Studies of Thermal Transitions of Phospholipids in Water: Effect of Chain Length and Polar Groups of Single Lipids and Mixtures

Abstract

Several phospholipids and mixtures of phospholipids in water-rich systems were studied by differential thermal analysis and polarized light microscopy. Sharp transition temperatures were observed for all lipids which were single molecular types. This transition is associated with the melting of the hydrocarbon chains and formation of the liquid crystalline form. For mixtures of lecithin of different hydrocarbon chain lengths, the transition temperature range was broadened and increased with the concentration of the long chain component. The transition temperatures of a series of lipids, which had the same hydrocarbon chains but different polar groups, phosphatidyl ethanolamine, N, N-dimethyl phosphatidyl ethanolamine, and lecithin, were lowered as the methylation of the amine increased. Mixtures of lecithin and phosphatidyl ethanolamine with the same hydrocarbon chains showed that the transition temperature of the phosphatidyl ethanolamine is lowered by the presence of lecithin and in some compositions two transition temperatures were observed. The mixing of an acidic lipid, dicetyl phosphoric acid, with lecithin showed that small concentrations of the acid lower the transition temperature of lecithin. At concentrations of the acid above 21 mole percent, the transition temperature of dicetyl phosphoric acid is observed.

This work was supported by Multiple Sclerosis Grant 503A and U.S. Department of the Interior, Office of Saline Water Grant 14-01-0001-1277.