Silphinene sesquiterpenes are established chrysomelid antifeedants. In this work, nine silphinene analogs, 11-acetoxy-5-angeloyloxysilphinen-3-one (1), 11-acetoxy-5-tigloyloxysilphinen-3-one (2), 11-acetoxy-5-iso- butyryloxysilphinen-3-one (3), 11-hydroxy-5-angeloyloxysilphinen-3-one (4), 11,5-dihydroxysilphinen-3-one (5), 11,5-diacetoxysilphinen-3-one (6), 5,11-diisobutyryloxysilphinen-3-one (7), silphinen-3,5,11-trione (8), and O-methyl-5-epicantabrenolic acid methyl ester (10), and a presilphiperfolane sesquiterpene (9) were tested against several divergent insect species, including the lepidopteran Spodoptera littoralis, the chrysomelid Leptinotarsa decemlineata, and five aphid species, and their antifeedant effects were compared with those of picrotoxinin, a GABA-antagonist, and thymol, an allosteric modulator for insect GABA receptors. All insects tested responded to at least one silphinene analog and/or GABA antagonist. Compound 3 and thymol were effective antifeedants against all species tested except S. littoralis, with varying potencies according to their feeding ecologies. The toxicity of these compounds was species-dependent and did not correlate with their antifeedant effect. 相似文献
A key component of a hydrogen fuel cell is a catalyst to dissociate dihydrogen to hydrogen atoms. In the present study, the adsorption of hydrogen on Pt/C fuel cell catalysts has been investigated by inelastic neutron scattering spectroscopy.
Monitoring a clean Pt(50%)/C catalyst with low energy neutron spectroscopy, after exposure to dihydrogen at 20 K, as it was heated to room temperature, showed three distinct temperature regimes: (i) a decrease in intensity from 10 to 60 K, (ii) a rise to a maximum between 60 and 120 K and then (iii) a slow fall-off towards room temperature. We assign the three regions as: (i) desorption of physisorbed dihydrogen, (ii) dissociation of dihydrogen to give an adsorbed layer and (iii) damping of the response by an increasing Debye–Waller factor.
The vibrational INS spectra of a series of Pt/C catalysts prepared under varying conditions were similar indicating that the same types of site are common to all the catalysts, although the relative proportions of each site are sample dependent. Features at 520, 950 and part of the intensity at 1300 cm−1 are assigned to hydrogen on (1 1 1) faces, in good agreement with single crystal data. The mode at 640 cm−1 is assigned as the doubly degenerate asymmetric stretch of Pt(1 0 0) faces with the symmetric stretch near 550 cm−1.
We assign the bending mode of the on-top site to the feature at 470 cm−1. The Pt–H stretch mode was observed at 2079 cm−1. This is a significant result: this is the first time that hydrogen on the on-top sites has been observed on nanosized platinum particles supported on high surface area carbon black. The width of the INS peak is surprisingly large and may give additional information on the type and relative proportions of the crystallographic faces present on the catalyst particles. 相似文献
The purpose of the present study was to investigate the physiological control of the main regulatory enzymes of cholesterol
metabolism in isolated enterocytes obtained from chick duodenum, jejunum and ileum. Cholesterol feeding resulted in an inhibition
of 3-hydroxy-3-methylglutaryl-CoA reductase and mevalonate 5-pyrophosphate decarboxylase, while cholestyramine feeding increased
reductase activity in all the regions studied and decarboxylase activity only in duodenum. Cholesterol feeding markedly increased
acyl-CoA:cholesterol acyltransferase, but the effects of cholestyramine were less clear. The effects on transferase activity
cannot be due to differences in the availability of acyl-CoA as exogenous substrate as no significant differences were found
in acyl-CoA hydrolase activity after any of the dietary treatments. The effects of cholesterol feeding were related to changes
in the cholesterol content of epithelial cells, whereas in the case of cholestyramine this relationship was less apparent. 相似文献
Chiral compounds can be produced efficiently by using biocatalysts. However, wild-type enzymes often do not meet the requirements of a production process, making optimization by rational design or directed evolution necessary. Here, we studied the lipase-catalyzed hydrolysis of the model substrate 1-(2-naphthyl)ethyl acetate both theoretically and experimentally. We found that a computational equivalent of alanine scanning mutagenesis based on QM/MM methodology can be applied to identify amino acid positions important for the activity of the enzyme. The theoretical results are consistent with concomitant experimental work using complete saturation mutagenesis and high-throughput screening of the target biocatalyst, a lipase from Bacillus subtilis. Both QM/MM-based calculations and molecular biology experiments identify histidine 76 as a residue that strongly affects the catalytic activity. The experiments demonstrate its important influence on enantioselectivity. 相似文献
The radial density distributions of fractal clusters generated in both DLCA and RLCA conditions by Monte-Carlo off-lattice cluster-cluster aggregation have been investigated. It has been computed by averaging a large number of clusters of same mass to form an average cluster, which is then considered as spherically symmetric. It is found that the radial density distribution, calculated using the mass center of the cluster as the center point, does not follow the fractal scaling, as sometimes assumed in the literature. An empirical model has been proposed to describe the dependence of the radial density distribution on the number of particles in the cluster. The obtained radial density distribution is used to compute permeability profile of the fractal cluster using several literature models, which is then applied to estimate the cluster hydrodynamic radius, Rh, by considering the cluster as a porous permeable object and using the solution of Brinkman equations for the fluid flow inside the cluster. The so obtained Rh values are compared to those in the literature, computed using the Kirkwood-Riseman (KR) theory. It has been found that, among the five permeability models examined, only the model proposed by Davis (Proceedings of the Institution of Mechanical Engineers Part B 1, 185) provides results in good agreement with those obtained using the KR model. Furthermore, it has been verified that the Rh values are insensitive to the model used for the density distribution. 相似文献
