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991.
A high-performance liquid chromatographic method is developed for use in the electrospray mass spectrometric (MS)-MS analysis of alkaloids contained in Cephalotaxus harringtonia leaves. Nine alkaloids having ester groups can be separated and detected with good sensitivity. The MS and MS-MS spectra obtained provides information on their chemical structures. Supercritical fluid extraction is also applied in order to improve the extraction efficiency of Cephalotaxus alkaloids such as cephalotaxine, harringtonine, homoharringtonine, and isoharringtonine. When carbon dioxide-methanol-water (80:18:2, v/v) is used the extraction yield is found to be higher than that using the other supercritical solvents evaluated and conventional organic solvent extraction. 相似文献
992.
An efficient synthesis of a configurationally stable l-serinal derivative 8 was achieved using an N-hydroxymethyl group in about 50% overall yield in four steps from l-serine. Not more than 1% racemization was observed during the preparation of 8. Its enantiomeric integrity was maintained for at least 15 days at room temperature, and it was stable on silica gel. The orthogonal protective groups of 8 would make it a useful chiral synthon. 相似文献
993.
Newkome GR Yoo KS Kim HJ Moorefield CN 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(14):3367-3374
Routes for the syntheses of isomeric, zwitterionic, bisterpyridine-Ru(II)-based macromolecules are described. Access to these novel architectures is facilitated by the construction of terpyridine-modified, 1-->3 C-branched, ester-terminated building blocks. Constitutional isomers result from the interchangable placement of methyl and tert-butyl ester groups on both the branched framework near the Ru(II) centers and the termini of the branched construct. Water solubility is imparted to each isomer through selective transformation of the tert-butyl esters to their corresponding carboxylates. Along with the standard characterization techniques, electrochemical and spectroscopic data also support the structural formation. 相似文献
994.
Synthesis and physical properties of novel multibranched two-photon materials are reported. The compound with three units of 4-(p-diphenylaminostyryl)-2,5-dicyanostyryl moieties attached to the central triphenylamine core exhibits a very large two-photon absorption cross-section. 相似文献
995.
Dong-Wan Tcha Hyung-Bong Ro Chun-Beon Yoo 《The Journal of the Operational Research Society》1988,39(9):873-878
This paper presents a heuristic method for solving the uncapacitated facility-location problem (UFLP), which is similar to Erlenkotter's ‘dual ascent’ procedure. The heuristic is of the ‘add’ type, which progressively selects facilities to open according to a certain criterion derived from the analysis of the linear programming dual. Computational experience with both (static) UFLPs and dynamic UFLPs reveals that the heuristic method yields solutions in most cases superior in quality to those achieved by the dual-ascent procedure, with barely noticeable additional computation time. 相似文献
996.
Wan-Suk Yoo Jeong-Han Lee Su-Jin Park Jeong-Hyun Sohn Oleg Dmitrochenko Dmitri Pogorelov 《Nonlinear dynamics》2003,34(1-2):3-29
Many papers have studied computer-aided simulations of elastic bodies undergoing large deflections and large deformations. But there have not been many attempts to check the validity of the numerical formulations used in these studies. The main aim of this paper is to demonstrate the validity of one such numerical formulation, the absolute nodal coordinate formulation (ANCF), by comparing the results it generates with the results of real experiments. Large oscillations of a thin cantilever beam are studied in this paper to numerically model the beam, which also accounts for the effects of an attached end-point weight and damping forces. The experiments were carried out using a high-speed camera and a data acquisition system. 相似文献
997.
P. Allenspach B. W. Lee D. A. Gajewski V. B. Barbeta M. B. Maple G. Nieva S. -I. Yoo M. J. Kramer R. W. McCallum L. Ben-Dor 《Zeitschrift für Physik B Condensed Matter》1995,96(4):455-464
Low temperature heat-capacity measurements have been performed to study the magnetic ordering in NdBa2Cu3O
x
and DyBa2Cu3O
x
. In order to investigate the type of ordering, magnetic fields up to 6 T and hydrostatic pressures up to 4.5 kbar were applied. While for NdBa2Cu3O
x
and DyBa2Cu3O
x
both the fully oxidized and the fully oxygen reduced samples display long rang antiferromagnetic ordering, only magnetic short range interaction could be found for samples in an intermediate range of oxygen concentration. For NdBa2Cu3O
x
, where superexchange is believed to be the dominant magnetic interaction,T
N
is three times higher in the sample withx=6.26 compared to the one withx=6.98. In contrast,T
N
of DyBa2Cu3O
x
is almost oxygen independent as expected for this mainly dipolar interacting system. 相似文献
998.
A highly sensitive assay method for phenylethanolamine N-methyltransferase in rat adrenal medulla and brain is described which employs high-performance liquid chromatography with fluorescence detection. Epinephrine formed enzymatically from the substrate norepinephrine and isoproterenol (internal standard), after chromatography on a small cartridge of a cation exchanger, Toyopak SP, are converted into the corresponding fluorescent compounds by reaction with 1,2-diphenylethylenediamine, a selective fluorescence derivatization reagent for catechol compounds. The derivatives are separated by reversed-phase chromatography on TSK gel ODS-120T. The detection limit for epinephrine formed enzymatically is 0.66 pmol per assay tube. 相似文献
999.
Luis Echegoyen George W. Gokel Lourdes E. Echegoyen Zhi-Hong Chen Hyunsook Yoo 《Journal of inclusion phenomena and macrocyclic chemistry》1989,7(2):257-266
The evolution of lariat ethers from relatively simple, substituted crown ethers into electrochemically sensitive ligands is presented. Although nitrogen-pivot lariats were observed to be better binders than the corresponding parent crowns and to retain considerable flexibility after complexation, overall stability constants were not favorable for cation transport applications. This led to the syntheses of nitrobenzene- and anthraquinone-substituted systems capable of reversible redox behavior and drastically enhanced cation binding abilities when reduced. Application of these in enhanced cation transport processes has been demonstrated. 相似文献
1000.
J. Y. Yoo J. H. Shin Y. H. Lee H. -H. Park B. -S. Yoo 《Optical and Quantum Electronics》1995,27(5):421-425
Precision etch-depth control is realized by a chemically assisted ion-beam etching system incorporated within situ laser reflectometry. By counting the number of interference fringes, etch-depth control better than a quarter-wave thickness is easily obtained. Optimized etching conditions for highly anisotropic etching of bulk GaAs and AlGaAs/GaAs distributed Bragg reflectors are obtained. With the ability to etch-stop just below the active region by thein situ monitoring, InGaAs vertical-cavity surface-emitting lasers with CW threshold current density as low as 380 A cm-2 with output power >11 mW are fabricated. Spatial uniformity is 5% over a 1-cm2 sample, which corresponds to one pair over 20 pairs of quarter-wave stacks of AlGaAs/GaAs distributed Bragg reflectors. 相似文献