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41.
Millot N Cox A Santini CC Molard Y Basset JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(6):1438-1442
The reaction of the Lewis acid B(C(6)F(5))(3) with silanol groups of silica surfaces, dehydroxylated at different temperatures (300, 500, 700, and 800 degrees C), has been investigated in presence of the Br?nsted base NEt(2)Ph. The structure of the resulting modified silica supports [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+) (1) has been carefully identified by IR and multinuclear solid-state NMR spectroscopies, isotopic (2)H and (18)O labeling, elemental analysis, molecular modeling, and comparison with synthesized molecular models. Highly dehydroxylated silica surfaces were required to transform selectively each silanol group into unique [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+) fragments. For lower dehydroxylation temperatures, two sorts of surface sites were coexisting on silica: the free silanol groups [triple bond SiOH] and the ionic species 1. 相似文献
42.
Oxidation of a variety of compounds, including methionine (Met), using a complex formed between dirhodium(II) acetate and the lacunary form of phosphotungstic acid as the catalyst is effective over a wide range of conditions, including pH 2–10. Thus, amperometric detection at a composite in which this complex is immobilized in a sol–gel material does not place restrictions on selection of conditions for separations by reverse-phase HPLC. A demonstration of this point is shown by a study of Met, Met–Phe, Phe–Met, Met–Met, and Gly–Met–Gly (Phe, phenylalanine; Gly, glycine). Using a 0.05 M phosphate buffer at pH 6.7, a C18 column, and a flow rate of 1 ml min−1, capacity factors for Met, Gly–Met–Gly, Met–Met, and Phe–Met were 1.4, 2.1, 5.6, and 34, respectively. Phe–Met and Met–Phe co-eluted. 相似文献
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The cytolysin is a novel, two-peptide lytic toxin produced by some strains of Enterococcus faecalis. It is toxic in animal models of enterococcal infection, and associated with acutely terminal outcome in human infection. The cytolysin exerts activity against a broad spectrum of cell types including a wide range of gram positive bacteria, eukaryotic cells such as human, bovine and horse erythrocytes, retinal cells, polymorphonuclear leukocytes, and human intestinal epithelial cells. The cytolysin likely originated as a bacteriocin involved with niche control in the complex microbial ecologies associated with eukaryotic hosts. However, additional anti-eukaryotic activities may have been selected for as enterococci adapted to eukaryotic cell predation in water or soil ecologies. Cytolytic activity requires two unique peptides that possess modifications characteristic of the lantibiotic bacteriocins, and these peptides are broadly similar in size to most cationic eukaryotic defensins. Expression of the cytolysin is tightly controlled by a novel mode of gene regulation in which the smaller peptide signals high-level expression of the cytolysin gene cluster. This complex regulation of cytolysin expression may have evolved to balance defense against eukaryotic predators with stealth. 相似文献
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Microwave transitions up to J = 53 in the ground vibrational state of deuterothioformaldehyde, CD2S, were studied between 8 and 40 GHz. A detailed centrifugal distortion analysis yields accurate constants for comparison with force field values. The isotopic species 13CH2S, CH234S, CH233S, 13CD2S, CD234S, and CD233S were studied in natural abundance. Accurate average zero-point structures were determined for both CD2S and CH2S: Changes in the zero-point geometry for deuterium substitution were established. Quadrupole fine structure arising from the 33S nucleus has been measured in CH233S and CD233S. Analysis gives the following coupling constants (for both molecules) as χaa = ?11.7 and χbb - χcc = 88.1 MHz. The dipole moment of CD2S was measured to be 1.6588(8)D and an accurate comparison with CH2S was made; the ratio of dipole moments was found to be 1.0062(4). The spectroscopic and bonding properties of CH2S will be compared with formaldehyde and other molecules. 相似文献