Impact of boron complexation by Tris buffer on the initial dissolution rate of borosilicate glasses.

Tris(hydroxymethyl)aminomethane is a commonly used buffer for leaching studies on glasses. In this work, we demonstrate that it plays a role on the alteration kinetics of borosilicate glasses. Comparative dissolution experiments on a soda-lime silicate glass and a sodium borosilicate glass, in several solutions with or without Tris at neutral pH, are carried out in order to assess the specific effects of the ionic strength and of the Tris molecules on the initial dissolution kinetics. Tris has no effect on the dissolution of the soda-lime silicate glass, while it significantly enhances the dissolution of the borosilicate glass, by increasing the B, Na, and Si release rates. This specific effect on the borosilicate glass is attributed to the Tris-boron complexation and discussed. The bidentate complexation of boron by Tris(hydroxymethyl)aminomethane (Tris) with a 1:1 stoichiometry is directly demonstrated by infrared and NMR spectroscopies. Complexation constants are determined for the Tris-borate complex and its protonated form Tris-boric, from the (11)B and (1)H NMR spectra. This complexation should be taken into account when using the Tris/HCl buffer in alteration experiments of borosilicate glasses.

Biomimetic copper(I)--CO complexes: a structural and dynamic study of a cali.

Four novel calix[6]arene-based cuprous complexes are described. They present a biomimetic tris(imidazole) coordination core associated with a hydrophobic cavity that wraps the apical binding site. Each differs from the other by the methyl or ethyl substituents present on the phenoxyl groups (OR1) and on the imidazole arms (NR2) of the calix[6]arene structure. In solution, stable CO complexes were obtained. We have investigated their geometrical and dynamic properties with respect to the steric demand. IR and NMR studies revealed that, in solution, these complexes adopted two distinct conformations. The preferred conformation was dictated only by the size of the OR1 group. When R1 was an ethyl group, the complex preferentially adopted a flattened C3-symmetrical structure. The corresponding helical enantiomers were in conformational equilibrium, which, however, was slow on the 1H NMR time scale at -80 degrees C. When R1 was a methyl group, the low-temperature NMR spectra revealed the partial inclusion of one tBu group. The complex wobbled between three dissymmetric but equivalent conformations. Hence, small differences in the steric demand of the calixarene's skeleton changed the geometry and dynamics of the system. Indeed, this supramolecular control was promoted by the strong conformational coupling between the metal center and the host structure. Interestingly, this was not only the result of a covalent preorganization, but also stemmed from weak interactions within the hydrophobic pocket. The vibrational spectra of the bound CO were revealed to be a sensitive gauge of this supramolecular behavior, similar to copper proteins in which allosteric effects are common.

31P-NMR and 13C-NMR studies of mannose metabolism in Plesiomonas shigelloides. Toxic effect of mannose on growth.

The metabolism of mannose was examined in resting cells in vivo using 13C-NMR and 31P-NMR spectroscopy, in cell-free extracts in vitro using 31P-NMR spectroscopy, and by enzyme assays. Plesiomonas shigelloides was shown to transport mannose by a phosphoenolpyruvate-dependent phosphotransferase system producing mannose 6-phosphate. However, a toxic effect was observed when P. shigelloides was grown in the presence of mannose. Investigation of mannose metabolism using in vivo 13C NMR showed mannose 6-phosphate accumulation without further metabolism. In contrast, glucose was quickly metabolized under the same conditions to lactate, ethanol, acetate and succinate. Extracts of P. shigelloides exhibited no mannose-6-phosphate isomerase activity whereas the key enzyme of the Embden-Meyerhof pathway (6-phosphofructokinase) was found. This result explains the mannose 6-phosphate accumulation observed in cells grown on mannose. The levels of phosphoenolpyruvate and Pi were estimated by in vivo 31P-NMR spectroscopy. The intracellular concentrations of phosphoenolpyruvate and Pi were relatively constant in both starved cells and mannose-metabolizing cells. In glucose-metabolizing cells, the phosphoenolpyruvate concentration was lower, and about 80% of the Pi was used during the first 10 min. It thus appears that the toxic effect of mannose on growth is not due to energy depletion but probably to a toxic effect of mannose 6-phosphate.

Six novel tetraterpenoid ethers, lycopanerols B-G, and some other constituents from the green microalga Botryococcus braunii.

