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At least 631 records · Page 35Linked to original sources
PubMed1993

Permeation of superoxide anion through the bilayer of vesicles of a synthetic amphiphile.

Large unilamellar vesicles, prepared with dioctadecyldimethylammonium chloride, entrap nitroblue tetrazolium. Addition of solid KO2, or production of superoxide anion by riboflavin photolysis, to nitroblue tetrazolium-containing dioctadecyldimethylammonium vesicles results in the formation of monoformazan above the phase-transition temperature of the bilayer. Below the phase-transition temperature the yield of monoformazan is negligible. These results demonstrate that superoxide anion permeates vesicles above the phase-transition temperature of the bilayer.

Anions↗
PubMed1984

Interaction of dolichol and dolichyl phosphate with phospholipid bilayers.

The thermotropic phase transition of dipalmitoylphosphatidylcholine vesicles reconstituted with dolichol or dolichyl phosphate was investigated as a function of the lipid-to-polyisoprenoid ratio by means of differential scanning calorimetry and fluorescence depolarization of the embedded probe 1,6-diphenyl-1,3,5-hexatriene. At the concentrations studied, dolichol and dolichyl phosphate lowered and broadened the transition temperature of dipalmitoylphosphatidylcholine bilayers. Dolichol was found to increase the motional freedom of the bilayer both below and above the transition temperature as determined by fluorescence depolarization. In contrast, low concentrations of dolichyl phosphate decreased the bilayer motional freedom below the transition temperature while high concentrations increased the motional freedom. Above the transition temperature, dolichyl phosphate decreased bilayer 'fluidity' at all concentrations. The data suggest that these polyisoprenoids perturb the bilayer lattice, with the neutral species dolichol increasing membrane 'fluidity', while dolichyl phosphate acts to 'stiffen' the membrane.

Calorimetry, Differential Scanning↗
PubMed2004

Study of transport behavior for Fe-doping La(0.67)Ca(0.33)MnO3 perovskite manganese.

Systematic studies of the transport properties of La(0.67)Ca(0.33)Mn(1-x)Fe(x)O3 (x=0-0.3) systems showed that with increasing Fe-doping content x the resistance increases and the insulator-metal transition temperature moves to lower temperature. For small doping content, the transport property satisfies metal transport behavior below the transition temperature, and above the transition temperature it satisfies the small polaron model. This behavior can be explained by Fe3+ doping, which easily forms Fe(3+)-O(2-)-Mn4+ channel, suppressing the double exchange Mn(3+)-O(2-)-Mn4+ channel and enhancing the spin scattering of Mn ions induced by antiferromagnetic clusters of Fe ions.

Journal Article↗
PubMed2003

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite.

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.

Journal Article↗
PubMed1978

Effect of basic protein from human central nervous system myelin on lipid bilayer structure.

The effect of myelin basic protein from normal human central nervous system on lipid organization has been investigated by studying model membranes containing the protein by differential scanning calorimetry or electron spin resonance spectroscopy. Basic protein was found to decrease the phase transition temperature of dipalmitoyl phosphatidylglycerol, phosphatidic acid, and phosphatidylserine. The protein had a greater effect on the freezing temperature, measured from the cooling scan, than on the melting temperature, measured from the heating scan. These results are consistent with partial penetration of parts of the protein into the hydrocarbon region of the bilayer in the liquid crystalline state and partial freezing out when the lipid has been cooled below its phase transition temperature. The effect of the protein on fatty acid chain packing was investigated by using a series of fatty acid spin labels with the nitroxide group located at different positions along the chain. If the protein has not yet penetrated, it increases the order throughout the bilayer in the gel phase, probably by decreasing the repulsion between the lipid polar head groups. Above the phase transition temperature, when parts of it are able to pentrate, it decreases the motion of the lipid fatty acid chains greatly near the polar head group region, but has little or no effect near the interior of the bilayer. Upon cooling again the protein still decreases the motion near the polar head group region but increases it greatly in the interior. Thus, the protein penetrates partway into the bilayer, distorts the packing of the lipid fatty acid chains, and prevents recrystallization, thus decreasing the phase transition temperature. The magnitude of the effect varied with the lipid and was greatest for phosphatidic acid and phosphatidylglycerol. It could be reversed upon cooling for phosphatidylglycerol but not phosphatidic acid. The protein was only observed to decrease the phase transition temperature of phosphatidylserine upon cooling. It had only a small effect on phosphatidylethanolamine and no effect on phosphatidylcholine. Thus, the protein may penetrate to a different extent into different lipids even if it binds to the polar head group region by electrostatic interactions.