A microkinetic model for methanol decomposition on platinum is presented. The model incorporates competitive decomposition
pathways, beginning with both O–H and C–H bond scission in methanol, and uses results from density functional theory (DFT)
calculations [Greeley and Mavrikakis, J. Am. Chem. Soc. 124 (2002) 7193, Greeley and Mavrikakis, J. Am. Chem. Soc. 126 (2004)
3910]. Results from reaction kinetics experiments show that the rate of H2 production increases with increasing temperature and methanol concentration in the feed and is only nominally affected by
the presence of CO or H2 with methanol. The model, based on the values of binding energies, pre-exponential factors and activation energy barriers
derived from first principles calculations, accurately predicts experimental reaction rates and orders. The model also gives
insight into the most favorable reaction pathway, the rate-limiting step, the apparent activation energy, coverages, and the
effects of pressure. It is found that the pathway beginning with the C–H bond scission (CH3OH→H2COH→HCOH→CO) is dominant compared with the path beginning with O–H bond scission. The cleavage of the first C–H bond in methanol
is the rate-controlling step. The surface is highly poisoned by CO, whereas COH appears to be a spectator species. 相似文献
In this paper the sol-gel preparation of hybrid materials made of metal oxide and polytorganophosphazenel components is described. The main problem focused on during this research was to avoid phase separation in order to get homogeneous materials. This problem was pursued looking at the formation of genuine chemical bonds between the exploited polyphosphazenes and the inorganic composite networks. Investigations on the thermal, mechanical, and electroconductive properties of the synthesized, phosphazene-containing composite materials showed that these matrices presented improved mechanical and thermal features with respect to those of the original phosphazene macromolecule, while the ionic conductivities of the prepared molecular hybrids doped with lithium or silver trillate are of the same order of magnitude as those measured for the neat, original phosphazene substrate.Presented at the Ist Italian Workshop on Cyclo- and Polytphosphazenel Materials. February 15–16, 1996. at the CNR Research Area in Padova, Italy. 相似文献
4D Video Textures (4DVT) introduce a novel representation for rendering video‐realistic interactive character animation from a database of 4D actor performance captured in a multiple camera studio. 4D performance capture reconstructs dynamic shape and appearance over time but is limited to free‐viewpoint video replay of the same motion. Interactive animation from 4D performance capture has so far been limited to surface shape only. 4DVT is the final piece in the puzzle enabling video‐realistic interactive animation through two contributions: a layered view‐dependent texture map representation which supports efficient storage, transmission and rendering from multiple view video capture; and a rendering approach that combines multiple 4DVT sequences in a parametric motion space, maintaining video quality rendering of dynamic surface appearance whilst allowing high‐level interactive control of character motion and viewpoint. 4DVT is demonstrated for multiple characters and evaluated both quantitatively and through a user‐study which confirms that the visual quality of captured video is maintained. The 4DVT representation achieves >90% reduction in size and halves the rendering cost. 相似文献
A business process is a set of activities performed in a coordinated manner within an organizational and technical environment that is aimed toward a business goal. The flexibility of a process is related to an understanding of the unexpected events that occur when people, systems and resources interact and require adjustments. Thus, business processes must be designed to respond to information about different events and their specificity. This information defines what the literature calls “context”. To broaden the perception of context in the case of a business process, this work proposes an approach to characterize the context of a business process activity in a given domain through conceptual models structured in layers. A case study was conducted to evaluate the proposal, which provided evidence of the applicability of the model. 相似文献
Summary: The effectiveness of some thermoplastic elastomers grafted with maleic anhydride (MA) or with glycidyl methacrylate (GMA) as compatibilizer precursors (CPs) for blends of low density polyethylene (LDPE) with polyamide‐6 (PA) has been studied. The CPs were produced by grafting different amounts of MA or GMA onto a styrene‐block‐(ethylene‐co‐1‐butene)‐block‐styrene copolymer (SEBS) (KRATON G 1652), either in the melt or in solution. A commercially available SEBS‐g‐MA copolymer with 1.7 wt.‐% MA (KRATON FG 1901X) was also used. The effect of the MA concentration and of other characteristics of the SEBS‐g‐MA CPs was also studied. The specific interactions between the CPs and the blends components were investigated through characterizations of the binary LDPE/CP and PA/CP blends, in the whole composition range. It was demonstrated that the SEBS‐g‐GMA copolymers display poor compatibilizing effectiveness due to cross‐linking resulting from reactions of the epoxy rings of these CPs with both the amine and the carboxyl end groups of PA. On the contrary, the compatibilizing efficiency of the MA‐grafted elastomers, as revealed by the thermal properties and the morphology of the compatibilized blends, was shown to be excellent. The results of this study confirm that the anhydride functional groups possess considerably higher efficiency, for the reactive compatibilization of LDPE/PA blends, than those of the ethylene‐acrylic acid and ethylene‐glycidyl methacrylate copolymers investigated in previous works.
SEM micrograph of the 75/25 LD08/PA blend (with 2 phr SEBSMA1). 相似文献