Six novel tetraterpenoid ethers, lycopanerols B-G, were isolated from lipidic extracts of the green microalga Botryococcus braunii (L race), along with a series of phytyl esters and alpha- and beta-tocopherols. The structures of the compounds were determined by means of spectral analyses including 2D NMR techniques. A biogenetic relationship is proposed between lycopanerols and lycopadiene, the acyclic diunsaturated tetraterpenoid hydrocarbon synthesized by the alga.

31P and 13C nuclear magnetic resonance studies of metabolic pathways in Pasteurella multocida characterization of a new mannitol-producing metabolic pathway.

Glucose metabolism of Pasteurella multocida was examined in resting cells in vivo using 13C NMR spectroscopy, in cell-free extracts in vitro using 31P NMR spectroscopy and using enzyme assays. The NMR data indicate that glucose is converted by the Embden-Meyerhof and pentose phosphate pathways. The P. multocida fructose 6-phosphate phosphotransferase activity (the key enzyme of the Embden-Meyerhof pathway) was similar to that of Escherichia coli. Nevertheless, and in contrast to that of E. coli, its activity was inhibited by alpha glycerophosphate. This inhibition is consistent with the very low fructose 6-phosphate phosphotransferase activity found in cell-free extracts of P. multocida using a spectrophotometric method. The dominant end products of glucose metabolism were mannitol, acetate and succinate. Under anaerobic conditions, P. multocida was able to constitutively produce mannitol from glucose, mannose, fructose, sucrose, glucose 6-phosphate and fructose 6-phosphate. We propose a new metabolic pathway in P. multocida where fructose 6-phosphate is reduced to mannitol 1-phosphate by fructose 6-phosphate reductase. Mannitol 1-phosphate produced is then converted to mannitol by mannitol 1-phosphatase.

Unusual Me-O Bond Cleavage in a Metalated Crown-Ether: X-ray Molecular Structure of (5-Methoxy-4,6-dimethyl-1,3-xylylene-2-one)-15 Crown-4 Complex of Pentamethylcyclopentadienyl Iridium.

The reaction of [(C(5)Me(5))Ir(Solvent)(3)][BF(4)](2) (1) with (2,5-dimethoxy-4,6-dimethyl-1,3-xylylene)-15 crown-4 (2) affords the metalated crown-ether complex [(eta(5)-C(5)Me(5))Ir(eta(6)-C(18)H(28)O(6))][BF(4)](2) (3) in 88% yield. Complex 3 undergoes a facile Me-O bond cleavage to give the related semiquinone form of the metalated crown-ether [(eta(5)-C(5)Me(5))Ir(eta(5)-C(17)H(25)O(6))][BF(4)] (4). A single-crystal X-ray structure determination of complex 4 is reported. Complex 4 crystallizes in the monoclinic space group P2(1)/m with a = 8.187(5) Å, b = 17.193(4) Å, and c = 10.900(3) Å, alpha = 90 degrees, beta = 109.68(1) degrees, gamma = 90 degrees, and Z = 2. The structure provides us with valuable information about the nature of the eta(5)-semiquinone form of the metalated crown-ether and reveals that, surprisingly, the Me-O unit close to the crown chain is the one that undergoes hydrolysis. A rationale consistent with the experimental results is advanced.

Metabolism of a 5-nitroimidazole in susceptible and resistant isogenic strains of Bacteroides fragilis.

We investigated the metabolism of dimetridazole (1,2-dimethyl-5-nitroimidazole) (DMZ) by the resting cell method in a susceptible strain of Bacteroides fragilis and in the same strain containing the nimA gene, which conferred resistance to 5-nitroimidazole drugs. In both cases, under strict anaerobic conditions DMZ was metabolized without major ring cleavage or nitrate formation. However, one of two distinct metabolic pathways is involved, depending on the susceptibility of the strain. In the susceptible strain, the classical reduction pathway of nitroaromatic compounds is followed at least as far as the nitroso-radical anion, with further formation of the azo-dimer: 5,5'-azobis-(1,2-dimethylimidazole). In the resistant strain, DMZ is reduced to the amine derivative, namely, 5-amino-1,2-dimethylimidazole, preventing the formation of the toxic form of the drug. The specificity of the six-electron reduction of the nitro group, which is restricted to 4- and 5-nitroimidazole, suggests an enzymatic reaction. We thus conclude that nimA and related genes may encode a 5-nitroimidazole reductase.