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At least 19 recordsLinked to original sources
PubMed1979

Properties of bilayer membranes in the phase transition or phase separation region.

The increase in passive permeability of bilayer membranes near the phase transition temperature is usually explained as caused by either the increase in the amount of 'boundary lipid' present in the membrane, or by the increase in lateral compressibility of the membrane. Since both the amount of 'boundary lipid' and the lateral compressibility show a similar anomaly near the transition temperature, it is difficult to distinguish experimentally between the two proposed mechanisms. We have examined some details of both of the proposed pictures. The fluid-solid boundary energy, neglected in previous work, has been computed as a function of the domain size. For a single component uncharged lipid bilayer, the results rule out the existence of even loosely defined solid domains in a fluid phase, or vice versa. Thermodynamic fluctuations, which are responsible for anomalous behaviour near the phase transition temperature, are not intense enough to approximate the formation of a domain of the opposite phase. Turning next to lateral compressibility of bilayer membranes we have considered two-component mixtures in the phase separation region. We present the first calculation of lateral compressibility for such systems. The behaviour shows interesting anomalies, which should correlate with existing and future data on transport across membranes.

Lipid Bilayers↗
PubMed2002

First-order phase transition and phase coexistence in a spin-glass model.

We study the mean-field static solution of the Blume-Emery-Griffiths-Capel model with quenched disorder, an Ising-spin lattice gas with random magnetic interaction. The thermodynamics is worked out in the full replica symmetry breaking scheme. The model exhibits a high temperature/low density paramagnetic phase. As temperature decreases or density increases, a phase transition to a full replica symmetry breaking spin-glass phase occurs. The nature of the transition can be either of the second order or, at temperature below a given critical value, of the first order in the Ehrenfest sense, with a discontinuous jump of the order parameter, a latent heat, and coexistence of phases.

Journal Article↗
PubMed1977

A theory of phase transitions and phase diagrams for one- and two-component phospholipid bilayers.

A statistical mechanical partition function for phospholipid bilayers is constructed to obtain a theoretical description of the chain melting phase transition in lipid bilayer membranes and of the phase diagrams for two-component bilayers. In addition to providing an accurate representation of the transition temperatures and enthalpies of one-component bilayers composed of 1,2-diacylphosphatidylcholines, the theory can also account for the shapes of the phase diagrams observed for bilayers which are binary mixtures of these compounds with two different hydrocarbon chain lenghts.

Kinetics↗
PubMed1975

Phase transitions and phase separations in phospholipid membranes induced by changes in temperature, pH, and concentration of bivalent cations.

Differential scanning calorimetry (DSC) and fluorescence polarization of embedded probe molecules were used to detect phase behavior of various phospholipids. The techniques were directly compared for detecting the transition of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidic acid (DPPA) dispersed in aqueous salt solutions. Excellent agreement occurred in the case of phosphatidylcholine; however, in the case of phosphatidic acid, at pH 6.5, transitions detected by fluorescence polarization using the disc-like perylene molecule occurred about 10 degrees lower than those detected by DSC. Discrepancy between fluorescence and DSC methods is eliminated by using a rod-like molecule, diphenylhexatriene (DPH). Both techniques show that doubly ionizing the phosphate group reduces the Tc by about 9 degrees. Direct pH titration of fluidity can be accomplished and this effect is most dramatic when membranes are in their transition temperature range (ca. 50 degrees). Phosphatidic acid transitions occur at higher temperatures, and have appreciably lower transition enthalpies and entropies than phosphatidylcholine. These effect could not be explained simply on the basis of double layer electrostatics and several other factors were discussed in an attempt to rationalize the results. Addition of monovalent cations (0.01-0.5 M) is shown to increase the Tc of dipalmitoylphosphatidylglycerol by less than 3 degrees. However, addition of (1 x 10-3 M) Ca2+ abolishes the phase transition of both phosphatidyglycerol and phosphatidylserine in the range 0-70 degrees. Preliminary X-ray evidence indicates the phosphatidylserine-Ca2+ bilayers are in a crystalline state at 24 degrees. In contrast, 5 x 10-3 M Mg2+ only broadens the transition and increases the Tc indicating a considerable difference between the effects of Ca2+ and Mg2+. Neutralization of PS increases the Tc from 6 degrees (at pH 7.4) to 20-26 degrees (at pH 2.5-3.0) but does not abolish the transition, suggesting the Ca2+ effect involves more than charge neutralization. Addition of Ca2+ to mixed phosphatidylserine-phosphatidylcholine dispersions, induces a phase separation of the dipalmitoyl- (and also distearoyl-) phosphatidylcholine as seen by the appearance of a new endothermic peak at 41 degrees (58 degrees). Similarly, in mixed (dipalmitoyl) phosphatidic acid-phosphatidylcholine (2:1) dispersions, Ca2+ again can separate the phosphatidylcholine component.

