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21.
Abachi S Baringer P Bylsma BG DeBonte R Koltick D Loeffler FJ Low EH McIlwain RL Miller DH Ng CR Rangan LK Shibata EI Derrick M Gan KK Kooijman P Loos JS Musgrave B Price LE Repond J Sugano K Weiss JM Wood DE Blockus D Brabson B Brom J Jung C Ogren H Rust DR Akerlof C Chapman J Errede D Kesten P Meyer DI Nitz D Seidl AA Thun R Willutzky M Cork B 《Physical review letters》1986,57(16):1990-1993
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A simple and efficient preparation of gramicidin S and its analogues is described. It involves solid-phase peptide synthesis and on-resin macrolactamization without side chain protection, affording cyclic products in high yield and high purity. The high specificity of the cyclization reaction was shown to originate in the formation of a pre-organized conformation of the linear biosynthetic precursor of gramicidin S. This facile method will provide convenient access to the analogues of the natural product for functional optimization to counter microbial resistance. 相似文献
24.
S.C. Ng 《European Polymer Journal》1982,18(10):917-919
A kinetic study of the aqueous polymerisation of methacrylamide initiated by hydrogen peroxide has shown that the polymerisation proceeds in two stages. The rate of polymerisation up to about 16% conversion is represented by the equation Within this period, the hydrogen peroxide is completely consumed and a hydroperoxide of methacrylamide is formed. Subsequently this hydroperoxide initiates polymerisation. 相似文献
25.
A new analytical procedure was developed for the extraction of degradation products of chemical warfare agents from water and for in-situ derivatisation prior to analysis by gas chromatography-mass spectrometry (GC-MS). With this new procedure, degradation products of the chemical warfare agents can be analysed and identified without going through laborious sample preparation. Parameters such as fiber selection, pH, salt content, derivatisation temperature, extraction and derivatisation periods, and sequence of adsorption/derivatisation were experimented with, to optimise the efficiency of this method. The detection limit is in the ppb to sub-ppb range with GC-MS in the full scan mode. Based on six injections of the devised procedure, a relative standard deviation from 10-35% can be achieved, depending on the compound. This method was demonstrated during the 4th International Interlaboratory Proficiency Test organised by the Organisation for the Prohibition of Chemical Weapons to be comparable to existing recommended operating procedures for verification of degradation products of chemical warfare agents. 相似文献
26.
A kinetic study of the aqueous persulphate initiated polymerisation of methacrylamide has shown that the rate of polymerisation is represented by the equation where the overall rate constant, . Chain transfer with monomer, where CM = 5.4 × 10?3 at 60°, is shown to be the dominant transfer step. Comparison with kinetic studies of the analogous acrylamide polymerisation shed doubt on the ‘cage effect’ interpretation of complex orders with respect to monomer. An alternative explanation is proposed. 相似文献
27.
Four compounds in the system (Li, Na)VO3 were synthesized and their structures refined in the space group . Site population analysis showed that their compositions are (Na0.5Li0.5)VO3, (Na0.62Li0.38)VO3, (Na0.71Li0.29)VO3 and (Na0.84Li0.16)VO3. All have the structure of LiVO3 and α-NaVO3 which are related to the silicate pyroxenes. Structural data of eight compounds in the system (K, Na, Li)VO3 were compiled, and correlations were established by multiple regression analyses between the effective ionic radii rM1 and rM2 of the alkali metal ions and various structural parameters. The size of the M2 site and the relative displacement of the (VO3)∞ chains are found to depend primarily on rM2 only. The size of the M1 site and the amount of chain rotation are affected by both rM1 and rM2. Changes in lattice parameters are related to the chain movements. The anomalous chain configuration of LiVO3 is also discussed. 相似文献
28.
Poon KW Liu W Chan PK Yang Q Chan TW Mak TC Ng DK 《The Journal of organic chemistry》2001,66(5):1553-1559
Up to eight redox-active ferrocenyl units have been incorporated, through the unsaturated ethynyl linkers, on the periphery of a series of cyclic tetrapyrrole derivatives including zinc(II) phthalocyanine and 2,3-naphthalocyanine, and nickel(II) meso-diphenylporphyrin. The synthesis of the former two macrocycles 4 and 7 involves the Sonogashira coupling reaction of ferrocenylethyne with 4,5-dichlorophthalonitrile (1) or 6,7-dibromonaphthalonitrile (5), respectively, followed by a base-promoted cyclization. The meso-bis(ferrocenylethynyl)porphyrin 11 has been prepared from the dibromo analogue 10 also by a palladium-catalyzed coupling reaction. These novel macrocyclic compounds have been spectroscopically and electrochemically characterized. As revealed by cyclic voltammetry, the ferrocenyl moieties appear to be electrochemically independent in these complexes and there is no significant electronic coupling among the iron(II) centers. 相似文献
29.
The elastic vibrations of spherical nanoparticles have been widely studied by Raman scattering. However, there exist more than one set of selection rules for Raman scattering from these vibrations. For instance, Kanehisa has stated that only torsional modes with angular momentum l = 2 are Raman-active, while Tanaka et al. proposed that spheroidal modes of even l and torsional modes of odd l are Raman-active. These contradict selection rules of Duval which states that only spheroidal modes of l = 0 or 2 are Raman-active. Our present calculations based on a macroscopic model show that all torsional modes have vanishing intensity in the Raman spectrum. 相似文献
30.
A novel chiral stationary phase (CSP) was prepared by immobilizing mono(6A-N-allylamino-6A-deoxy)-perphenylcarbamoylated beta-cyclodextrin onto the surface of silica gel via hydrosilylation. The chromatographic properties of this column were tested with a wide range of structurally diverse racemic compounds and drugs under reverse phases. Separation mechanisms involved are also discussed. 相似文献