Phospholipids transition temperature

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. 

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

In aqueous systems, membrane lipids may exist in a gel-like solid state or as a two-dimensional liquid. In the case of pure phospholipids, these states interconvert at a well-defined transition temperature, Tc, that increases with alkyl chain length and decreases with introduction of alkyl chain unsaturation. In cell membranes, which have marked heterogeneity in both the polar and nonpolar domains of the bilayer, this state is described as liquid disordered . The presence of sufficient sphingolipids, with... 

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). 

The chloroform solution of lipids (Solution A) is placed in a 50-mL round-bottomed spherical Quick-fit flask. Following evaporation of the solvent in a rotary evaporator at about 37°C, a thin lipid film is formed on the walls of the flask. The film is flushed for about 60 seconds with oxygen-free nitrogen (N2) to ensure complete solvent removal and to replace air. Two milliliters of distilled water and a few glass beads are added into the flask, the stopper is replaced, and the flask shaken vigorously by hand or mechanically until the lipid film has been transformed into a milky suspension. This process is carried out above the liquid-crystalline transition temperature (7/) of the phospholipid component of liposomes (> 7/) by prewarming the water... 

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... 

Phosphatidylcholine (PC) is widely used in liposomes, and at physiological pH has a zwitterionic structure, is only slightly able to form salts with divalent cations such as Ca2+, and has a lower transition temperature from liquid to crystalline than other phospholipids. 

EPR spectrum of tetramethylpiperidine-l-oxyl dissolved in an aqueous dispersion of phospholipids. (Top) above the main bilayer transition temperature Tt (center) between Tt and pretransition temperature (bottom) below pretransition temperature. From Shimshick and McConnell/... 

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