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81.
Enantioseparations of (R)- and (S)-6-O-desmethylnaproxens as O-ethoxycarbonyl/(R)-(+)-1-phenylethylamides and (R)- and (S)-naproxens as (R)-(+)-1-phenylethylamides were achieved by achiral gas chromatography in a single run within 11 min. The characteristic mass spectral patterns facilitated easier peak identification. The method for determinations of (R)-naproxen (1.0 to 50 ng) and (S)-naproxen (100 to 2000 ng) was linear with acceptable repeatability and accuracy. The enantiomeric composition ratios between (R)- and (S)-6-O-desmethylnaproxens and between (R)- and (S)-naproxens in urine sample collected six hours after oral administration of (S)-naproxen tablets were measured to be 2.2 (± 0.2):97.8 (± 0.2) and 2.1 (± 0.1):97.9 (± 0.1), respectively. 相似文献
82.
A. Daoudi A. Anakkar J. -M. Buisine F. Bougrioua N. Isaert H. T. Nguyen 《Journal of Thermal Analysis and Calorimetry》1996,46(2):337-345
Effects of high pressures on phase sequences of the two homologousn=16 andn=18 of the (3-fluoro-4 ((R) or (S)-methylheptyloxy) 4, (4-fluoro benzoyloxy) tolans) series have been studied. Both compounds exhibit the new twisted Smectic A phase called Twist Grain Boundary TGBA phase. The high pressure experiments were performed by thermobarometric analysis. The order of the transitions (first or second) have been determined in respect to pressure and temperature. The pressure-temperature phase diagrams show that the TGBA phase is stabilized by increasing pressure; an inducedN
* phase appears under high pressure for n=18 leading to the observation of TGBA-N
*-I triple point. 相似文献
83.
Nguyen Van Suc H. B. Desai R. Parthasarathy S. Gangadhaŕan 《Journal of Radioanalytical and Nuclear Chemistry》1992,164(5):321-325
Individual rare earth impurities in high purity La2O3 (99.9%) have been determined by NAA after pre-separation of the matrix (La). The separation is carried out on an anion exchanger (Dowex 1×8) using different mixtures of methanol/nitric acid as eluants. The rare earth elements from Dy to Lu are eluted quantitatively using a 10% 1M HNO3-90% methanol mixture, while the light rare earths from Ce to Gd are eluted quantitatively using a 10% 0.05M HNO3-90% methanol mixture. La, which is retained on the column, is eluted using 0.1M HNO3. The recoveries of the various rare earth elements have been checked using radiotracers and also by spiking the sample with known amount of elements, and the recoveries are found to be quantitative. Results obtained on a typical high purity lanthanum oxide are reported here. 相似文献
84.
85.
C. Decker H. Le Xuan T. Nguyen Thi Viet 《Journal of polymer science. Part A, Polymer chemistry》1995,33(16):2759-2772
Low molecular weight epoxidized natural rubber has been crosslinked within seconds by UV irradiation in the presence of a triarylsulfonium salt. The photoinitiated cationic ring-opening polymerization was studied quantitatively by infrared spectroscopy and shown to proceed with surprisingly long kinetic chains in such solid medium. The high conversion (60%) needed for complete insolubilization, together with the presence of tetrahydrofuran structures, argue in favor of an intramolecular polymerization process involving neighboring epoxy groups. The photoinitiator concentration has a strong influence on the rate and extent of the reaction, as well as on the depth of cure profile. Because of an efficient dark process, close to 100% conversion was reached upon storage of the irradiated elastomer at ambient, with a concomitant increase of the gel fraction and the polymer hardness. The grafting of pendent acrylate groups onto the polymer chain leads to a three-fold decrease of the initial rate of polymerization of the epoxide. The photocuring of natural rubber bearing both epoxy and acrylate groups generates a dual polymer network which combines the properties of the two moieties. © 1995 John Wiley & Sons, Inc. 相似文献
86.
Heather A. Carlson Toan B. Nguyen Modesto Orozco William L. Jorgensen 《Journal of computational chemistry》1993,14(10):1240-1249
Absolute free energies of hydration have been computed for 13 diverse organic molecules using partial charges derived from ab initio 6-31G* wave functions. Both Mulliken charges and charges fit to the electrostatic potential surface (EPS) were considered in conjunction with OPLS Lennard–Jones parameters for the organic molecules and the TIP4P model of water. Monte Carlo simulations with statistical perturbation theory yielded relative free energies of hydration. These were converted to absolute quantities through perturbations to reference molecules for which absolute free energies of hydration had been obtained previously in TIP4P water. The average errors in the computed absolute free energies of hydration are 1.1 kcal/mol for the 6-31G* EPS charges and 4.0 kcal/mol for the Mulliken charges. For the EPS charges, the largest individual errors are under 2 kcal/mol except for acetamide, in which case the error is 3.7 kcal/mol. The hydrogen bonding between the organic solutes and water has also been characterized. © John Wiley & Sons, Inc. 相似文献
87.
Correction for pile-up losses in the amplifier is possible by the dead-time fraction indicator of the ADC in case of long-lived radionuclides. If the dead-time meter has been calibrated, an accuracy of 1.5% is feasible up to a dead-time fraction of 25%. The precision decreases from 1.5% at 10% dead-time fraction to 3% at a deadtime fraction of 30%. 相似文献
88.
Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier. 相似文献
89.
Marchand C Nguyen CH Ward B Sun JS Bisagni E Garestier T Hélène C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(9):1559-1563
Sequence-specific DNA recognition can be achieved by oligonucleotides that bind to the major groove of oligopyrimidine x oligopurine sequences. These intermolecular structures could be used to modulate gene expression and to create new tools for molecular biology. Here we report the synthesis and biochemical characterization of triple helix-specific DNA cleaving reagents. It is based on the previously reported triplex-specific ligands, benzo[e]pyridoindole (BePI) and benzo[g]pyridoindole (BgPI), covalently attached to ethylenediaminotetraacetic acid (EDTA). In the presence of iron, a reducing agent and molecular oxygen, BgPI-EDTA x FeII but not BePI-EDTA x FeII induced a double-stranded cut in a plasmid DNA at the single site where a triplex-forming oligonucleotide binds. At single nucleotide resolution, it was found that upon triplex formation BePI-EDTA x FeII led to cleavage of the pyrimidine strand and protection of the purine strand. BgPI-EDTA x FeII cleaved both strands with similar efficiency. The difference in cleavage efficiency between the two conjugates was rationalized by the location of the EDTA x FeII moiety with respect to the grooves of DNA (major groove: BePI-EDTA x FeII, minor groove: BgPI-EDTA x FeII). This work paves the way to the development of a new class of triple helix directed DNA cleaving reagents. Such molecules will be of interest for sequence-specific DNA cleavage and for investigating triple-helical structures, such as H-DNA, which could play an important role in the control of gene expression in vivo. 相似文献
90.
Institute of Chemistry, Moldavian SSR Academy of Sciences, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 759–760, September–October, 1988. 相似文献