A calorimetric study of the thermotropic behavior of aqueous dispersions of natural and synthetic sphingomyelins

Abstract

A recently developed differential scanning calorimeter was 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.degree. C. The transition is characterized by maxima in the heat capacity function at 31.2 and 37.1.degree. C and a total enthalpy change of 7.2 .+-. 0.4 kcal/mol. Beef brain sphingomyelin liposomes behave simiarly but exhibit heat capacity maxima at 30, 32, and 38.degree. C and a total enthalpy change of 6.9 kcal/mol. The thermotropic behavior of 4 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 Tm below that observed for the individual components. A 1:1 mixture of N-stearoylsphingosinephosphorylcholine and 1-palmitoyl-2-oleylphosphatidylcholine exhibits 3 maxima in the heat capacity function. The thermotropic behavior of sphingomyelin-containing liposomes is a complex function of the exact composition. It appears that the behavior of the liposomes derived from natural sphingomyelins cannot be explained in terms of phase separation of the individual components.