Calorimetric investigations of saturated mixed-chain phosphatidylcholine bilayer dispersions.

A series of saturated mixed-chain phosphatidylcholines were prepared whose sn-2 acyl chains are two, four, six, and eight carbon atoms shorter than the sn-1 acyl chain. The calorimetric behavior of multilamellar bilayers of these phosphatidylcholines in excess water is investigated. The phosphatidylcholines display cooperative phase transitions which are dependent upon both the difference in chain length and the position of the acyl chains on the glycerol backbone of the phospholipid. A model is proposed which suggests that the thermotropic behavior of the mixed-chain phosphatidylcholines results from progressively greater interdigitation of the acyl chains of the phospholipid across the bilayer center, in the gel state, as the chain-length difference is increased beyond a minimum value. The disruptive effect of the terminal methyl groups of the fatty acyl chains upon the bilayer packing stability is also stressed. Dispersions of some of the mixed-chain phosphatidylcholines display transition endotherms which appear to be composites of two or more individual transition peaks. The dependence of this behavior on the thermal history of the dispersions is investigated. It is proposed that these peaks arise from the ability of the phosphatidylcholines' acyl chains to pack in more than one interdigitated conformation in the gel state.