排序方式: 共有21条查询结果,搜索用时 15 毫秒
1.
Deerenberg S Schrekker HS van Strijdonck GP Kamer PC van Leeuwen PW Fraanje J Goubitz K 《The Journal of organic chemistry》2000,65(16):4810-4817
A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities. 相似文献
2.
Gardolinski JE Ramos LP de Souza GP Wypych F 《Journal of colloid and interface science》2000,221(2):284-290
Well-crystallized kaolinite (K) was initially reacted at 60 degrees C with a water/dimethylsulfoxide (DMSO) mixture and the resulting intercalation derivative (K-DMSO) was characterized by powder X-ray diffractometry (PXRD), thermal analysis (simultaneous TG and DSC), and Fourier-transformed infrared spectroscopy (FTIR). Benzamide crystals were then melted with the K-DMSO derivative at 140 degrees C for 4 days, when a gradual displacement of DMSO by benzamide was observed within the interlayer spacing of the modified kaolinite. The resulting material, after extensive washing with acetone, was characterized and compared to the results obtained previously for the K-DMSO composite. Benzamide intercalation proceeded by gradual displacement of DMSO molecules until completion. The structural stabilization of the K-BZ derivative was explained through the establishment of hydrogen bonds between the carbonyl oxygen atoms of the intercalated benzamide and aluminol groups present at the surface of the kaolinite layer. The interlamellar spacing of K-BZ was shown to be possibly occupied by benzamide molecules that were located at a 68 degrees orientation in relation to the layer surface. Unlike most intercalation molecules such as DMSO, variations in the interplanar spacing of kaolinite were consistent with the nonkeying of any other part of the molecule between the aluminosilicate interlayers. Copyright 2000 Academic Press. 相似文献
3.
M. M. Mason J. S. Morris V. L. Spate C. K. Baskett T. A. Nichols T. L. Horsman L. Le Marchand L. N. Kolonel S. Yukimoto 《Journal of Radioanalytical and Nuclear Chemistry》1998,232(1-2):29-34
Haem peroxidases are globular proteins with an iron-porphyrin complex as prosthetic group. They catalyze the oxidation of
substrate by peroxides, frequently via free-radical intermediates. The catalytic cycle involves changes in the redox states
of the prosthetic group, that can be monitored by changes in the intense absorption spectra. During the past decades, considerable
scientific effort has been put into the elucidation of the mechanisms of reactions catalyzed by these enzymes. Radiation-chemical
techniques have made an important contribution by providing information on the redox states of the enzymes and their interconversions,
as well as on the properties of the free-radical intermediates involved. 相似文献
4.
T. A. Nichols J. S. Morris V. L. Spate C. J. Tharp C. K. Baskett T. L. Horsman M. M. Mason T. P. Cheng 《Journal of Radioanalytical and Nuclear Chemistry》1998,235(1-2):65-69
A global radionuclide monitoring system is being engineered as part of a multi-technology verification system for the Comprehensive
Nuclear Test Ban Treaty. The system detects airborne radioactive aerosols and gases that can indicate nuclear weapons test
debris. The backbone of the system is a network of 80 remote detection stations that utilize high-volume air sampling and
high-resolution gamma spectrometry to provide in-situ assay and near-real time reporting. These stations are linked to the
International Data Centre, which is a central data processing hub where raw spectral data is automatically processed, analyzed,
and disseminated to the states parties. Measurements are categorized based on spectral content to determine which contain
anomalous anthropogenic radionuclides that require intensive radiochemical analysis at a certified laboratory. The resulting
system has the capability to measure microbecquerel concentrations of radionuclides and provide accessible data products within
minutes of field measurements. During the past year of international operations, the minimum detectable concentrations and
spectroscopy processing statistics were recorded as a function of geographical location and time. The results show that this
system is an effective tool for nuclear test monitoring, as well as other applications such as radiological emergency response,
public health monitoring, and scientific research. 相似文献
5.
Intravenous administration of Gd-DTPA prior to DWI does not affect the apparent diffusion constant 总被引:2,自引:0,他引:2
Chen G Jespersen SN Pedersen M Pang Q Horsman MR Stødkilde-Jørgensen H 《Magnetic resonance imaging》2005,23(5):685-689
MRI measurements of water diffusion and blood perfusion are increasingly used for the evaluation of organ functionality and tissue viability (e.g., in tumors). While diffusion-weighted imaging is performed without contrast agents, measurement of blood perfusion is normally performed based on the administration of paramagnetic substances such as gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). Simultaneous measurements of these two parameters are often preferred. However, it may be argued that Gd-DTPA causes constriction of small blood vessels or alters hemodynamic parameters such as blood viscosity, thereby corrupting subsequent measurements of the apparent diffusion constant (ADC). The objective of the current study was to investigate the possible changes in the ADC in tumors following intravenous administration of 0.2 and 0.4 mmol/kg of Gd-DTPA in mice. The study was conducted with C3H mouse mammary carcinomas inoculated in the right foot of the animal subjects. The results were compared with findings in a sham group, demonstrating that Gd-DTPA had no significant impact on the ADC as measured in a 7-T animal system. 相似文献
6.
Macrocyclic polyketide natural products are an indispensable source of therapeutic agents. The final stage of their biosynthesis, macrocyclization, is catalyzed regio- and stereoselectively by a thioesterase. A panel of substrates were synthesized to test their specificity for macrocyclization by the erythromycin polyketide synthase TE (DEBS TE) in vitro. It was shown that DEBS TE is highly stereospecific, successfully macrocyclizing a 14-member ring substrate with an R configured O-nucleophile, and highly regioselective, generating exclusively the 14-member lactone over the 12-member lactone. 相似文献
7.
