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排序方式: 共有913条查询结果,搜索用时 15 毫秒
881.
The formal [2sigma + 2sigma + 2pi] cycloaddition of quadricyclane, 1, with dimethyl azodicarboxylate, 2, in water has been studied using DFT methods at the B3LYP/6-31G** and MPWB1K/6-31G** levels. In the gas phase, the reaction of 1 with 2 has a two-stage mechanism with a large polar character and an activation barrier of 23.2 kcal/mol. Inclusion of water through a combined discrete-continuum model changes the mechanism to a two-step model where the first nucleophilic attack of 1 to 2 is the rate-limiting step with an activation barrier of 14.7 kcal/mol. Analysis of the electronic structure of the transition state structures points out the large zwitterionic character of these species. A DFT analysis of the global electrophilicity and nucleophilicity of the reagents provides a sound explanation about the participation of 1 as a nucleophile in these cycloadditions. This behavior is reinforced by a further study of the reaction of 1 with 1,1-dicyanoethylene. 相似文献
882.
Driss Mountassif Pierre Andreoletti Zakaria El Kebbaj Adnane Moutaouakkil Mustapha Cherkaoui-Malki Norbert Latruffe M'hammed Saïd El Kebbaj 《BMC biochemistry》2008,9(1):26
Background
The interconversion of two important energy metabolites, 3-hydroxybutyrate and acetoacetate (the major ketone bodies), is catalyzed by D-3-hydroxybutyrate dehydrogenase (BDH1: EC 1.1.1.30), a NAD+-dependent enzyme. The eukaryotic enzyme is bound to the mitochondrial inner membrane and harbors a unique lecithin-dependent activity. Here, we report an advanced purification method of the mammalian BDH applied to the liver enzyme from jerboa (Jaculus orientalis), a hibernating rodent adapted to extreme diet and environmental conditions. 相似文献883.
Laszlo P. Csernai Astrid Sk?lvik D. J. Wang Daniel Strottman Csaba Anderlik Yun Cheng Yu L. Yan Ben H. Sa 《Central European Journal of Physics》2012,10(6):1271-1273
Collective flow is studied in a 3+1D fluid dynamical model with globally symmetric, peripheral initial conditions, taking into account the shear flow. At $\sqrt {s_{NN} } $ = 2.76 TeV in semi-peripheral Pb+Pb collisions this leads to rotation, while at more peripheral collisions with high resolution and low numerical viscosity, the initial development of a Kelvin-Helmholz instability is observed. This effect provides a precision tool for studying the viscosity of Quark-Gluon Plasma. 相似文献
884.
Oxidized nanocellulose (ONC) have been synthesized and grafted with tryptophan-based peptides of varying lengths using a two
step coupling method. The ONC was first activated by N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide hydrochloride, forming a stable active ester in the presence of N-hydroxysuccinimide. Then, the active ester was reacted with the amino groups of the peptide forming an amide bond between
ONC and peptide. Using this method, the intermolecular connection of Trp-based peptides (Trp-Ps) was avoided and uniform coupling
of peptides on ONC was achieved. The coupling reaction was very fast in mild conditions and without alteration of the polysaccharide.
The obtained products (ONC-Trp-Ps) were characterized by transmission electron microscopy and by different spectroscopic techniques. 相似文献
885.
Park J Li JR Sañudo EC Yuan D Zhou HC 《Chemical communications (Cambridge, England)》2012,48(6):883-885
A metal-organic framework (MOF) with helical channels has been constructed by bridging helical chain secondary building units with 2,6-di-p-carboxyphenyl-4,4'-bipyridine ligands. The activated MOF shows permanent porosity and gas adsorption selectivity. Remarkably, the MOF exhibits a facile transition from micro- to meso-porosity. 相似文献
886.
Growth of p-type hematite by atomic layer deposition and its utilization for improved solar water splitting 总被引:1,自引:0,他引:1
Lin Y Xu Y Mayer MT Simpson ZI McMahon G Zhou S Wang D 《Journal of the American Chemical Society》2012,134(12):5508-5511
Mg-doped hematite (α-Fe(2)O(3)) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 10(15) cm(-3). When grown on n-type hematite, the p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions. A nominal 200 mV turn-on voltage shift toward the cathodic direction was measured, which is comparable to what has been measured using water oxidation catalysts. This result suggests that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations. Similar approaches may be used to mitigate problems caused by energy mismatch between water redox potentials and the band edges of hematite and many other low-cost metal oxides, enabling practical solar water splitting as a means for solar energy storage. 相似文献
887.
Stefan Saß Maria Kadow Kristian Geitner Mark L. Thompson Lea Talmann Dominique Böttcher Marlen Schmidt Uwe T. Bornscheuer 《Tetrahedron》2012,68(37):7575-7580
An assay for the spectrophotometric determination of Baeyer–Villiger monooxygenase (BVMO) activity is described. Baeyer–Villiger oxidation of p-nitroacetophenone generates the corresponding acetate and subsequent hydrolysis of this ester by an esterase or NaOH results in the formation of p-nitrophenolate. This chromophore can be easily quantified spectrophotometrically at 410 nm. The assay can be performed in a microtiter plate format and is applicable to whole Escherichia coli cells containing recombinant BVMO, crude cell extract as well as using purified enzyme as exemplified for the 4-hydroxyacetophenone monooxygenase (HAPMO) from Pseudomonas putida JD1. Furthermore, the assay was used to identify more active HAPMO variants within enzyme mutant libraries generated by error-prone PCR or site-saturation mutagenesis. 相似文献
888.
Ohkanda J Strickland CL Blaskovich MA Carrico D Lockman JW Vogt A Bucher CJ Sun J Qian Y Knowles D Pusateri EE Sebti SM Hamilton AD 《Organic & biomolecular chemistry》2006,4(3):482-492
A series of imidazole-containing peptidomimetic PFTase inhibitors and their co-crystal structures bound to PFTase and FPP are reported. The structures reveal that the peptidomimetics adopt a similar conformation to that of the extended CVIM tetrapeptide, with the imidazole group coordinating to the catalytic zinc ion. Both mono- and bis-imidazole-containing derivatives, 13 and 16, showed remarkably high enzyme inhibition activity against PFTase in vitro with IC50 values of 0.86 and 1.7 nM, respectively. The peptidomimetics were also highly selective for PFTase over PGGTase-I both in vitro and in intact cells. In addition, peptidomimetics and were found to suppress tumor growth in nude mouse xenograft models with no gross toxicity at a daily dose of 25 mg kg(-1). 相似文献
889.
890.
Synthesis of 3-hydroxy-6-oxo[1,2,4]triazin-1-yl alaninamides, a new class of cyclic dipeptidyl ureas
Cyclohexyl or benzyl isocyanide, benzoyl-, or 4-methoxybenzoylformic acid and semicarbazones underwent Ugi reactions in methanol for 3 days to give the Ugi adducts, which were then stirred with sodium ethoxide in ethanol for 12 h to give 3-hydroxy-6-oxo[1,2,4]triazin-1-yl alaninamides. The X-ray diffraction structure of the first example showed the tautomer having the proton in the O2 atom that was fixed in the crystal by packing in dimers with a H-bond distance of 1.9 A. Selected [1,2,4]triazines were treated with diazomethane for 12 h to get the O-methyl derivatives. Both hydroxy and O-methyl derivatives obtained by this method constitute a new class of pseudopeptidic [1,2,4]triazines composed of two different amino acids, arylglycine and alanine derivatives, in which the N-terminal arylglycine and the peptidic amide nitrogen atoms are bonded through a urea moiety. 相似文献