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191.
Henry J. Pownall Joel D. Morrisett James T. Sparrow Louis C. Smith James Shepherd Richard L. Jackson Antonio M. Gotto Jr. 《Lipids》1979,14(4):428-434
The human plasma high density lipoproteins (HDL) are a heterogeneous ensemble of five proteins associated with both neutral
and polar lipids. The sequences of all five proteins are known. ApoA-I and apoA-II are the major protein components; apoC-I,
apoC-II and apoC-III are the minor protein components. All these apoproteins spontaneously recombine with phospholipids to
give stable lipid-protein complexes and freely exchange between the two major HDL subclasses, HDL2 and HDL3. In addition, ApoC-I, apoC-II, and apoC-III exchange between HDL and very low density lipoproteins. Furthermore, certain
HDL apoproteins are activators for plasma enzymes that are important in lipid metabolism. ApoA-I and apoC-I activate lecithin/cholesterol
acyltransferase; apoC-II is an activator of lipoprotein lipase. The regions of apoC-I and apoC-II that are involved in the
activation of these enzymes have been localized with synthetic peptides. Studies of synthetic and native fragments of apoA-II,
apoC-I, apoC-II, and apoC-III as well as model lipid-binding peptides have identified specific regions with structural features
common to lipid-binding proteins. These special properties, which include helical potential, sequences with a critical amphipathic
length, and high hydrophobicity of the nonpolar side of the amphipathic helix, are the determinants of HDL structure and metabolism. 相似文献
192.
Poly(aryl ether oxadiazoles) 总被引:1,自引:0,他引:1
James L. Hedrick 《Polymer Bulletin》1991,25(5):543-550
A general method for the preparation of poly(aryl ether oxadiazoles) has been developed where the generation of an aryl ether
linkage was the polymer-forming reaction. Two synthetic approaches were investigated based on either an oxadiazole-activated
halo displacement with phenoxides or a hydrazide-activated halo-displacement with phenoxides. The hydrazide may be subsequently
thermally cyclodehydrated to the oxadiazole heterocyclic. In each case, the negative charge developed in the transition state
could be stabilized through a Meisenheimer Complex, analogous to conventional activating groups (e.g., sulfone and ketone),
and the electron affinity, as judged by 1H NMR, was comparable to that of other activating groups. An appropriately substituted diarylfluoro oxadiazole was prepared
and polymerized with various bisphenols in an NMP/CHP solvent mixture in the presence of K2CO3. High molecular weight poly(aryl ether oxadiazoles) were synthesized with viscosities ranging from 0.44 to 0.76 dL/g and
Tg's in the 200°C range. Conversely, the hydrazide activated halo-displacement as a means of preparing poly(aryl ether hydrazides)
was not as successful, since it appears that the hydrazide was of sufficient acidity to form a salt with K2CO3 preventing solubility and polymerization. 相似文献
193.
James N. Seiber Paul M. Tuskes Lincoln P. Brower Carolyn J. Nelson 《Journal of chemical ecology》1980,6(2):321-339
Adult monarch butterflies,Danaus plexippus L. (Lepidoptera: Danaidae), store only some of the cardenolides present in the larval milkweed (Asclepiadaceae) host. Feeding known doses of individual cardenolides to 4th instar monarch larvae led to more efficient larval tissue incorporation at low doses than at high ones, and favored storage of cardenolide glycosides over genins. A qualitative regulation also occurs during larval feeding; calactin and calotropin were stored as such but uscharidin was rapidly converted to a mixture of calactin and calotropin which were the forms stored by the larvae. Two genins, uzarigenin and digitoxigenin, were stored by larvae as polar cardenolide metabolites.Research supported by National Science Foundation grants DEB 7514266 and DEB 7514266-AO2 (U.C. Davis) and DEB 7514265 (Amherst College). 相似文献
194.
This paper proposes an improvement to the delay‐dependent stability of discrete systems with time‐varying delays. The approach is based on the observation that the positive definiteness of a chosen Lyapunov–Krasovskii functional does not necessarily require all the involved symmetric matrices to be positive definite, which has been overlooked in the literature. The derived delay‐dependent stability conditions are in terms of linear matrix inequalities. It is theoretically proved that our results are less conservative than the corresponding ones obtained by requiring the positive definiteness of all the symmetric matrices in a chosen Lyapunov–Krasovskii functional. The importance of the present approach is that a great number of delay‐dependent analysis and synthesis results obtained by the aforementioned requirement in the literature can be improved by the present approach without introducing any new decision variables. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
195.
An experimental investigation of the rheological properties of glass fiber-reinforced polycarbonate melts and the extrusion of such compounds through capillary and slit dies is presented. The viscosity–shear rate function seems independent of instrument for cone-plate and capillary investigations. The presence of fibers increases the level of the viscosity. Normal stresses at fixed shear stress are also increased by the presence of fibers. The extrudate swell is decreased by the presence of fibers and surface roughness is increased. Fiber orientation increases and surface roughness decreases with increasing extrusion rate. 相似文献
196.
197.
The potential hazard of a carbonate-based, phosphate-free detergent was compared with that of a variety of granular and liquid
phosphate detergents. Evaluations were made using methods prescribed by the Federal Hazardous Substances Act (FHSA) and others
designed to more closely approximate exposure in actual use. Some phosphate products produced esophageal and gastric mucosal
irritation similar to that produced by the nonphosphate detergent, while others caused varying, lesser degrees of injury.
