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1.
The aim of this article is to give a new dynamical proof of the Ferrand–Obata theorem when the manifold is compact. This will give us a generalisation of this theorem to transversally conformal foliations ofcodimension greater than three and constant basic functions. 相似文献
2.
Ramnial T McKenzie I Gorodetsky B Tsang EM Clyburne JA 《Chemical communications (Cambridge, England)》2004,(9):1054-1055
One-electron oxidation of N-heterocyclic carbenes (NHCs) has been carried out using oxidising agents such as tetracyanoethylene (TCNE) and ferrocenium [Cp(2)Fe](+); the formation of carbene radical cations is postulated. 相似文献
3.
Although food is a necessity, compounds within food products can be dangerous and life-threatening for people with food allergies. These allergy-causing compounds, such as proteins from eggs and milk, must be identified on the labels of commercial products. Unintentional contamination of food with these compounds occurs as a result of storage, manufacturing procedures, or cleaning procedures. A sensitive, specific, and rapid method to identify foods containing allergens is required by the food industry. The array biosensor, a rapid detection system, may provide a solution to this need. The array biosensor performs fluorescent immunoassays on the surface of a planar waveguide by first running samples, then fluorescently labeled antibodies, over a surface patterned with capture antibodies. An optical image is captured by a charged-coupled device camera and converted into fluorescence values. Signal intensity and spot location provide information on the compound and its concentration. The array biosensor has been successfully demonstrated for toxin, bacteria, and virus detection at low levels in under 20 min in food, clinical samples, and environmental matrixes. An assay for detection of ovalbumin as an indicator of egg contamination has been developed with limits of detection of 25 pg/mL in buffer and 1.3 ng/mL (13 ng/g) in non-egg pasta extract (buffer:pasta 10:1, v/w). 相似文献
4.
The atomistic origin of fatigue failure in micron-sized silicon devices is not fully understood. Two series of density-functional theory calculations on cubic diamond Si explore the effect of surface bond formation on crack healing in systems which exhibit strong surface reconstruction. Both series introduce a separation between Si(100) layers (i.e., the crack) and allow the ions to relax to their minimum-energy configuration. The initial surface ionic positions are either bulk terminated or 2 x 1 reconstructed. A plot of the energy versus the introduced separation reveals that once the surfaces reconstruct, the crack is no longer able to return to the equilibrium configuration. Rather, the healed crack interface contains defects which places the flawed energy minimum at a finite strain of 3% and an increased energy of 1.13 Jm2 relative to the equilibrium configuration. The irreversible plastic deformation supports the mechanism proposed by Kahn et al. [Science 298 1215 (2002)] that invokes mechanically induced subcritical cracking to explain the delayed onset of failure. 相似文献
5.
We have studied gramicidin A, an environmentally sensitive polymorphic pentadecapeptide, fully 15N-labelled and dispersed in a highly deuterated phospholipid bilayer system. By submitting the sample to fast magic angle spinning, we were able to reduce the polypeptide amide hydrogen linewidths to 160 Hz, and hence to partially resolve them. By correlating these resonances with the 40 Hz wide dipolar coupled 15N in a 2D-CROPSY (cross-polarization spectroscopy) experiment, it was possible to observe the 20 partially overlapping 1H-15N signal pairs from the polypeptide backbone and sidechains. Both chemical shift distributions closely match those of the same peptide in SDS micelles, but only poorly match those of conformationally different gramicidin A in trifluoroethanol, dimethylsulfoxide, or methanol/chloroform mixture. Our results are indicative of the N-to-N right-handed beta6.3-helix conformation of gramicidin A and offer sufficient resolution to encourage development of experiments to measure orientational or distance restraints using through-space dipolar couplings. 相似文献
6.
Bulloch EM Jones MA Parker EJ Osborne AP Stephens E Davies GM Coggins JR Abell C 《Journal of the American Chemical Society》2004,126(32):9912-9913
(6S)-6-Fluoroshikimate has antimicrobial activity. The molecular basis of this effect had not been identified, but there was speculation that (6S)-6-fluoroshikimate is first converted in vivo into 2-fluorochorismate, which then could inhibit 4-amino-4-deoxychorismate synthase (ADCS). 2-Fluorochorismate was prepared from E-fluorophosphoenolpyruvate and erythose-4-phosphate by the sequential reactions of DAHP synthase, dehydroquinate synthase, dehydroquinase, shikimate dehydrogenase, EPSP synthase, and chorismate synthase. Inhibition studies on ADCS showed that it was inhibited rapidly and irreversibly by 2-fluorochorismate. Electrospray mass spectrometry of the inactivated enzyme showed an additional mass of 198 +/- 10 Da. A novel peptide of 1087.6 Da was identified in the HPLC trace for the tryptic digest of 2-fluorochorismate-inactivated ADCS. Sequencing of this peptide by MS/MS showed that the peptide corresponded to residues 272-279 with a modification of 206.1 Da on Lys-274. This observation is particularly exciting in the context of a recent proposal for the catalytic mechanism of ADCS. 相似文献
7.
