Analysis of the chainlength dependence of lipid phase transition temperatures: main and pretransitions of phosphatidylcholines; main and non-lamellar transitions of phosphatidylethanolamines.

The dependence on chainlength, n, of the temperatures at which the various thermotropic phase transitions in phospholipid bilayers take place can be fitted to high accuracy with an expression of the form: Tt = Tt infinity (1-A/(n-d)) where Tt infinity, A and d are the constants to be fitted. This expression works well with literature values for the main and pretransitions of saturated straight-chain phosphatidylcholines, for the main transition of isobranched- and anteisobranched-chain phosphatidylcholines, and for the main and non-lamellar phase transitions of saturated straight-chain phosphatidylethanolamines, and also for diacylglycerol glycolipids. The parameters in the fit can be related thermodynamically to the calorimetric properties of the phase transition. The constants A and d are related to the end effects in the chainlength dependence of the transition enthalpy and transition entropy, respectively, and the transition temperature extrapolated to infinite chainlength, Tt infinity, is determined by the ratio of the incremental transition enthalpy and transition entropy per methylene group. A reasonable correspondence is found between the chainlength dependence of the transition temperature and of the transition enthalpy and transition entropy for saturated straight-chain diacylphosphatidylcholines. It is likely, however, that the expression for the chainlength dependence of the transition temperature may be of more general applicability, because, on the one hand of the greater inherent precision of the transition temperature measurements, and on the other hand because the expression for the transition temperature is of more general validity, provided that the chainlength dependence of the incremental transition enthalpy and entropy are the same.