Lipid Bilayer Phase Research Articles

Theory of lipid bilayer phase transitions as detected by fluorescent probes

We present a quantitative theory that relates the fluorescence intensityvs. temperature (I vs. T) profile of a fluorescent-labeled two-component lipid bilayer to the phase diagram of the bilayer and the partition coefficientK of the fluorophore between fluid and solid phases of the bilayer. We show how the theory can be used to evaluateK from experimentalI vs. T profiles and the appropriate phase diagrams as well as to understand the different shapes ofI vs. T profiles obtained with particular fluorophores and phase diagrams. Using calculatedI vs. T graphs, we discuss the meaning of parameters, such as midpoint of the phase transition and onset and termination of a transition, which are often used to characterize phase transitions on the basis of fluorescence intensityvs. temperature profiles.

Redetermination of the pressure dependence of the lipid bilayer phase transition

The effect of pressure on the phase transition temperature for the dipalmitoyllecithin bilayer was redetermined by following the volume change accompanying the transition. These measurements were carried out isothermally with the transition from the ordered to the disordered phase induced by decreasing the pressure. This contrasts with our previous measurements which were carried out at constant pressure and increasing temperature. The transition at every temperature was sharp and confirmed our previous observation that the volume change associated with the transition (0.033 mL g-1) is invariant with pressure. However, our present measurements, in contrast to our previous results, indicate that dP m/dTm at all pressures is in agreement with the 1 atm value of delta H/Tm delta V within experimental error where Tm and Pm are the temperature and pressure of the phase transition, respectively. These results, which are now in agreement with all other known pressure data, indicate that the entropy change associated with the transition is invariant with pressure.

Semiphenomenological model for the lipid bilayer phase transition: Finite chains in three dimensions

The exactly solvable model of the Nagle is reinterpreted and is used to form the basis of a Landau phenomenology of phospholipid bilayers with finite hydrocarbon chains. The properties of the model depend on the chain length. The results of the model calculation are in agreement with important experimental trends. It is found that infinite chain bilayers in the reinterpreted model should have transition temperatures of about 700 °K, while very short chain bilayers may have no phase transition.

Theory of lipid monolayer and bilayer phase transitions: effect of headgroup interactions.

Headgroup and soft core interactions are added to a lipid monolayer-bilayer model and the surface pressure-area phase diagrams are calculated. The results show that quite small headgroup interactions can have biologically significant effects on the transition temperature and the phase diagram. In particular, the difference in transition temperatures of lecithins and phosphatidyl ethanolamines is easy to reproduce in the model. The phosphatidic acid systems seem to require weak transient hydrogen bonding which is also conjectured to play a role in most of the lipid systems. By a simple surface free energy argument it is shown that monolayers under a surface pressure of 50 dynes/cm should behave as bilayers, in agreement with experiment. Although the headgroup interactions are biologically very significant, in fundamental studies of the main phase transition in lipids they are secondary in importance to the hydrocarbon chain interactions (including the excluded volume interaction, the rotational isomerism, and the attractive van der Waals interaction).

Phosphorylation and dephosphorylation of membrane proteins as a possible mechanism for structural rearrangement of membrane components

A correlation was found between dephosphorylation of chicken erythrocyte membrane proteins, aggregation of intramembrane particles, increase in the lipid bilayer phase of the membrane and exposure of membrane phospholipids toward phospholipase A and trinitrobenzene sulfonic acid. Most of the covalently bound phosphate of the membrane proteins turns over and is associated with 5 major bands. It is suggested that phosphorylation and dephosphorylation of these proteins causes changes in their charge and conformation. Such changes might affect the interaction of these proteins with the neighbouring lipids or lipoprotein complexes and results in the aggregation of intramembrane particles and relative increase in the exposed free lipid bilayer phase of the membrane.

Correlation between changes in the membrane organization and susceptibility to phospholipase C attack induced by ATP depletion of rat erythrocytes

About 20 and 43% of the total membrane phospholipids are hydrolized in fresh rat erythrocytes by treatment with phospholipase C ( Bacillus cereus), or both sphingomyelinase and phospholipase C, respectively, without causing cell lysis. Treatment of ATP-depleted cells with phospholipase C alone results in 50% hydrolysis and extensive lysis. Depletion of ATP causes a marked increase in the aggregation of intramembranous particles accompanied by a similar increase in the smooth area between the particle clusters as revealed by the freeze-etch technique. Such changes are not induced by extensive phospholipid hydrolysis in absence of cell lysis in fresh cells. Based on these and additional data, it is suggested that the membrane phospholipid organization can be divided into 3 types: phospholipids exposed to phospholipase C; phospholipids protected against phospholipase C by presence of sphingomyelin; phospholipids which can be exposed following alteration of the proteinlipid interactions. Such alterations which might be induced by a variety of means, including ATP depletion, might result in clustering of intramembranous particles and increase of the free lipid bilayer phase of the membrane.

Some models for lipid bilayer and biomembrane phase transitions

Biological membranes consist to a large extent of lipid bilayers, some of which exhibit fairly sharp phase transitions near body temperatures. We describe here a model for the main observed phase transition in pure lipid monolayers and bilayers. The model contains approximate treatments of hard−core repulsions and weak attractive forces between molecules. We believe that the model accounts qualitatively for many of the experimental properties that the model accounts qualitatively for many of the experimental properties of of monolayers and bilayers. Improvements which should enhance quantitative agreement are also discussed.

The pressure dependence of the lipid bilayer phase transition.

The pressure dependence of the lipid bilayer phase transition.

Lipid bilayer phase transition: density measurements and theory.

The overall change of density for dipalmitoyl lecithin bilayers agrees with a general order-disorder theory and yields about seven gauche rotations per molecule for the biologically relevant high-temperature phase. The shape of the curve of density against temperature is similar to the result of an exact calculation on a specific model, which gives a 3/2-order phase transition.

The phase behavior of monogalactosyl, digalactosyl, and sulphoquinovosyl diglycerides

The phase behavior in water of mono- and digalactosyl diglycerides from pelargonium leaves and sulpholipid from an algal source have been defined by X-ray diffraction methods. The monogalactosyl diglyceride forms an hexagonal lipid-water phase (water cylinders in a lipid matrix) over an extended partof its phase diagram. Incorporation of water into the hexagonal lattice is limited (about 22 weight%), at which point the cylinder-to-cylinder separation, at 20 °C, is 60.5 Å. The rod-like structure of this phase is confirmed by freeze-etch electron microscopy and calculations of the cylinder separation are in good agreement with those shown by X-ray diffraction. The addition of a second galactose unit completely alters the phase behavior and digalactosyl diglyceride, over an equivalent temperature-composition range, forms only a lamellar lipid bilayer phase, again with limited uptake of water (about 22 weight%). At 20°C the lamellar repeat distance, at maximum water uptake, is 54 Å with a lipid thickness of 41.6 Å and a surface area per molecule of 75 Å 2, these values being similar to those of hydrated phospholipids. Algal sulpholipids, with much higher contents of saturated fatty acids, show a more complex behavior in that the lamellar phase formed appears to exhibit limited swelling behavior at low temperatures, whereas raising the temperature results in a gradual increase in interbilayer water uptake. At high temperatures the swelling behavior resembles that of phospholipids with a net charge ( e.g. phosphatidylserine) where all the water added is incorporated between the lipid bilayers. The phase behavior of glycolipids and phospholipids are compared and considered in terms of their respective roles in plant and animal cell membranes.