Design and evaluation of a Dean vortex-based micromixer 总被引:2,自引:0,他引:2
A mixer, based on the Dean vortex, is fabricated and tested in an on-chip format. When fluid is directed around a curve under pressure driven flow, the high velocity streams in the center of the channel experience a greater centripetal force and so are deflected outward. This creates a pair of counter-rotating vortices moving fluid toward the inner wall at the top and bottom of the channel and toward the outer wall in the center. For the geometries studied, the vortices were first seen at Reynolds numbers between 1 and 10 and became stronger as the flow velocity is increased. Vortex formation was monitored in channels with depth/width ratios of 0.5, 1.0, and 2.0. The lowest aspect ratio strongly suppressed vortex formation. Increasing the aspect ratio above 1 appeared to provide improved mixing. This design has the advantages of easy fabrication and low surface area. 相似文献
8.
Proteases are key regulators of many physiological and pathological processes [1,2], and are recognized as important and tractable drug candidates. Consequently, knowledge of protease substrate recognition and specificity promotes identification of biologically relevant substrates, helps elucidating a protease's biological function, and the design of specific inhibitors. Traditional methods for establishing substrate recognition profiles involve the identification of the scissile bond within a given protein substrate by proteomic methods such as Edman degradation. Then, synthetic peptide variants of this sequence can be screened in an iterative fashion to arrive at more optimized substrates. Even though it can be fruitful, this iterative strategy is biased toward the original substrate sequence and it is also tremendously cumbersome. Furthermore, it is not amenable to high throughput analysis. In 1993, Matthew & Wells presented a method for the use of monovalent "substrate phage" libraries for discovering peptide substrates for proteases, in which more than 10(7) potential substrates can be tested concurrently [3]. A library of fusion proteins was constructed containing randomized substrate sequences placed between a binding domain and the gene III coat protein of the filamentous phage, M13, which displays the fusion protein and packages the gene coding for it inside. Each fusion protein was displayed as a single copy on filamentous phagemid particles (substrate phage). This method allows one to rapidly survey the substrate recognition and specificity of individual or closely related members of proteases. Over the past decade, substrate phage screening has shown terrific utility in rapidly determining protease specificity and characterization of substrate recognition profile of proteases. In some cases, the structural insights of the catalytic domain were obtained from comparison of substrate specificity among closely related family of proteases [4-6]. The number of proteases (from various classes) characterized by this approach testifies to its power. Since the initial development of substrate phage library, different versions of the substrate phage cloning vectors have been constructed to further improve the utility of substrate phage display. This review will provide an overview of the construction of substrate phage display libraries, screening of substrate phage libraries, examples of application, summary and future directions. 相似文献
9.
Deleterious gases such as CO and H(2)S can cause degradation of steel by reacting with the metal surface. Here we consider whether alloying the steel surface might be able to inhibit these damaging surface reactions by raising the barriers to molecular dissociation. We employ first-principles density functional theory techniques to investigate the elementary reaction pathways and barriers for CO and H(2)S on FeAl and Fe(3)Si surfaces and compare them with pure Fe surfaces (as a model for steel). We find that H(2)S dissociates on iron surfaces much more easily than CO does. Although FeAl surfaces raise the barriers for H(2)S dissociation, they significantly lower the barriers for CO dissociation. On the other hand, Fe(3)Si surfaces raise the barriers for CO dissociation, but they are as vulnerable as Fe surfaces to H(2)S dissociation. Our findings suggest that alloying iron with Al or Si is unlikely to simultaneously increase its resistance to the initial stages of chemical degradation by CO and H(2)S. 相似文献
10.
The through-shell borane reduction and methyllithium addition to benzaldehyde (1), benzocyclobutenone (2), and benzocyclobutenedione (3) incarcerated inside a hemicarcerand (4) with four tetramethylenedioxy bridges are reported. All guests could be reduced and methylated. Selective monoreduction and monomethylation were observed for 3. In the methyllithium addition to 4[symbol: see text]3, the initially formed lithium alcoholate underwent a Moore rearrangement. The reactivity of the incarcerated guests toward methyllithium increased in the order 1 < 2 < 3 and toward borane in the order 1 < 2 approximately equal 3. Guest reactivity was correlated with the inner-phase location of the reacting carbonyl group in the preferred guest inner-phase orientation. The latter was determined from the X-ray structures of 4[symbol: see text]1, 4[symbol: see text]2, and 4[symbol: see text]3, from molecular mechanical calculations, and from the hemicarcerand-induced upfield shift of the guest proton resonances. In the methyllithium and n-butyllithium addition to 4[symbol: see text]1 and 4[symbol: see text]3 at elevated temperatures, selective cleavage of a host's spanner or tetramethylenedioxy bridge, respectively, was observed. The cleavage of one spanner also took place in the methyllithium addition to the 1-methyl-2-pyrrolidinone hemicarceplex. These scission reactions are initiated by the initially formed lithium alcoholates, which show enhanced basicity and nucleophilicity in the inner phase as compared to the bulk phase. Mechanisms for the host scission reactions are discussed. 相似文献