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The fluorides KF, RbF and CsF have been known to serve as catalysts for the reaction CF2O + F2→ CF3OF. The list of catalysts for this process has now been enlarged to include NaF, MgF2, CaF2, SrF2, BaF2 and LaF3. Lithium fluoride and thorium fluoride also give CF3OF but are less active. Perhaps the substances CsF·HF, KAgF4 and NiF2 should be included in this list. Silver fluoride, usually as a mixture of AgF2 with AgF, has been known to catalyze the reaction of CF2O with F2 to give both CF3OF and CF3OOCF3. The proportion of the latter in the mixture of products increases with decreasing temperature. At 25°, the reaction is slow and the yield of CF3OOCF3 is very high. It has now been shown that TIF3 behaves like silver fluoride. It has also been shown that many other fluorides of metals give higher yields of CF3OOCF3 than of CF3OF but require higher temperatures than AgF2 (100-ca. 150°) to be effective. Various possible mechanisms for these catalytic processes are discussed. 相似文献
3.
Becker-Szendy R Bratton CB Cady R Casper D Dye ST Gajewski W Goldhaber M Haines TJ Halverson PG Jones TW Kielczewska D Kropp WR Learned JG LoSecco JM Matsuno S McGrew C Mudan MS Price L Reines F Schultz J Sobel HW Stone JL Sulak LR Svoboda R Wittel F 《Physical review D: Particles and fields》1991,43(4):1413-1415
4.
Observation of unusual surface ordering in a uniaxial Sm-A phase formed by a highly biaxial compound
We have studied one banana-shaped compound using null-transmission ellipsometry. By studying free-standing films of various thicknesses we confirm that this compound exhibits the Sm-A phase. This is the first banana-shaped compound with a relatively small bend angle (approximately 140 degrees) in which a uniaxial phase has been observed. At the lower temperature end of the Sm-A phase we observe unusual ordering at the surface of the film. 相似文献
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6.
E. N. Brown R. B. Willms G. T. Gray III P. J. Rae C. M. Cady K. S. Vecchio J. Flowers M. Y. Martinez 《Experimental Mechanics》2007,47(3):381-393
The current work presents the characterization and comparison of the mechanical response of three different industrial forms
of polyethylene. Specifically, high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross-linked
polyethylene (PEX) were tested in compression as a function of temperature (−75 to 100°C) and strain-rate (10−4 to 2,600 s−1). The responses of UHMWPE and PEX are very similar, whereas HDPE exhibits some differences. The HDPE samples display a significantly
higher yield stress followed by a flat flow behavior. Conversely UHMWPE and PEX both exhibit significant strain hardening
after yield. The temperature and strain-rate dependence are captured by simple linear and logarithmic fits over the full range
of conditions investigated. The yield behavior is presented in terms of an empirical mapping function that is extended to
analytically solve for the mapping constant. The power-law dependence on strain-rate observed in some polymers is explained
using this mapping function. 相似文献
7.
A. Cady R. Pindak W. Caliebe P. Barois W. Weissflog H. T. Nguyen C. C. Huang 《Liquid crystals》2002,29(8):1101-1104
We have performed resonant X-ray scattering on two bent-core liquid crystal compounds exhibiting the B2 phase using three sample geometries: free-standing film and two others with free surface arrangements. The results conclusively demonstrate the two-layer orientational periodicity in this phase suggested by optical studies. We have performed the first resonant scattering experiments on liquid crystals at the chlorine K-edge, opening up a new class of compounds for resonant X-ray scattering studies. Furthermore, we have achieved an excellent alignment of the B2 phase with a free surface. 相似文献
8.
