Determining the absolute configuration of two marine compounds using vibrational chiroptical spectroscopy

Chiroptical techniques are increasingly employed for assigning the absolute configuration of chiral molecules through comparison of experimental spectra with theoretical predictions. For assignment of natural products, electronic chiroptical spectroscopies such as electronic circular dichroism (ECD) are routinely applied. However, the sensitivity of electronic spectral parameters to experimental conditions and the theoretical methods employed can lead to incorrect assignments. Vibrational chiroptical methods (vibrational circular dichroism, VCD, and Raman optical activity, ROA) provide more reliable assignments, although they, in particular ROA, have been little explored for assignments of natural products. In this study, the ECD, VCD, and ROA chiroptical spectroscopies are evaluated for the assignment of the absolute configuration of a highly flexible natural compound with two stereocenters and an asymmetrically substituted double bond, the marine antibiotic Synoxazolidinone A (SynOxA), recently isolated from the sub-Arctic ascidian Synoicum pulmonaria. Conformationally averaged nuclear magnetic resonance (NMR), ECD, Raman, ROA, infrared (IR) and VCD spectral parameters are computed for the eight possible stereoisomers of SynOxA and compared to experimental results. In contrast to previously reported results, the stereochemical assignment of SynOxA based on ECD spectral bands is found to be unreliable. On the other hand, ROA spectra allow for a reliable determination of the configuration at the double bond and the ring stereocenter. However, ROA is not able to resolve the chlorine-substituted stereogenic center on the guanidinium side chain of SynOxA. Application of the third chiroptical method, VCD, indicates unique spectral features for all eight SynOxA isomers in the theoretical spectra. Although the experimental VCD is weak and restricted by the limited amount of sample, it allows for a tentative assignment of the elusive chlorine-substituted stereocenter. VCD chiroptical analysis of a SynOxA derivative with three stereocenters, SynOxC, results in the same absolute configuration as for SynOxA. Despite the experimental challenges, the results convincingly prove that the assignment of absolute configuration based on vibrational chiroptical methods is more reliable than for ECD.     

Cyto-mechanoresponsive polyelectrolyte multilayer films

Cell adhesion processes take place through mechanotransduction mechanisms where stretching of proteins results in biological responses. In this work, we present the first cyto-mechanoresponsive surface that mimics such behavior by becoming cell-adhesive through exhibition of arginine-glycine-aspartic acid (RGD) adhesion peptides under stretching. This mechanoresponsive surface is based on polyelectrolyte multilayer films built on a silicone sheet and where RGD-grafted polyelectrolytes are embedded under antifouling phosphorylcholine-grafted polyelectrolytes. The stretching of this film induces an increase in fibroblast cell viability and adhesion.     

The effect of clay on the morphology of multiphase latex particles

The effect of montmorrilonite clay (MMT) platelets on the morphology of polystyrene/poly(methyl methacrylate) (PMMA) composite latex particles prepared via PMMA-seeded (semi-) batch emulsion polymerization of styrene was studied. It was found that the particle morphology obtained greatly depended on the ability of the clay platelets to diffuse through the polymer particle. Indeed, when the reactions were strictly under kinetic control, i.e., where the clay platelets were unable to diffuse during polymerization, anisotropic core-shell-like morphologies with split core were observed. A better mobility of the clay platelets could more or less restrict the diffusion of the second-stage polymers within the host polymer, leading to original kinetically controlled morphologies. In the case of a full migration of the clay platelets to the particle surface, the penetration of the second-stage polymer species in the seed latex was found to be more limited, enhancing the formation of secondary particles.     

Preparative high-performance liquid chromatography of peptides on a new reversed-phase packing material, Kromasil C18

Preparative reversed-phase high-performance liquid chromatography has found wide use in the production of peptides for pharmaceutical formulations. Purity of the substance and overall economy of the chromatographic system are the most important criterias. In this sense optimized, silica particles and production process with capability to separately control parameters important to chromatography, are essential to high-performance chromatography. Kromasil C18 packing material was tested and evaluated in respect of its selectivity, flow and pressure properties, resolution, load capacity, recovery, adsorption effects, mechanical strength and chemical degradation.     

