Thermotropic behavior of bilayers formed from mixed-chain phosphatidylcholines

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Abstract
The 6 possible phosphatidylcholines [PC] containing 2 different chains derived from myristic, palmitic and stearic acids were synthesized, and their bilayer structures were investigated by high-sensitivity differential scanning microcalorimetry. Chain migration during the syntheses caused each of the lipids to contain about 10% of the corresponding positional isomer. A phase diagram for each pair of isomers was constructed to permit estimation of the transition properties of the pure mixed-length phospholipids. The phase transitions of these lipids were similar to those of saturated like-chain PC. The main transition temperatures and enthalpies fall within the range of those for the like-chain lipids. In each pair of positional isomers the isomer having the longer chain at position 2 on the glycerol backbone has the higher transition temperature and enthalpy. The transition curves of the pure mixed-chain lipids with myristic acid at position 2 and either palmitic or stearic acid at position 1 exhibited 2 partially separated peaks for the main transition. No satisfactory interpretation of this unexpected phenomenon was developed.