Chemical Phenomena↗
PubMed1983

Thermodynamic investigations of cytochrome b5 unfolding. II. Detergent-solubilized cytochrome b5 in solution and in a reconstituted system with dimyristoyl phosphatidylcholine.

Thermal unfolding of the detergent-solubilized cytochrome b5 was investigated by scanning calorimetry. The protein shows different thermostability in the presence and absence of detergent, and it achieves the maximal transition temperature after incorporation into dimyristoyl phosphatidylcholine liposomes. However, transition temperature and Gibbs energy change at unfolding are still lower than that of the tryptic fragment of cytochrome b5 in aqueous solution. Cytochrome b5 undergoes in aqueous solution in the absence of detergent an irreversible, complicated transition, but it remains in the associated state after thermal denaturation. Half transition temperature, enthalpy and heat capacity changes of cytochrome b5 unfolding under various external conditions are reported and compared with the corresponding values of the tryptic fragment of the protein. The thermodynamic data and independent results are suitable for detailing a model proposed by Tanford (The Hydrophobic Effect (1980), pp. 205-211, John Wiley & Sons, New York) for the spatial arrangement of the protein within the membrane.

Animals↗
PubMed1980

Ethanol effects on synaptic glutamate receptors and on liposomal membrane structure.

Exposure of synaptic plasma membranes to 50 mM ethanol in vitro brought about a 3.5 degrees C decrease in the transition temperature of the high affinity glutamate binding process in these membranes. Ethanol had no effect on the energy of activation of glutamate binding below the transition temperature but decreased the energy of activation above the transition temperature. Electron paramagnetic resonance (EPR) studies of lipid organization of egg lecithin and bovine brain phospholipid liposomes indicated that ethanol at low concentrations (0.04--2 mM) caused small increases in the rigidity of the membrane near the surface. At higher concentrations (0.04--2 M) ethanol brought about increasing fluidization of both the surface and inner areas of the bilayer. Even at 4 mM concentration ethanol enhanced the ordered to fluid state transition of liposome membranes as shown by a 5.2 degrees C and 1.9 degrees C decrease in the transition temperatures of the membrane determined with the cholestane EPR probe.

Animals↗
PubMed2004

Effect of sugar-phosphate mixtures on the stability of DPPC membranes in dehydrated systems.

The stabilizing role of sugars on dehydrated membranes is well established. The formation of a glassy matrix and the direct interaction between the sugars and the lipids are some of the mechanisms proposed to be involved in this stabilizing effect. Phospholipidic systems have been studied extensively as models for biological membranes and also due to the practical applications of liposomes as vehicles for drug delivery. In this work, we evaluate the effect of sugar-phosphate mixtures on the transition temperature of dehydrated 1,2-dipalmitoylphosphatidylcholine, and also examine some physical characteristics of these mixtures, such as the glass transition temperature and water sorption properties. The addition of phosphate salts to sugar systems has several interesting features that merit its consideration in formulations to protect dehydrated labile biomaterials. In particular, sucrose-phosphate mixtures provide an interesting alternative to pure saccharide formulations due to their high glass transition temperatures and their increased ability to maintain a low melting transition temperature in the presence of small amounts of water.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1976

Changes in (Na+ + K+)-ATPase activity of Ehrlich ascites tumor cells produced by alteration of membrane fatty acid composition.

The fatty acid composition of plasma membrane derived from Ehrlich ascites tumor cells was altered in vivo by changing the dietary lipid of the tumor-bearing mice. The activity of (sodium + potassium)-adenosinetriphosphatase ((Na+ + K+ATPase), in partially purified plasma membranes, was measured ass a function of temperature. Arrhenius plots of the data were biphasic. Striking differences, dependent on the membrane fatty acid composition, were observed in the transition temperature and in the energies of activation below the transition temperature. The transition temperatures for the (Na+ + K+)-ATPase of plasma membrane derived from tumor cells grown in mice fed a regular chow diet containing a mixture of fatty acids (PMC), a 16% sunflower oil diet (PMSU), or a 4% tristearin diet (PMTS) were 20, 21, and 13.5 degrees C, respectively...

Adenosine Triphosphatases↗
PubMed1981

Vesicle- vesicle interactions in sonicated dispersions of dipalmitoylphosphatidylcholine.