Binding Sites↗
PubMed1979

A theoretical model for lipid mixtures, phase transitions, and phase diagrams.

We present a new model for the thermodynamic properties of lipid bilayers. The model consists of a system of hard cylinders of varying radii that correspond to the different molecular radii of lipids having different numbers of gauche rotations in their chains. Scaled particle theory is used to provide an accurate estimate of the entropy of packing of the cylinders. To apply the model to bilayers we introduce a semiempirical attractive potential energy. Once the form of this potential is chosen, we adjust one parameter, the interaction strength, so that the model fits the transition temperatures and entropies for various phospholipids. The model then agrees quite well with other published data for these systems. We also directly generalize our model to lipid mixtures, and we obtain phase diagrams that we compare to existing data for these systems. We use the model to describe lipid protein interactions in bilayers as well.

Lipid Bilayers↗
PubMed1991

General features of phospholipid phase transitions.

The phase transitions that take place in hydrated phospholipid dispersions are reviewed in terms of the lyotropic and thermotropic mesomorphism. The thermodynamics of the phase transitions are discussed, including the various contributions to the shifts in phase transition temperatures. Particular attention is given to the phase transitions involving the lamellar or lipid bilayer phase, in view of the relevance to the lipid component of biological membranes. These transitions include especially the chain-melting transition and the transformation to non-lamellar phases.

Calorimetry↗
PubMed1990

Superoxide anion radical induces lipid phase transitions that mimic temperature-induced phase-transition phenomenon of membrane lipids.

A biological membrane is shown to undergo lipid-phase transition leading to increased membrane fluidity when exposed to extraneously generated superoxide anion radical. This phase-transition is several folds higher in magnitude when compared to the temperature-induced fluidity change at the transition temperatures. This finding could have significant importance since an altered membrane configuration may involve aspects of biochemistry, biophysics or physiology.

Animals↗
PubMed2002

Scaling of entanglement close to a quantum phase transition.

Classical phase transitions occur when a physical system reaches a state below a critical temperature characterized by macroscopic order. Quantum phase transitions occur at absolute zero; they are induced by the change of an external parameter or coupling constant, and are driven by quantum fluctuations. Examples include transitions in quantum Hall systems, localization in Si-MOSFETs (metal oxide silicon field-effect transistors; ref. 4) and the superconductor-insulator transition in two-dimensional systems. Both classical and quantum critical points are governed by a diverging correlation length, although quantum systems possess additional correlations that do not have a classical counterpart. This phenomenon, known as entanglement, is the resource that enables quantum computation and communication. The role of entanglement at a phase transition is not captured by statistical mechanics-a complete classification of the critical many-body state requires the introduction of concepts from quantum information theory. Here we connect the theory of critical phenomena with quantum information by exploring the entangling resources of a system close to its quantum critical point. We demonstrate, for a class of one-dimensional magnetic systems, that entanglement shows scaling behaviour in the vicinity of the transition point.

Journal Article↗
PubMedDate not supplied

[Phase transition studies of model and biological membranes. I. Use of hydrophobic fluorescent probes for studying phase transitions in liposomes].

Phase transition induced by temperature changes in liposomes prepared from synthetic dimyristoyl-(DML), dipalmytoyl-(DPL) and distearyl-(DSL) lecithins and also from equimolar mixture of DML and DPL and mixtures of DPL and cholesterol were studied with fluorescent probes such as 3-metoxybenzantron (MBA) and dimetylaminocholkone (DMC). The significant changes of fluorescence intensity and maximum position of MBA and DMC were found in the regions of phase transitions at 23, 42, 54 and 31c in liposomes from DML, DPL, DSL and mixture of DML and DPL, respectively. Cholesterol incorporation into liposomes from DPL led to a decrease of transition temperature and cooperativity for these liposomes. At cholesterol concentration of 20 mol. percent or more the transition disappeared completely. It is concluded that MBA and DMC can be used for the investigation of biological membranes phase transitions.

Cholesterol↗