Temperature-dependent Phase Behavior Research Articles

Temperature-dependent morphological and phase behavior of sphingomyelin

Aqueous dispersions of bovine brain sphingomyelin were studied as a function of temperature. 31P-NMR, X-ray diffraction, and negative-stain and freeze-fracture electron microscopy were used to determine the morphology and phase structure at several temperatures. 31P-NMR indicated a change in phase structure with an increase in temperature. Evidence was found only for the lamellar phase at all temperatures studied with X-ray diffraction. Electron microscopy unexpectedly revealed the spontaneous development of small unilamellar vesicles at elevated temperatures, consistent with the 31P-NMR data, in the absence of any outside disturbances.

The polymorphic phase behaviour of phosphatidylethanolamines of natural and synthetic origin. A 31P NMR study

1. 1. The polymorphic phase behaviour of aqueous dispersions of phosphatidylethanolamines isolated from human erythrocytes, hen egg yolk and Escherichia coli have been investigated employing 31P NMR techniques. All species exhibit well defined, reversible bilayer to hexagonal ( H 11) phase transitions as the temperature is increased. The temperatures at which these transitions take place (10, 25–30 and 55–60°C for erythrocyte, egg yolk and E. coli phosphatidylethanolamine, respectively) are sensitive to the fatty acid composition, occurring at a temperature up to 10°C above the high temperature end of the hydrocarbon phase transition as detected by differential scanning calorimetry. In some cases the bilayer to hexagonal ( H 11) transitions may also be detected employing calorimetric techniques. 2. 2. The addition of equimolar concentrations of cholesterol to these naturally occurring phosphatidylethanolamines does not dramatically affect the bilayer-hexagonal ( H 11) transition temperature, producing changes of up to 10°C. 3. 3. 18 : 1 t 18 : 1 t phosphatidylethanolamine undergoes the bilayer to hexagonal ( H 11) phase transition as the temperature is increased through the interval 50–55°C. Alternatively, hydrated 12 : 0 12 : 0 phosphatidylethanolamine remains in the bilayer phase at temperatures up to 90°C (50°C above the hydrocarbon phase transition temperature). 4. 4. The presence of 100 mM NaCl or 10 mM CaCl 2 in aqueous dispersions of egg yolk phosphatidylethanolamine does not alter the temperature-dependent polymorphic phase behaviour significantly. However, at 40°C, increasing the p 2H above 8.0 results in progressive inhibition of the hexagonal ( H 11) phase and the appearance of a phase possibly of cubic structure at p +H 9.0. At p 2H 10.0 the bilayer phase is preferred, 5. 5. It is suggested that in biomembranes containing phosphatidylethanolamine as a majority species (such as that of E. coli) the fatty acid composition may primarily reflect the need to maintain bilayer structure. Alternatively, it is pointed out that in mammalian membranes such as that of the erythrocyte, phosphatidylethanolamine tends to destabilize bilayer structure. The resulting possibility that transitory non-bilayer lipid configurations may occur may be directly related to many important properties of biological membranes.

Effects of cholesterol on the properties of equimolar mixtures of synthetic phosphatidylethanolamine and phosphatidylcholine. A 31P NMR and differential scanning calorimetry study

1. 1. The interaction of cholesterol with equimolar mixtures of 18 : 1 c 18 : 1 c phosphatidylethanolamine and 16 : 0 16 : 0 phosphatidylcholine has been investigated employing differential scanning calorimetry and 31P NMR. At temperatures where lateral phase separation occurs cholesterol interacts preferentially with the (gel state) 16 : 0 16 : 0 phosphatidylcholine component, in agreement with previous studies (van Dijck et al. (1976) Biochim. Biophys. Acta 455, 576–587). Bilayer structure is maintained in such situations, and the effects do not therefore arise due to a preference of cholesterol for phospholipids in bilayer or non-bilayer phases. Further, the 31P NMR results indicate that when both species are in the liquid crystalline state the preferential interaction of cholesterol with the phosphatidylcholine component is maintained. 2. 2. The addition of cholesterol to (liquid crystalline) equimolar mixtures of 18 : 1 c 18 : 1 c phosphatidylethanolamine and 16 : 0 16 : 0 phosphatidylcholine stabilizes the bilayer phase. Alternatively, cholesterol destabilizes the bilayer phase in equimolar mixtures of 18 : 1 c 18 : 1 c phosphatidylethanolamine and 18 : 1 c 18 : 1 c phosphatidylcholine, reducing the temperature at which non-bilayer phases are formed. Intermediate cholesterol concentrations encourage formation of a phase which possibly has cubic structure, whereas higher cholesterol contents (and higher temperatures) encourage formation of the hexagonal ( H 11) phase. 3. 3. The temperature-dependent polymorphic phase behaviour of equimolar mixtures of 18 : 1 c 18 : 1 c phosphatidylethanolamine and 18 : 1 c 18 : 1 c phosphatidylcholine exhibits a very pronounced hysteresis which is progressively reduced by addition of cholesterol.