STRUCTURAL ORGANIZATION OF N-ACYL DERIVATIVES OF EGG PHOSPHATIDYLETHANOLAMINE INTO BILAYERS - POLYMORPHIC AND THERMOTROPIC STUDIES

The present paper focuses on the thermotropic and polymorphic behaviou r of a series of N-acylphosphatidylethanolamines of natural origin. He adgroup and N-acyl chain modifications have been introduced, and the m iscibility of the synthesized compounds with the main membrane phospho lipids phosphatidylcholine and phosphatidylethanolamine has been studi ed. The compounds N-oleoylphosphatidylethanolamine and N-stearoylphosp hatidylethanolamine of natural origin, and two different types of stru ctural analogues have been synthesized and studied: N-oleoylphosphatid ylpropanolamine with one extra methylene group between the phosphate a nd amino groups, and the N-alkyl derivative N-methyl-N-oleoylphosphati dylethanolamine. The N-acylation of transphosphatidylated phosphatidyl ethanolamine (PE) with saturated acyl chains caused a significant incr ease in the gel-to-liquid-crystalline phase transition temperature, de termined by differential scanning calorimetric analysis, but all the N -oleoyl-containing glycerophospholipids assayed showed lower transitio n temperatures than their precursor aminophospholipids. The presence o f a third hydrophobic chain, the possibility of intermolecular hydroge n bonding between the amide groups of N-acyl PEs and the loss of the h ydrogen binding ability of aminophospholipids would account for this t hermotropic behaviour. The miscibility of the N-acylphospholipids with egg phosphatidylcholine and transphosphatidylated egg yolk phosphatid ylcholine has also been shown by these calorimetric studies, although non-ideal mixing of these phospholipid molecules has been revealed. P- 31 NMR studies have shown that the ability of the different N-acylated analogues to adopt lamellar structures is significantly different. Wh en the N-acylation reactions were carried out with unsaturated acyl ch ains, the tendency to form nonlamellar phases was dramatically increas ed. Analysis of the P-31 NMR spectra of mixtures of the different N-ac ylglycerophospholipids with the main membrane constituents phosphatidy lcholine and phosphatidylethanolamine has shown that these lipid molec ules are miscible, in agreement with the calorimetric data.