Thermotropic behavior of fatty acid ethyl esters in phospholipid liposomes

The thermotropic behavior of a series of synthetic fatty acyl ethylesters (FAEE) in multilamellar liposomes has been studied by differential scanning calorimetry and monitoring the changes in polarization emitted by the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Their thermotropic behaviour has been compared to that of the homologous fatty acids (FA) from which they are synthesized in vivo in the presence of ethanol. Compared to the correspondent FA, saturated FAEE show, depending on the chain length, a minor rigidifying effect or even a slight fluidizing effect on phospholipid bilayers. Unsaturated FAEE show, compared to the homologous FA, slightly greater fluidizing properties. The difference between FA and FAEE is more evident in single component phospholipid liposomes in the gel phase, and in mixed liposomes of two lipids at temperatures at which microdomains of gel and liquid zones coexist. The calorimetric data suggest that FAEE are completely miscible with phospholipids both in the gel and liquid phases; they appear to destabilize the bilayer wherein the ethoxy head group interferes with the intrinsic phospholipid interactions.