Yijun Yan Jing Yang Li Wang Dongdong Xu Zhiyin Yu Xiaowei Guo Geoff P. Horsman Shuangjun Lin Meifeng Tao Sheng-Xiong Huang 《Chemical science》2020,11(15):3959
Rubrolones, isatropolones, and rubterolones are recently isolated glycosylated tropolonids with notable biological activity. They share similar aglycone skeletons but differ in their sugar moieties, and rubterolones in particular have a rare deoxysugar antiarose of unknown biosynthetic provenance. During our previously reported biosynthetic elucidation of the tropolone ring and pyridine moiety, gene inactivation experiments revealed that RubS3 is involved in sugar moiety biosynthesis. Here we report the in vitro characterization of RubS3 as a bifunctional reductase/epimerase catalyzing the formation of TDP-d-antiarose by epimerization at C3 and reduction at C4 of the key intermediate TDP-4-keto-6-deoxy-d-glucose. These new findings not only explain the biosynthetic pathway of deoxysugars in rubrolone-like natural products, but also introduce RubS3 as a new family of reductase/epimerase enzymes with potential to supply the rare antiarose unit for expanding the chemical space of glycosylated natural products.Rubrolones, isarubrolones, and rubterolones are recently isolated glycosylated tropolonids with notable biological activity. 相似文献
8.
Horsman GP Liu AM Henke E Bornscheuer UT Kazlauskas RJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):1933-1939
Directed evolution combined with saturation mutagenesis identified six different point mutations that each moderately increases the enantioselectivity of an esterase from Pseudomonas fluorescens (PFE) towards either of two chiral synthons. Directed evolution identified a Thr230Ile mutation that increased the enantioselectivity from 12 to 19 towards methyl (S)-3-bromo-2-methylpropanoate. Saturation mutagenesis at Thr230 identified another mutant, Thr230Pro, with higher-than-wild-type enantioselectivity (E=17). Previous directed evolution identified mutants Asp158Asn and Leu181Gln that increased the enantioselectivity from 3.5 to 5.8 and 6.6, respectively, towards ethyl (R)-3-phenylbutyrate. In this work, saturation mutagenesis identified other mutations that further increase the enantioselectivity to 12 (Asp158Leu) and 10 (Leu181Ser). A homology model of PFE indicates that all mutations lie outside the active site, 12-14 A from the substrate and suggests how the distant mutations might indirectly change the substrate-binding site. Since proteins contain many more residues far from the active site than close to the active site, random mutagenesis is strongly biased in favor of distant mutations. Directed evolution rarely screens all mutations, so it usually finds the distant mutations because they are more common, but probably not the most effective. 相似文献
9.
Ruzzini AC Ghosh S Horsman GP Foster LJ Bolin JT Eltis LD 《Journal of the American Chemical Society》2012,134(10):4615-4624
Meta-cleavage product (MCP) hydrolases are members of the α/β-hydrolase superfamily that utilize a Ser-His-Asp triad to catalyze the hydrolysis of a C-C bond. BphD, the MCP hydrolase from the biphenyl degradation pathway, hydrolyzes 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to 2-hydroxypenta-2,4-dienoic acid (HPD) and benzoate. A 1.6 ? resolution crystal structure of BphD H265Q incubated with HOPDA revealed that the enzyme's catalytic serine was benzoylated. The acyl-enzyme is stabilized by hydrogen bonding from the amide backbone of 'oxyanion hole' residues, consistent with formation of a tetrahedral oxyanion during nucleophilic attack by Ser112. Chemical quench and mass spectrometry studies substantiated the formation and decay of a Ser112-benzoyl species in wild-type BphD on a time scale consistent with turnover and incorporation of a single equivalent of (18)O into the benzoate produced during hydrolysis in H(2)(18)O. Rapid-scanning kinetic studies indicated that the catalytic histidine contributes to the rate of acylation by only an order of magnitude, but affects the rate of deacylation by over 5 orders of magnitude. The orange-colored catalytic intermediate, ES(red), previously detected in the wild-type enzyme and proposed herein to be a carbanion, was not observed during hydrolysis by H265Q. In the newly proposed mechanism, the carbanion abstracts a proton from Ser112, thereby completing tautomerization and generating a serinate for nucleophilic attack on the C6-carbonyl. Finally, quantification of an observed pre-steady-state kinetic burst suggests that BphD is a half-site reactive enzyme. While the updated catalytic mechanism shares features with the serine proteases, MCP hydrolase-specific chemistry highlights the versatility of the Ser-His-Asp triad. 相似文献
10.
The SgcC5 condensation enzyme catalyzes the attachment of SgcC2-tethered (S)-3-chloro-5-hydroxy-β-tyrosine (2) to the enediyne core in C-1027 (1) biosynthesis. It is reported that SgcC5 (i) exhibits high stereospecificity toward the (S)-enantiomers of SgcC2-tethered β-tyrosine and analogues as donors, (ii) prefers the (R)-enantiomers of 1-phenyl-1,2-ethanediol (3) and analogues, mimicking the enediyne core, as acceptors, and (iii) can recognize a variety of donor and acceptor substrates to catalyze their regio- and stereospecific ester bond formations. 相似文献