THE INFLUENCE OF N-ACYL CHAIN-LENGTH ON THE PHASE-BEHAVIOR OF NATURAL AND SYNTHETIC N-ACYLETHANOLAMINE PHOSPHOLIPIDS

The influence of N-acyl chain length on the thermotropic phase behaviour of N-acylphosphatidylethanolamines of natural origin was studied by DSC, P-31-NMR and IR-FT spectroscopy, The obtained data were also compared with the known behaviour of N-acyl derivates of dipalmitoylphosphatidylethanolamine. All the compounds analyzed form multilamellar liposomes and their enthalpies of calorimetric transition always exceed the corresponding values for phosphatidylcholines (PC) and phosphatidylethanolamines (PE). The transition temperature (T-m) for both series of compounds shows a V-shaped behaviour with an apparent minimum value at N-C6 and N-C8 chain length for natural and synthetic N-acylamino phospholipids, respectively. This behaviour can be accounted for by the disruptive effect of the terminal methyl group of the N-acyl chain in the bilayer packing, For long N-acyl chain (C18 to C14), they pack in the entire hydrocarbon width of bilayer and the T-m is higher than that of the corresponding PE. On decreasing the N-acyl chain length (C12 to C6), the T-m value decreases and becomes lower than that of the corresponding PC. The hydrocarbon chain packing in the gel state is perturbed by the N-acyl chain inequivalence and the disordering effect reaches a maximum value. For shorter N-acyl chains (C4 and C2), the T-m approaches that of the corresponding PC. This can be related to the smaller disruptive effect due to the location of the methyl terminus in the vicinity of the bilayer interface region, The results obtained by P-31-NMR and infrared spectroscopy confirm these observations.