On the bilayer phase transition temperatures for monoenoic phosphatidylcholines and phosphatidylethanolamines and the interconversion between them.

The dependence of the phase transition temperature, Tm, on the structural parameters of the lipid molecule has been firmly established for lipid bilayers composed of saturated phosphatidylcholines (PC) or phosphatidylethanolamines (PE). It is from such a dependence that conversions of Tm values from saturated PC to PE and vice versa have been successfully demonstrated. In this communication, similar chain-melting-structure relationships are sought for monoenoic PC and PE in which the long hydrocarbons linked at the sn-1 and sn-2 positions are saturated and cis-mono-unsaturated chains, respectively. Specifically, various molecular species of monoenoic PC and PE were first semisynthesized and their Tm values were subsequently determined by high-resolution differential scanning calorimetry. Based on the crankshaft-like kink motif model, the synthesized monoenoic PC and PE can each be divided into two groups, each group being characterized by three structural parameters. For each group of monoenoic PC or PE, a specific equation correlating the changes in Tm with variations in the structural parameters was derived using the multiple regression analysis. Furthermore, within each group of lipids including both PC and PE, an explicit equation describing the interconversion of Tm values between PC and PE with the same fatty acyl chain composition was established. The present work not only extends the earlier work relating the chain-melting temperature of saturated PC and PE to their structural parameters, but also indicates that the basic topology of the sn-2 mono-unsaturated acyl chain of PC and PE in the gel-state bilayer can be approximated by the crankshaft-like kink motif with the longer segment being packed much more orderly than the shorter segment.