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61.
Under O2, the Aspidosperma alkaloid tabersonine ( 1 ) was converted by a crude enzyme preparation from leaves of mature plants of Catharanthus roseus G DON into the trimeric 3-hydroxy-14′-(3α″-tabersonyl)voafrine B ( 4 ) which was easily reduced by NaBH4 to 14′-(3α″-tabersonyl)voafrine B ( = tertabersonine; 5 ). Compounds such as 4 and 5 are the first trimeric alkaloids in the series of monoterpenoid indole alkaloids. 相似文献
62.
Picolinyl ester derivatives of common fatty acids can be readily desorbed by fast atom bombardment (FAB) as positive ions and then collisionally activated. Collisionally activated spectra of the (M + H)+ ions of the derivatives reveal that structurally informative remote-charge-site fragmentations occur. The presence of substitutents such as double bond, branch points, cyclopropane rings, hydroxy groups, and epoxy rings interrupts the fragmentation process in such a way that the substituent can be identified and its location on the alkyl chain can be determined. This method is also applicable to the picolinyl esters of short-chain fatty acids and to the analysis of mixtures of fatty acid derivatives. The approach is advantageous becasue the epicolinyl ester derivatives are also amenable to gas chromatography/mass spectrometry (GC/MS). Therefore, the FAB-MS/MS approach developed here is complementary to GC/MS. 相似文献
63.
Stereoselective molecularly imprinted polymers (MIPs) have been synthesised via ring-opening metathesis polymerisation, in essentially, quantitative yield. A covalent imprinting strategy was followed during the network formation of the chiral sorbent. Recognition of the substrate however involved non-covalent interactions; a combination of hydrogen bonding and the chiral environment presented by the imprinted cavities. The enantiomeric excess achievable with these new MIPs is solvent dependent and stereoselectivities of up to 20% e.e. (separation factor α=2.2) were found in batch equilibrations. 相似文献
64.
Van T.H. Nguyen Joachim T. Anders Qingjun Ma Regine Herbst-Irmer Peter Langer 《Tetrahedron》2007,63(52):12975-12985
Acid-mediated ring-transformations of 5-alkylidene-2,5-dihydropyrrol-2-ones, available by cyclization of 1,3-diketone dianions with bis(imidoyl) dichlorides of oxalic acid, resulted in formation of functionalized pyran-4-ones, such as 5,6-dihydrobenzo[h]chromones and 5,6,7,8-tetrahydrochromones. 相似文献
65.
Prässler F Hoffmann V Schumann J Wetzig K 《Analytical and bioanalytical chemistry》1996,355(7-8):840-846
An algorithm is presented as a concept for the quantification in direct current and radiofrequency glow discharge (GD) modes for GD optical emission spectroscopy. The algorithm is divided into excitation and sputtering part and thus it is possible to distinguish between the different excitation processes and to consider equivalent sputtering crater formations in both modes. Intensity-time profiles are affected corresponding to the method by several effects. One important effect is that sputtering occurs at a single time in different depths because of curved crater bottoms, this is usually called crater effect. The main purpose is to introduce an iterative deconvolution technique which for the quantification numerically takes into account the curved sputtering crater bottom. Input data for the deconvolution technique are the calibrated mass-time profile, the partial densities of the sample constituents and the measured final shape of the sputtering crater. Using a relatively simple model for ion sputtering the deconvolution technique improves iteratively the calculated layer structure by means of information on crater formation. The mathematical handling is illustrated for the quantification of a depth profile of a multilayer sample of ten 100 nm layers. The resulting concentration-depth profile reflects excellently the real elemental distribution of the multilayer system. 相似文献
66.
We present a density functional scheme for calculating the frequency-dependent linear response of superconductors. The central result is a set of integral equations determining the linear response of the normal and anomalous densities to external perturbations. Analytic solutions of these integral equations are obtained for homogeneous systems with separable effective interactions. For inhomogeneous superconductors, the formalism leads to a scheme for calculating the critical temperature without explicitly solving the gap equation. © 1995 John Wiley & Sons, Inc. 相似文献
67.
68.
Miniaturizing all dimensions of apparatus, such as electronics and computers, is the current trend followed by scientists
in various fields. The idea of Lab-on-a-Chip has significantly expanded and found its broad applications in analytical chemistry.
Microplasmas can act as a sample excitation source and are the miniaturized versions of full-sized plasmas. These can be created
in various forms, such as direct current, microwave induced, capacitively coupled and inductively coupled plasmas. Scaling
down the size would reduce the amount of gases, liquids and consumables required, as well as the sample analysis time, which
in turn would decrease the operating costs. Therefore, several research groups are involved in the development of microplasmas
for utilisation in analytical instruments. 相似文献
69.
Joachim Angelkort Sander van Smaalen Prof. Dr. Sven Oliver Hauber Mark Niemeyer 《无机化学与普通化学杂志》2007,633(7):1031-1035
Single crystal X‐ray diffraction of Eu(SC36H49)2 has revealed a first order phase transition at a temperature of 119 K, that involves a reduction of the specific volume by 1 %. The transition corresponds to a doubling of the unit cell, as it is the result from reorientations of isopropyl groups that appear at peripheral sites of the organic ligands. It is argued that a denser packing is achieved at the expense of a less favourable conformation of one of the two crystallographically‐independent complexes in the low‐temperature phase. The Bond‐Valence method is used to show that interactions of equal strengths are present between europium and both sulphur atoms and both coordinating phenyl rings. 相似文献
70.
In the present investigation we report selection of the N-methyl-N-(tert.-butydimethylsilyl)trifluoroacetamide (MTBSTFA) reagent as the most comprehensive derivatization protocol among 17 tested reactions covering trifluoroacetylation, pentafluorobenzylation, methylations, and trimethylsilylations. MTBSTFA allowed easy and robust tert.-butyldimethylsilyl derivatization of 1-aminocyclopropane-1-carboxylic acid, indole-3-acetic acid, (+/-)-jasmonic acid, salicylic acid, (+/-)-abscisic acid, meta-topolin, and trans-zeatin. Detection limits as analysed by selected ion monitoring quadrupole GC-MS were 0.2, 0.01, 1.0, 0.02, 0.3, 0.3, and 0.9 pmol of injected substance, respectively. Analysis of gibberellic acid A3, trans-zeatin riboside and (+/-)-abscisic acid-beta-D-glucopyranosyl ester was best when coupled by splitting extracts and trimethysilylation. The MTBSTFA derivatization protocol was optimised, and validated. The preparation was insensitive to 2% residual water and to < or = 1 day storage at room temperature. The final scheme was highly reproducible and successfully applied to extracts from approximately 300 mg (fresh mass) of tobacco (Nicotiana tabacum) root and Arabidopsis thaliana seedling. 相似文献