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71.
Dieter Seebach H. Michael Bürger Hans-Martin Müller Urs D. Lengweiler Albert K. Beck Kathy E. Sykes Phil A. Barker Peter J. Barham 《Helvetica chimica acta》1994,77(4):1099-1123
Repetitive treatment of the biopolymer P(3-HB) (molecular weight > 105 Dalton, storage or s-P(3-HB)), with lithium hexamethyl disilazanid (LHMDS) at ?70° in THF leads to a mixture of oligomers with increasingly sharp distribution around a 15-, 30-, and 45mer. Discrete fragments are also isolated when P(3-HB) is heated under reflux (89°) in neat Et3N. Linear oligo(3-HB) derivatives ( 3-7 ) containing up to 96 3-HB units are synthesized using an exponential segment-coupling strategy. These oligomers are used to calibrate size-exclusion chromatography columns for the analysis of oligo(3-HB) samples from the different sources. The linear oligo-(3-HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3-HB) and were investigated as such by transmission electron microscopy (TEM) and by small- and wide-angle X-ray scattering (SAXS and WAXS). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Å, respectively, indicating that the 32mer molecules are folded once, very tightly, into a ‘hair-pin’-type conformation. High-molecular-weight P(3-HB), which was crystallized in a similar way, also has a lamellar crystallite thickness of ca. 50–65 Å. Thus, the treatment of P(3-HB) with LHMDS at low temperature causes etching of the amorphous regions, an effect well known in polymer science for studying the regularity of chain folding. The ca. 50-Å packing within the tight folds of P(3-HB) is discussed in view of its possible function in ion transport through cell membranes. 相似文献
72.
Patricia E. Simmons Tom Albers Debbie Barco Michelle Barker Ron Beer Cassandra Belton Betty Burris Hal Donovan Pili Dressel Ron Giesler Terri Gipson Neal Gleditsch Marian Heimbouch Lea Heintz Linda Kalinowski Kathy Ketterer Kit Klein Lebee Meehan Diane Miller Donna Northcott Barb Pollman Tom Potter Jeffrey Powell Dan Prater Doug Raney Chris Roman Beryl Rosenthal Brian Schaffer Joe Seidler Laura Sturmfels Mary Stewart Carey Tisdal Carol Valenta Suzanne Walton Ralph Wilke 《School science and mathematics》1998,98(7):354-356
73.
Sandra J. Bell Eugene P. Mazzola Michael J. Dinovi William F. Reynolds Kathy W. Nielsen 《Journal of heterocyclic chemistry》1991,28(3):641-645
Pyrazolone T and three derivatives have been characterized by 13C and, in part, 15N nmr at several pH values. The 13C chemical shifts have been assigned at, or near, the equivalence points and pKa values of these four compounds. Closely situatued quaternary carbon signals were assigned by means of a heteronuclear chemical shift correlation (FLOCK) experiment which is sensitive to, and was optimized for, 3-bond C-H couplings. The 13C chemical shift data indicate the existence of both tautomeric and acid-base equilibria and demonstrate that the four congeners exist in surprisingly different forms at certain common pH values. 相似文献
74.
Lawrence W. Baggett Herbert A. Medina Kathy D. Merrill 《Proceedings of the American Mathematical Society》1996,124(1):89-93
Two results are obtained about the topological size of the set of irrationals for which a given function is a trivial cocycle. An example of a continuous function which is a coboundary with non- cobounding function is constructed.
75.
Guindon Y Ogilvie WW Bordeleau J Cui WL Durkin K Gorys V Juteau H Lemieux R Liotta D Simoneau B Yoakim C 《Journal of the American Chemical Society》2003,125(2):428-436
Johnson-type acetals derived from dimethyl tartrate give, after opening with Me(2)BBr and cuprate displacement, secondary alcohols with high diastereoselectivity (>30:1). The mechanism proposed for the induction of diastereoselectivity is downstream from the ring fission. It implies a direct participation of the Lewis acid as a source of nucleophile and the stereospecific transformation of the resulting bromo acetal through an invertive and temperature-dependent process. The acetals are prepared by reaction of the desired aldehyde with dimethyl tartrate. Removal of the auxiliary is accomplished through SmI(2) reduction or by an addition-elimination protocol using methoxide. 相似文献
76.
Whalen CT Hanson GD Putzer KJ Mayer MD Mulford DJ 《Journal of chromatographic science》2002,40(4):214-218
In this study, a sensitive, specific assay for the determination of TNP-470 and its two major metabolites M-IV (also know as AGM-1883) and M-II in human plasma is reported. The assay involves liquid-liquid extraction of acidified plasma followed by reversed-phase high-performance liquid chromatography-atmospheric pressure chemical ionization-tandem mass spectrometry. A liquid-liquid extraction using an organic solvent mixture (methyl-tert-butyl-ether-hexane, 1:1, v/v) is used in place of solid-phase extraction because it provides consistent recoveries for all analytes, including the internal standard. Retention times for the analytes and internal standard are less than 7 min. Within- and between-day precision is < or = 6.5% and < or = 13.3% relative standard deviation, respectively, for the three analytes. The lower limits of quantitation are 0.25, 0.5, and 1.0 ng/mL for TNP-470, M-IV, and M-II, respectively. 相似文献
77.
Derek H.R. Barton Jean Boivin Nubar Ozbalik Kathy M. Schwartzentruber Krzysztof Jankowski 《Tetrahedron letters》1985,26(4):447-450
The high selectivity of the Gif system for hydrocarbon oxidation is shown to depend upon the capture of radicals by pyridine; the mechanism for the secondary oxidation products has only a minor radical component as judged by competitive trapping. 相似文献
78.
Moore AA Jacobson ML Belabas N Rowlen KL Jonas DM 《Journal of the American Chemical Society》2005,127(20):7292-7293
The role and the nature of the continuum in Surface Enhanced Raman Spectroscopy (SERS) are unclear. Here, two-dimensional (2D) covariance and correlation analysis is applied to single molecule SERS spectra on silver colloids with and without rhodamine 6G (native colloid). The resulting 2D covariance and correlation maps show that the sharp molecular Raman peaks from rhodamine 6G and the molecule responsible for the SERS peaks from the native colloid are correlated to different continua even though both continua are present in each data set. This suggests that two distinct active sites on the silver colloids produce the two different continua, and that each site has some molecular specificity. 相似文献
79.
80.
Clostridium botulinum organisms generally produce 1 of 4 neurotoxin types (A, B, E, and F) associated with human illness. Neurotoxin type determination is important in identification of the bacterium. A polymerase chain reaction (PCR) method was developed to identify 24 h botulinal cultures as potential types A, B, E, and F neurotoxin producers as well as other clostridial species which also produce neurotoxins. Components of the PCR and amplification conditions were adjusted for optimal amplification of toxin gene target regions to enable simultaneous testing for types A, B, E, and F in separate tubes using a single thermal cycler. Each primer set was specific for its corresponding toxin type. A DNA extraction procedure was also included to remove inhibitory substances that may affect amplification. This procedure is rapid, sensitive, and specific for identification of toxigenic C. botulinum. 相似文献