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101.
Jian‐Hua Zhao Hsuan‐Liang Liu Hsin‐Yi Lin Chih‐Hung Huang Hsu‐Wei Fang Shau‐Wei Tsai 《中国化学会会志》2007,54(4):835-842
In this study, we present the results from the semiempirical molecular orbital calculations for the acylation step in the lipase‐catalyzed ester hydrolysis. The results reveal that the lowest energy path for the formation of the tetrahedral intermediate is for the serine residue of the catalytic triad to attack the substrate, followed by coupling heavy atom movement and proton transfer. The calculations of four active site models show that the cooperation of the aspartate group and the oxyanion hole is capable of lowering the activation energy by about 16 kcalmol?1. Our results further suggest that the lipase‐catalyzed ester hydrolysis adopts the single proton transfer mechanism. 相似文献
102.
The first use of plasma polymerization technique to modify the surface of a glass chip for capillary isoelectric focusing (cIEF) of different proteins is reported. The electrophoresis separation channel was machined in Tempax glass chips with length 70 mm, 300 microm width and 100 microm depth. Acetonitrile and hexamethyldisiloxane monomers were used for plasma polymerization. In each case 100 nm plasma polymer films were coated onto the chip surface to reduce protein wall adsorption and minimize the electroosmotic flow. Applied voltages of 1000 V, 2000 V and 3000 V were used to separate mixtures of cytochrome c (pI 9.6), hemoglobin (pI 7.0) and phycocyanin (pI 4.65). Reproducible isoelectric focusing of each pI marker protein was observed in different coated capillaries at increasing concentration 2.22-5 microg microL(-1). Modification of the glass capillary with hydrophobic HMDS plasma polymerized films enabled rapid cIEF within 3 min. The separation efficiency of cytochrome c and phycocyanin in both acrylamide and HMDS coated capillaries corresponded to a plate number of 19600 which compares favourably with capillary electrophoresis of neurotransmitters with amperometric detection. 相似文献
103.
A general method to find the rate constant and particle self-diffusion coefficient is suggested for a heterogeneous isotopic exchange reaction which is controlled by surface mass reaction or controlled by a combination of surface mass reaction and intraparticle diffusion. The values of the kinetic parameter, ξ1 (ratio of the forward surface mass reaction rate to the intraparticle diffusion rate), particle self-diffusion coefficient D and rate constant k are obtained by the proposed method for the isotopic exchange reaction systems CaCO3(s)/Ca2+(aq) and CaC2O4(s)/Ca2+(aq). 相似文献
104.
The commercial hair conditioners and fabric softeners were analyzed for the content of alkyltrimethylammonium compounds (ATMACs) by gas chromatography-mass spectrometry (GC-MS) with electron impact (EI) and low-pressure positive-ion chemical ionization (PICI) modes. The method involves mixed diluted samples (adjust pH to 10.0) with potassium iodide to enhance the extraction of iodide-ATMA+ ion pairs by direct liquid-liquid extraction. The iodide-ATMA+ pairs were then demethylated to their corresponding nonionic alkyldimethylamines (ADMAs) by thermal decomposition in a GC injection-port. A high abundance of ADMAs was detected at the temperature above 300 degrees C in the GC injection-port. The enhanced selectivity of quasi-molecular ion chromatograms of C12-C18-ADMA, obtained using methanol PICI-MS enables ADMAs to be identified. The accuracy and precision of the method was validated and was successfully applied to determine contents of ATMAC in commercial hair conditioners and fabric softeners. The contents of total measured ATMAC ranged from 0.4 to 6.9% for hair conditioners, and from 3.3 to 4.6% for fabric softeners. 相似文献
105.
