Phospholipid bilayer phase transition

Differential scanning calorimetry is a basic method to study the phospholipid bilayer phase transitions since these are accompanied by a positive enthalpy of fusion. The excess heat capacity,... 

Table 2.1. Thermodynamic Properties of the Phospholipid Bilayer Phase Transition...

Particular phospholipids display characteristic transition temperatures (Tm). As shown in Table 9.1, increases with chain length, decreases with unsaturation, and depends on the nature of the polar head group. For pure phospholipid bilayers, the transition occurs over a narrow temperature range. The phase transition for dimyristoyl lecithin has a peak width of about 0.2°C. 

Phospholipid molecules form bilayer films or membranes about 5 nm in thickness as illustrated in Fig. XV-10. Vesicles or liposomes are closed bilayer shells in the 100-1000-nm size range formed on sonication of bilayer forming amphiphiles. Vesicles find use as controlled release and delivery vehicles in cosmetic lotions, agrochemicals, and, potentially, drugs. The advances in cryoelec-tron microscopy (see Section VIII-2A) in recent years have aided their characterization [70-72]. Additional light and x-ray scattering measurements reveal bilayer thickness and phase transitions [70, 71]. Differential thermal analysis... 

Membranes are composed of phospholipids and proteins. The fatty acid composition of the phospholipids in a membrane influences how it is affected by the cold. In general, as the temperature of a cell is lowered the lipids in the membrane bilayer undergo a phase transition from a liquid crystalline (fluid) state to a gel (more solid) state. The temperature at which this transition takes place is very narrow for phospholipids composed of a simple mixture of fatty acids, but is quite broad for the phospholipids in cellular membranes. It is usually implied from various methods... 

TABLE 5 NMR Characterization of Thermal Phase Transition in Zones I III of Phospholipid Bilayers... 

Lee, A. G., Effects of charged drugs on the phase transition temperature of phospholipid bilayers, Biochim. Biophys. Acta 514, 95-104 (1978). 

Liposome Formation. The pioneering investigations of Bang-ham (5) have shown that thin films of natural phospholipids form bilayer assemblies if they are lyophilized in excess water by simple handshaking above the phase transition temperature. While this procedure results in the formation of large, multibilayered spherical structures, by ultrasonication of such lipid dispersions small unilamellar liposomes are formed (16), which are schematically shown in Figure 10. Additional metTiods for liposome preparation are described in a number of reviews (17,44,45,46). 

This review describes experimental techniques, then gives some selected results of H, and NMR studies of pressure effects on the structure, dynamics and phase transitions of phospholipid bilayers. Other examples deal with 2D-NOESY experiments on lipid vesicles and pressure effects on the interaction of anaesthetics with phospholipid bilayers. Furthermore, we discuss... 

There is an abrupt decrease in the lateral diffusion coefficient of DPPC upon the phase transition from the GI phase to the Gi phase. This is because the acyl-chain region is being packed even more efficiently in the Gi phase than in the GI phase, and the hydrocarbon volume in the Gi phase is smaller than in the GI phase. Also, in the Gi phase, the lipid acyl-chains from the opposing bilayer leaflets interdigitate. In order for a phospholipid molecule to diffuse it has to circumvent the nearby interdigitated molecules which hinder diffusion. 

LIPID PHASE TRANSITION KINETICS LIPID PHASE TRANSITION KINETICS LIPID TRACER KINETICS Lipid transfer across membrane bilayers, PHOSPHOLIPID FLIP-FLOP Lipoamide,... 

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Phospholipid vesicles (and bilayers) composed of phospholipids with well-defined fatty acid side chains undergo a sharp transition from a crystallinelike state to an amorphous state as the temperature is raised.107 The transition temperature depends on the nature of the fatty acid side chains. For example, for C12 saturated fatty acid chains on lecithin the transition temperature is 0° and for C18 saturated fatty acid chains it is 58°C for unsaturated lecithins the transition temperature is below zero.107 For real membranes sharp phase transitions are not observed, because of the heterogeneous composition of the membrane. In the case of /3 hydroxybutyrate dehydrogenase, the enzymic activity apparently is not influenced by this phase transition as judged by the temperature dependence of the reaction rate. However, for some membrane-bound proteins, a plot of the reaction rate versus the reciprocal temperature... 

The chemical compositions and isomeric structures of the fatty acid chains of phospholipids is well known to have large effects on the physical properties of lipid bilayers, such as the temperatures of endothermic chain melting phase transitions. Lipid vesicles sensitized with lipid haptens can be agglutinated with specific antibodies directed against the haptens (see Fig. 1). 

Hand-shaken dispersions of phospholipids thus may be a more reasonable choice for comparison with membranes, and except in rare cases of highly unsaturated systems the high resolution NMR spectra of such dispersions do not exhibit high resolution proton absorption. Hence, for most membranes, NMR can indicate immobilization of fatty acid chains but not whether chains are immobilized by binding onto proteins or by tight packing into bilayers. Broad line NMR may be useful, especially when a phase transition is present. 

The phase transitions in bilayers can be recognized in many ways. Differential scanning colorimetry has already been mentioned. Another approach is to measure the spacing between molecules by X-ray diffraction. The cross-sectional area occupied by a phospholipid in a bilayer is always greater than the 0.40 nm2 expected for closest packing of a pair of extended hydrocarbon chains.39 85... 

Procarione and Kaufmann (49) studied the electrical properties of phospholipid bilayers between metal contacts. They observed, for example, irregularities in current and capacitance vs. temperature data which may be the result of phase transitions in the lipid bilayer. They also observed that both temperature-independent (tunneling) and temperature-dependent conduction processes with an activation energy of 0.65 eV were important. 

When metals are available, the laminae are stabilized by bonding forces as shown in studies on phospholipid-water systems59. Even films of myristic acid and metal ions are solid up to a cross-section area of 50 A per molecule60,61. The numerous phase transitions of pure phospholipid-water systems62 and the polymorphism of phospholipid crystals63 suggest that even pure phospholipid bilayers — in their true nature — do not represent stable lattice structures but require metal ions for coordina-tive purposes. 

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