In the United States, fractions are an important part of the middle school curriculum, yet many middle school students struggle with fraction concepts. Teachers also have difficulty with the conceptual understanding needed to teach fractions and rely on textbooks when making instructional decisions. This reliance on textbooks, the idea that teaching and learning of fractions is a complex process, and that fraction understanding is the foundation for later topics such as proportionality, algebra, and probability, makes it important to examine the variation in presentation of fraction concepts in U.S. textbooks, especially the difference between traditional and standards‐based curricula. The purpose of this study is to determine if differences exist in the presentation of fractions in conventional and standards‐based textbooks and how these differences align with the recommendations of National Council of Teachers of Mathematics, Common Core State Standards, and the research on the teaching and learning of fractions. 相似文献
9.
Cady SD Wang J Wu Y DeGrado WF Hong M 《Journal of the American Chemical Society》2011,133(12):4274-4284
The transmembrane domain of the influenza M2 protein (M2TM) forms a tetrameric proton channel important for the virus lifecycle. The proton-channel activity is inhibited by amine-containing adamantyl drugs amantadine and rimantadine, which have been shown to bind specifically to the pore of M2TM near Ser31. However, whether the polar amine points to the N- or C-terminus of the channel has not yet been determined. Elucidating the polar group direction will shed light on the mechanism by which drug binding inhibits this proton channel and will facilitate rational design of new inhibitors. In this study, we determine the polar amine direction using M2TM reconstituted in lipid bilayers as well as dodecylphosphocholine (DPC) micelles. (13)C-(2)H rotational-echo double-resonance NMR experiments of (13)C-labeled M2TM and methyl-deuterated rimantadine in lipid bilayers showed that the polar amine pointed to the C-terminus of the channel, with the methyl group close to Gly34. Solution NMR experiments of M2TM in DPC micelles indicate that drug binding causes significant chemical shift perturbations of the protein that are very similar to those seen for M2TM and M2(18-60) bound to lipid bilayers. Specific (2)H-labeling of the drugs permitted the assignment of drug-protein cross peaks, which indicate that amantadine and rimantadine bind to the pore in the same fashion as for bilayer-bound M2TM. These results strongly suggest that adamantyl inhibition of M2TM is achieved not only by direct physical occlusion of the channel, but also by perturbing the equilibrium constant of the proton-sensing residue His37. The reproduction of the pharmacologically relevant specific pore-binding site in DPC micelles, which was not observed with a different detergent, DHPC, underscores the significant influence of the detergent environment on the functional structure of this membrane protein. 相似文献
10.
The influenza A M2 protein forms a proton channel for virus infection and also mediates virus assembly and budding. The minimum protein length that encodes both functions contains the transmembrane (TM) domain (roughly residues 22-46) for the amantadine-sensitive proton-channel activity and an amphipathic cytoplasmic helix (roughly residues 45-62) for curvature induction and virus budding. However, structural studies involving the TM domain with or without the amphipathic helix differed on the drug-binding site. Here we use solid-state NMR spectroscopy to determine the amantadine binding site in the cytoplasmic-helix-containing M2(21-61). (13)C-(2)H distance measurements of (13)C-labeled protein and (2)H-labeled amantadine showed that in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers, the first equivalent of drug bound S31 inside the M2(21-61) pore, similar to the behavior of M2 transmembrane peptide (M2TM) in DMPC bilayers. The nonspecific surface site of D44 observed in M2TM is disfavored in the longer peptide. Thus, the pharmacologically relevant drug-binding site in the fully functional M2(21-61) is S31 in the TM pore. Interestingly, when M2(21-61) was reconstituted into a virus-mimetic membrane containing 30% cholesterol, no chemical shift perturbation was observed for pore-lining residues, whereas M2TM in the same membrane exhibited drug-induced chemical shift changes. Reduction of the cholesterol level and the use of unsaturated phospholipids shifted the conformational equilibrium of M2TM fully to the bound state but did not rescue drug binding to M2(21-61). These results suggest that the amphipathic helix, together with cholesterol, modulates the ability of the TM helix to bind amantadine. Thus, the M2 protein interacts with the lipid membrane and small-molecule inhibitors in a complex fashion, and a careful examination of the environmental dependence of the protein conformation is required to fully understand the structure-function relation of this protein. 相似文献