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Biomembrane Phase Transitions

STUDIES OF LIPID-WATER SYSTEMS USING DIFFERENTIAL SCANNING CALORIMETRY

From the ¹ From the Chemistry Department, Sheffield University, Sheffield, England
² From the Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada

Differential scanning calorimetry has been applied to the study of some simple model biomembranes, i.e. lipid-water systems, and the endothermic phase transitions which they exhibit when transforming from the gel to liquid crystal phase have been studied.

These studies show that:

1. When lipids of the same class, e.g. lecithins with dissimilar chain lengths C₁₄ and C₁₆ are mixed, a continuous series of solid solutions are formed below the Tc line and co-crystallization occurs. No great change in the range of the thermal transition occurs as a result of the mixing.

2. When lipids of different classes, i.e. lecithin and phosphatidylethanolamine classes but of the same chain length, are mixed, a considerable increase in transition range occurs. Clusters of gel and liquid crystalline lipids can coexist within this temperature range.

3. Small amounts of 1,2-dimyristoyl phosphatidylethanolamine present in a lecithin bilayer cause pronounced effects associated with the head group transition of the lecithin. This appears to be an example of a more general effect, i.e. the presence of small amounts of a "foreign molecule" in a membrane surface can sometimes affect the packing of the polar groups of many other lipid molecules in the surface.

4. Interactions of divalent cations with the polar groups of the lipids raise the gel to liquid crystalline transition temperature. This behavior parallels the properties observed in monolayer systems.

5. Electrostatic binding of proteins and basic polypeptides to the (anionic) polar groups of phospholipids shifts the temperature of the gel-liquid crystal transition of the lipid and also affects the energy barrier for reorientation of a spin label in the hydrocarbon phase. This behavior may be relevant to the interpretation of thermal transitions observed with some biomembranes.

6. Admixture of the ionophore gramicidin A with dipalmitoyl lecithin at low concentrations affects the lipid polar group packing and at higher concentrations causes a marked decrease in the energy associated with the lipid endothermic phase transition. This effect may occur because of intercalation of gramicidine A molecules among the lipid chains.

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