Chan-Chen Su Hwei-Lien Tsai Shiou-Rong Li Sne-Lein Wang Chih-Yi Cheng 《Transition Metal Chemistry》1990,15(6):454-458
Summary The x-ray crystal structure of tris(N-methylformamide) (N, N, N', N'-tetramethylethylenediamine)copper(II) perchlorate has been determined using three-dimensional x-ray diffraction data. The complex crystallizes in space group P21/c with Z=4. The cell dimensions are=8.255(3),b=17.951(6),c=16.541(4) Å, and =91.03(2)°. Least-squares refinement of the structure has yielded a final R value of 4.52% for 2108 independent reflect ions. The square pyramidal structure consists ofN, N, N', N'-tetramethylethylenediamine and twoN-methylformamide ligands forming a basal square plane with Cu–N bond lengths of 2.029(5) and 2.033(5) and Cu–O 1.981(4) and 1.985(4) Å, and a thirdN-methylformamide on the apex with a Cu–O distance of 2.181(4)Å. Each of the molecular plances of the three N-methylformamides lies nearly coplanar with thexy, yz andxz molecular coordination planes. The bonding properties of the title compound and its i.r. and uv-vis spectra are discussed with reference to the structure. 相似文献
106.
Hon Man Lee Chi Ying Lu Chih Yuan Chen Wen Ling Chen Hung Ching Lin Pei Ling Chiu Pi Yun Cheng 《Tetrahedron》2004,60(27):5807-5825
A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)2 produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs. 相似文献
107.
Ah-lim Tsai Eva Strobel-Jager Kenneth K. Wu 《Journal of computer-aided molecular design》1991,5(2):135-148
Summary To elucidate the conformation of receptor-associated prostacyclin (PGI2), we first performed structure-activity correlation analysis of over 200 PGI2 analogues and derived from this analysis several crucial features pertaining to structural requirements for PGI2 activity [Ah-lim Tsai and Kenneth K. Wu, Eicosanoids, 2 (1989) 131–143]. These structural features proved to be useful guidelines for selecting model molecules for further investigations by molecular mechanics. By properly selecting four analogues with either rigid or uniquely oriented -side chain structure for geometric fitting, we succeeded in maximally minimizing the degree of freedom of the carboxylate terminus of PGI2. We were able to define the spatial relationship among the four critical functional groups, i.e., C1-COOH, C6a-O, C11-OH and C15-OH. More information is needed, however, to define the geometry of the -side chain, particularly for the moiety beyond C15. Nevertheless, results from structure-activity correlation analysis and molecular modeling provide useful information regarding the conformation of receptor-associated PGI2, which assumes an elongated conformation instead of the traditional hairpin structure. 相似文献
108.
109.
We have provided a model to interpret the non-quadratic-intensity dependence behavior commonly observed in the two-photon fluorescence (TPF) experiment excited with high laser intensity. The model also provides one with a different technique to measure the two-photon absorption cross section of an organic chromophore in solution. In contrast to the commonly used low intensity technique that depends on the quadratic-intensity law, the present technique is based on the non-quadratic-intensity dependence of two-photon fluorescence. Auxiliary data such as two-photon quantum efficiency and fluorescence collection efficiency, essential in the low intensity method, are not required in the present technique. TPF measurements of Rhodamine B in methanol are carried out to demonstrate the validity of the present method. The method is used to determine the two-photon absorption cross section of a new chromophore attached with tricyano-derivatized furan as the electron acceptor. The two-photon absorption cross section measured using this method is also compared with that using a conventional transmission technique. 相似文献
110.
[reaction: see text] Ester-containing nitrones, including 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide 5, have been reported to be robust spin traps for superoxide (O2*-). Using a chiral column, we have been able to isolate the two enantiomers of nitrone 5. With enantiomerically pure nitrone 5a and 5b we explored whether one of these isomers was solely responsible for the EPR spectrum of aminoxyl 6. Data obtained demonstrate that the spin trapping of O2*- by nitrone 5a and nitrone 5b affords the identical EPR spectra and lifetimes in homogeneous aqueous solution and exhibits the same ratio of cis and trans isomers. Quantum chemical modeling in vacuo also finds no difference, aside from the expected optical activity, arising from the difference in stereochemistry. 相似文献