A Calorimetric Study of the Thermotropic Behavior of Aqueous Dispersions of Natural and Synthetic Sphingomyelins

A recently developed differential scanning calorimeter has been used to characterize the thermotropic behavior of aqueous dispersions of liposomes containing sphingomyelin. Liposomes derived from sheep brain sphingomyelin exhibit a broad gel-liquid crystalline phase transition in the temperature range of 20-45 °C. The transition is characterized by maxima in the heat capacity function at 31.2 and 37.1 °C and a total enthalpy change of 7.2 ± 0.4 kcal/mol. Beef brain sphingomyelin liposomes behave similarly but exhibit heat capacity maxima at 30, 32, and 38 °C and a total enthalpy change of 6.9 kcal/mol. The thermotropic behavior of four pure synthetic sphingomyelins is reminiscent of multilamellar lecithin liposomes in that a single, sharp, main transition is observed. Results obtained for liposomes containing mixtures of different sphingomyelins are complex. A colyophilized mixture of N-palmitoylsphingosinephosphorylcholine, N-stearoylsphingosinephosphorylcholine, and N-lignocerylsphingosinephosphorylcholine in a 1:1:1 mol ratio exhibits a single transition with a T_m below that observed for the individual components. On the other hand a 1:1 mixture of N-stearoylsphingosinephosphorylcholine and 1-palmitoyl-2-oleylphosphatidylcholine exhibits three maxima in the heat capacity function. It is clear from these results that the thermotropic behavior of sphingomyelin-containing liposomes is a complex function of the exact composition. Furthermore, it appears that the behavior of the liposomes derived from natural sphingomyelins cannot be explained in terms of phase separation of theindividual components.