Thermal and 13C-NMR study of the dynamic structure of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-glycero-3-phosphocholine in aqueous dispersions.

Mixed-acid monounsaturated phosphatidylcholines containing palmitate and oleate chains have been synthesized by phospholipase A2 digestion of the appropriate single-acid phosphatidylcholine, followed by reacylation of the lysophosphatidylcholine with the desired fatty acid anhydride. The positional isomers 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-phosphocholine have been thus obtained. The thermotropic behavior of these lipids dispersed in excess water has been studied by differential scanning calorimetry. Positional isomers of mixed-acid monounsaturated phosphatidylcholines are found to have different gel to liquid-crystalline transition temperatures and enthalpies. It was found that mixtures of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine or 1,2-distearoyl-sn-glycero-3-phosphocholine exhibited inmiscibility in the phosphatidylcholine gel state. The dynamic structure of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-glycero-3-phosphocholine bilayers has been investigated by measuring the 13C nuclear spin-lattice relaxation times of sonicated aqueous dispersions. No difference was found between the two systems, suggesting that above the thermal transition the presence of the unsaturated acyl group in the 1 or 2 position does not affect significantly the dynamic structure of the bilayer.