The nonphosphate detergent, as well as several phosphate products, was seriously irritating to the unwashed eye at required
FHSA test levels. Under more realistic conditions, eye irritation was reduced significantly. Skin irritation by the nonphosphate
product was comparable to that by phosphate detergents, and in some cases was lower. In skin irritation tests with human subjects,
results with the carbonate detergent were equal to or lower than those obtained with phosphate detergents. The carbonate detergent
was found not to be phototoxic, nor a contact- nor photo-sensitizer. Total alkalinity was shown not to correlate with the
severity of tissue injury in either test animals or in man.
Presented at the AOCS Short Course, “Update on Detergents and Raw Materials,” Lake Placid, New York, June 1971. 相似文献
198.
Vasudev P. Saraf Wolfgang G. Glasser Garth L. Wilkes James E. McGrath 《应用聚合物科学杂志》1985,30(5):2207-2224
Hydroxypropyl lignin-based thermosetting polyurethanes were synthesized with excess hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI) by solution casting. Four polyethylene glycols (PEG) of molecular weight 400, 600, 1000, and 4000 were mixed with lignin polyol to incorporate different proportions of soft segment into the network prior to crosslinking. Neither thermal nor mechanical and limited small angle x-ray scattering (SAXS) analysis provided distinct evidence for phase separation and microstructure formation. The study examines the effect of the soft segment in relation to chain length and weight contribution on the thermal and mechanical properties of the final networks. A significant sensitivity of glass transition temperature (Tg), of swelling in DMF, and of the mechanical properties to soft segment content was observed. Some of this sensitivity must, however, be attributed to differences in crosslink density since the polyol to diisocyanate weight ratio was kept constant throughout the synthesis series. The magnitude of the change of the different properties was found to be influenced by both glycol content and glycol molecular weight. The Tg of the network decreased from 105°C to as low as 38°C (HDI), and from 158°C to 70°C (TDI), with incorporation of up to 17.8% glycol, and it was greater with lower molecular weight glycols than with higher ones at any weight fraction. Swelling in DMF increased as expected with soft segment content. Mechanical properties were affected most if HDI and lower molecular weight glycols were used. The uniformity in structure, reduction in brittleness, and considerable improvement in mechanical properties with inclusion of minor PEG contents indicates that lignin-based network polyurethanes can be synthesized with controllable performance characteristics. 相似文献
199.
A real‐time cell electronic sensing (RT‐CES) system has been used for label‐free dynamic measurements of cell responses to toxicant. Cells are grown onto the surfaces of the microelectronic sensors. Changes in cell number expressed as cell index (CI) have been recorded on‐line as time series. The CI data are used for dynamic modelling or parameter estimation for cell cytotoxicity process. We consider two dynamic modelling approaches, namely data‐based system identification and first principle modelling. It is shown that data‐based system identification can provide a quick solution for the cytotoxicity dynamic models and is effective for short‐term predictions. It, however, can be poor for long‐term predictions, particularly if there is no output correction, i.e., when the model is used for simulation. In view of this, the first principle modelling approach by considering fundamental physical principles such as toxicant transport is explored. For long‐term prediction or simulation, the prediction performance for some of cytotoxicity process is dramatically improved using the models obtained from the latter approach. This happens only if the underlying mechanism is truly understood. Through several cytotoxicity modelling and validation studies, it is shown that the black box modelling and first principle modelling both should be considered in challenging modelling problems such as the cytotoxicity. Pros and cons of the two modelling approaches are discussed. 相似文献
200.
Phosphatidylcholine acyltransferase (lecithin:cholesterol acyltransferase or LCAT; EC 2.3.1.43) activity was found to be present
in pig ovarian follicular fluid (POFF), in addition to pig serum (PS). The cholesterol esterification rate in both POFF and
PS is linear with incubation time up to 2 hr. The mean absolute rate of POFF-cholesterol esterification was 8.1±0.4 nmoles
per ml per hr approximately one-fourth of that in PS. However, the fractional rate (percent of labeled cholesterol esterified
per hr) of POFF-cholesterol esterification was similar to that observed in PS. There was little variation of absolute rate
of cholesterol esterification in the fluid obtained from different sizes of follicles. Fatty acid or triacylglycerol did not
participate in the reaction of cholesterol esterification in POFF. No appreciable change in enzymatic activity was found from
storing POFF at 4 C for periods of time up to 24 hr or at −70 C up to 2 months, but activity was lost thereafter. On the other
hand, PS showed a much longer period of stability (5 days at 4 C and 9 months at −70 C). A discrepancy between the fatty acid
composition of cholesteryl esters formed by the LCAT reaction and the fatty acid composition at the C-2 position of phosphatidylcholine
led us to propose a two-step mechanism for the LCAT reaction. It is concluded that the LCAT of POFF, as well as that of plasma,
is specific for individual fatty acids rather than for the fatty acid composition of phosphatidylcholine. The fatty acid concentration
of lysophosphatidylcholine decreased during prolonged incubation times (6 to 21 hr) suggesting that the increased lysophosphatidylcholine
formed as a product of the LCAT reaction may be reused as substrate for the LCAT reaction or for hydrolysis by lysophosphatidylcholine
hydrolase.
Presented at the AOCS Meeting, New York, May 1977. 相似文献