The satellite-shaped Co-15 Polyoxotungstate, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(beta-SiW8O31)3}]5-

The 15-cobalt-substituted polyoxotungstate [Co(6)(H(2)O)(30){Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}](5-) (1) has been characterized by single-crystal XRD, elemental analysis, IR, electrochemistry, magnetic measurements, and EPR. Single-crystal X-ray analysis was carried out on Na(5)[Co(6)(H(2)O)(30){Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}].37H(2)O, which crystallizes in the hexagonal system, space group P6(3)/m, with a = 19.8754(17) A, b = 19.8754(17) A, c = 22.344(4) A, alpha= 90 degrees, beta = 90 degrees, gamma = 120 degrees, and Z = 2. The trimeric polyanion 1 has a core of nine Co(II) ions encapsulated by three unprecedented (beta-SiW(8)O(31)) fragments and two Cl(-) ligands. This central assembly {Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}(17-) is surrounded by six antenna-like Co(II)(H(2)O)(5) groups resulting in the satellite-like structure 1. Synthesis of 1 is accomplished in a simple one-pot procedure by interaction of Co(II) ions with [gamma-SiW(10)O(36)](8-) in aqueous, acidic NaCl medium (pH 5.4). Polyanion 1 was studied by cyclic voltammetry as a function of pH. The current intensity of its Co(II) centers was compared with that of free Co(II) in solution. Our results suggest that 1 keeps its integrity in solution. Magnetic susceptibility results show the presence of both antiferro- and ferromagnetic coupling within the (Co(II))(9) core. A fully anisotropic Ising model has been employed to describe the exchange-coupling and yields g = 2.42 +/- 0.01, J(1) = 17.0 +/- 1.5 cm(-1), and J(2) = -13 +/- 1 cm(-(1). Variable frequency EPR studies reveal an anisotropic Kramer's doublet.     

Aldol condensations over reconstructed Mg-Al hydrotalcites: structure-activity relationships related to the rehydration method

Two different rehydration procedures in the liquid or gas phase have been applied to reconstruct mixed oxides derived from calcined hydrotalcite-like materials to be used as catalysts for aldol condensation reactions. The as-synthesized hydrotalcite, its decomposition product, as well as the reconstructed solids upon rehydration were characterized by XRD, N(2) adsorption, He pycnometry, FTIR, SEM, TEM, (27)Al MAS-NMR and CO(2)-TPD (TPD=temperature-programmed desorption). Compared to the Mg-Al mixed oxide rehydrated in the gas phase (HT-rg), that rehydrated in the liquid phase (HT-rl) exhibits a superior catalytic performance with respect to the aldol condensation of citral with ketones to yield pseudoionones and in the self-aldolization of acetone. The textural properties of HT-rl and HT-rg differ strongly and determine the catalytic behavior. A memory effect led to a higher degree of reconstruction of the lamellar structure when the mixed oxide was rehydrated in the gas phase rather than in the liquid phase, although liquid-phase rehydration under fast stirring produced a surface area that was 26 times greater. This contrasts to typical statements in the literature claiming a higher degree of reconstruction in the presence of large amounts of water in the medium. CO(2)-TPD shows that the number of OH(-) groups and their nature are very similar in HT-rg and HT-rl, and cannot explain the markedly different catalytic behavior. Accordingly, only a small fraction of the available basic sites in the rehydrated samples is active in liquid-phase aldol condensations. Our results support the model in which only basic sites near the edges of the hydrotalcite platelets are partaking in aldol reactions. Based on this, reconstructed materials with small crystallites (produced by exfoliation during mechanical stirring), that is, possessing a high external surface area, are beneficial in the reactions compared to larger crystals with a high degree of intraplatelet porosity.     

Selective pervaporation of water through a nonselective microporous titania membrane by a dynamically induced molecular sieving mechanism

Pervaporation experiments were performed on microporous titania membranes using several binary liquids containing 2-20 wt % water. The membrane was nonselective in the separation of water from alcohols and p-dioxane but showed a remarkably high selectivity in the separation of water from ethylene glycol/water mixtures with < or =15 mol % water. The absence of selectivity under most conditions is explained by the large pore size (0.9 nm) of microporous titania. The high selectivity for water in the separation from ethylene glycol can be explained by the formation of a hydrogen-bonded network of ethylene glycol in the micropores, which blocks transport of ethylene glycol, while water can still permeate through. These networks are disrupted by water at higher concentrations, leading to full loss of membrane selectivity.