trans-cis isomerization of geldanamycin

The affinity of geldanamycin (GA) for binding to heat shock protein 90 (HSP90) is 50- to 100-fold weaker than is the affinity of the structurally distinct natural product radicicol. X-ray crystallography shows that although radicicol maintains its free conformation when bound to HSP90, the conformation of GA is dramatically altered from an extended conformation with a trans amide bond to a kinked shape in which the amide group in the ansa ring has the cis configuration. We have performed ab initio quantum chemical calculations to demonstrate that the trans-cis isomeriztion of GA in solution is both kinetically and thermodynamically unfavorable. Thus, we propose that HSP90 catalyzes the isomerization of GA. We identify Ser113, a conserved residue outside the ATP binding pocket, as essential for the isomerization of GA. In support of this model, we show that radicicol binds equally well to both wild-type HSP90 and the Ser113 mutant, whereas the binding of GA to the Ser113 mutant is decreased significantly from its binding to wild-type HSP90. Based on this finding, a mechanism of keto-enol tautomerization of GA catalyzed by HSP90 is proposed. The added requirement of isomerization prior to tight binding may explain the enhanced binding affinity of GA for HSP90 in a cell extract versus in a purified form.     

Studies towards the total synthesis of contignasterol

The (17R,20S,22S,24S) C₂₀-C₂₉ segment of contignasterol has been stereoselectively prepared in 8 steps and 40% overall yield from (S)-carvone. Synthetic studies towards contignasterol's C/D ring functionalization/isomerization are also reported.     

Calculated structures and relative stabilities of furoxan,some 1,2-dinitrosoethylenes and other isomers

The structures and relative stabilities of furoxan and some of its isomers, e.g., the 1,2-dinitrosoethylenes, have been determined by means of ab initio Hartee–Fock and Møller–Plesset calculations. Geometries were optimized at the HF/3-21G, HF/6-31G* and MP2/6-31G* levels, and subsequently used for computing MP2/6-31G*, MP3/6-31G*, and MP4/6-31G* energies. The results are markedly affected by the inclusion of electronic correlation, which renders three of the isomers unstable. It also emphasizes the importance of a zwitterionic contribution to the structure of furoxan, which promotes ring-opening through a cis 1,2-dinitrosoethylene intermediate/transition state that has an MP4/6-31G*//MP2/6-31G* energy that is 31.6 kcal/mol above furoxan.