The time course of the size transformation of sonicated small unilamellar dipalmitoylphosphatidylcholine vesicles at 23 degrees C has been followed with (31)P and (1)H nuclear magnetic resonance (NMR) spectroscopy. Comparison of these results with turbidity measurements indicates that vesicle aggregation, monitored by turbidity, and size transformation, followed by NMR, occur on distinctly different time scales. For lipid concentrations in the 5-50 mM range, aggregation takes place on a time scale of minutes, whereas size transformation takes many hours. Aggregation, unlike size transformation, can be reversed by increasing the temperature above the phospholipid phase transition temperature. Analysis of the (31)P NMR line shapes provides evidence for a model in which the small vesicles transform into the product vesicles [characterized by Schullery, S.E., Schmidt, C.F., Felgner, P., Tillack, T.W., & Thompson, T.E. (1980) Biochemistry 19, 3919] without producing significant amounts of intermediate-size vesicles. Kinetic analysis indicates that the size transformation is apparently second order. (1)H NMR data indicate that the rate of transformation is decreased if trivalent ions are added to the dispersions and also if the temperature is periodically increased above the transition temperature. Analysis of the latter experiment provides some evidence that vesicle aggregation is a necessary precursor to size transformation. It was also found that increasing the average vesicle size decreases the extent of transformation.

Kinetics↗
PubMed1979

Time-dependent changes in the size distribution of distearoylphosphatidylcholine vesicles.

The results of transmission electron microscopic and ultracentrifugal studies of the size distributions of sonicated distearoylphosphatidylcholine vesicles are reported. Small vesicles (d approximately 300 A) were prepared by sonication of pure 1,2-distearoyl-3-sn-phosphatidylcholine in water and incubated at 4, 21, 40, 53, and 65 degrees C. The vesicle size distributions changed as a function of time at all temperatures below the phase-transition temperature but remained constant at the transition temperature and above. The sizes of structures to which the small vesicles are converted are the same at all temperatures, although the rates of conversion differ. The primary structures formed are identified as larger vesicles. The rate of loss of small vesicles is found to increase with decreasing temperature. At 4 and 21 degrees C small vesicles are converted to amorphous material, possibly irregular fragments of neat phase, in addition to being converted to larger vesicles. Trace amounts of an impurity commonly produced in the synthesis of 1,2-distearoyl-3-sn-phosphatidylcholine, 1,3-distearoyl-2-sn-phosphatidylcholine, are found to dramatically reduce the rate of loss of small vesicles at 21 degrees C.

Kinetics↗
PubMed1980

Fluorescence depolarization studies of phase transition and fluidity in lecithin liposomes containing alpha-tocopherol.

The effects of alpha-tocopherol and related compounds on the membrane fluidity of liposomes composed of dipalmitoyl-lecithin or egg lecithin were studied by measuring the fluorescence polarization of DPH(1,6-diphenyl-1,3,5-hexatriene), a hydrophobic fluorescent probe distributed in the hydrocarbon region of lipid bilayers. The transition temperature of dipalmitoyl-lecithin liposomes was lowered on incorporation of alpha-tocopherol into the membranes. A similar effect was observed on incorporation of phytol or phytanic acid into liposomes, but not on incorporation of 2,2,5,7,8-pentamethyl-1-hydroxychroman as an alpha-tocopherol model compound (TMC) which lacks an isoprenoid side chain. Above the phase-transition temperatures, incorporation of alpha-tocopherol or TMC dose-dependently increased the polarization values of DPH in dipalmitoyl-lecithin and egg lecithin liposomes, while incorporation of phytol or phytanic acid had little effect or slightly lowered the polarization. These results suggest that the effect of alpha-tocopherol in lowering the phase-transition temperature depends on its hydrophobic side chain, and that its effect in decreasing the membrane fluidity above the phase-transition temperature depends on its hydrophylic chroman ring portion. Cholesterol had an effect similar to alpha-tocopherol on membrane fluidity. alpha-Tocopherol and cholesterol had independent effects in increasing the polarization of DPH above the phase-transition temperature when the two compounds were incorporated together into liposomes.

Cholesterol↗
PubMed1985

The effect of head group structure on phase transition of phospholipid membranes as determined by differential scanning calorimetry.

The phase transition characteristics of cardiolipin and phosphatidylglycerol suspensions were investigated by differential scanning calorimetry. The phase transition temperatures for dimyristoylphosphatidylglycerol, tetramyristoylcardiolipin, dipalmitoylphosphatidylglycerol and tetrapalmitoylcardiolipin were 25.0, 47.0, 40.5 and 62.2 degrees C, respectively. The phase transition temperature for a mixture of two analogous phospholipids was higher than that for phosphatidylglycerol alone, but lower than that for cardiolipin alone. It increased along with cardiolipin content. The phase transition temperature for cardiolipin was increased in the presence of divalent cations, particularly Ca2+. The results indicate that the head group of cardiolipin by itself can increase the phase transition temperature.

Calorimetry, Differential Scanning↗
PubMed2000

Influence of induced interactions on the superfluid transition in dilute fermi gases

We calculate the effects of induced interactions on the transition temperature to the BCS state in dilute Fermi gases. For a pure Fermi system with two species having equal densities, the transition temperature is suppressed by a factor (4e)(1/3) approximately 2.2, and for nu fermion species, the transition temperature is increased by a factor (4e)(nu/3-1) approximately 2.2(nu-3). For mixtures of fermions and bosons the exchange of boson density fluctuations gives rise to an attractive interaction, and we estimate the increase of the transition temperature due to this effect.

Journal Article↗
PubMed1995

Molecular dynamics in stiff ionene below glass transition.

Temperature dependences of proton and fluorine second moments and spin-lattice relaxation time T1 below glass transition were measured in glassy "I-Do,Pip-Me-BF4" ionene. The existence of motions of methyl groups and segments linking the cationic centers, namely piperidinium rings and trimethylene groups, for the polymeric part of ionene were established. Isotropic rotation of the counter-ion was evidenced and its limited diffusion suggested. To interpret the proton and fluorine relaxation data, a Davidson-Cole distribution of correlation times was assumed.

Anions↗
PubMed1976

Interactions of proteins and cholesterol with lipids in bilayer membranes.

Mixtures of lipids and protein, the ATPase from rabbit sarcoplasmic reticulum, were studied by freeze-fracture electron microscopy and by measurement of the amount of fluid lipid with the spin label 2,2,6,6-tetramethylpiperidine-1-oxyl (TEM-PO). In dimyristoyl phosphatidylcholine vesicles the protein molecules were randomly distributed above the transition temperature, Tt, of the lipid and aggregated below Tt. For mixtures of dimyristoyl and dipalmitoyl phosphatidylcholine the existence of fluid and solid domains were shown in the temperature interval predicted from earlier TEMPO measurements. When protein was incorporated into this lipid mixture, freeze-fracture particles were randomly distributed in fluid lipids, or aggregated when only solid lipids were present. In mixtures of dimyristoyl phosphatidylcholine with cholesterol the protein was distributed randomly above the transition temperature of the phosphatidylcholine. Below that transition temperature the protein was excluded from a banded phase of solid lipid in the case of 10 mol% cholesterol. In mixtures containing 20 mol% cholesterol, protein molecules formed linear arrays, 50-200 nm in length, around smooth patches of lipid. Phase diagrams for lipid/cholesterol and lipid/protein systems are proposed which account for many of the available data. A model for increasing solidification of lipid around protein molecules or cholesterol above the transition temperature of the lipid is discussed.

Adenosine Triphosphatases↗
PubMed1983

A characterization of the low temperature structural transition of Escherichia coli 5 S RNA by partial enzymatic digestion.

The low temperature structural transition (low leads to high) of 5 S RNA from Escherichia coli is investigated by partial digestion with ribonuclease T1. In addition to a general masking of guanines from the nuclease, differential changes of accessibility are observed when Mg2+ and salt concentrations are increased to bring about the low leads to high transition. Residue G13 becomes more exposed in the high form while residues G54, G56, G61, G72, and G83-86 become less exposed. The observed cutting rate at other sites is unchanged. A possible conformational change is discussed which could explain the observed changes in RNase T1 digestion patterns as well as the physical chemical observations.

Escherichia coli↗
PubMed2001

Pseudoelasticity and thermoelasticity of nickel-titanium alloys: a clinically oriented review. Part II: Deactivation forces.

The purpose of this review was to organize a systematic reference to help orthodontists evaluate commonly used orthodontic nickel-titanium alloys. Part I of the article reviewed the data available in the literature regarding the temperature transitional ranges of the alloys. The thermomechanical behavior of these compounds is, in fact, strictly dependent upon the correlation between the temperature transitional range and the oral temperature range. Part II focuses on the mechanical characteristics of the alloys, such as the magnitude of the forces delivered and correlations with the temperature transitional range and oral temperature.

Body Temperature↗