Structure and thermotropic properties of hydrated N-stearoyl sphingomyelin bilayer membranes

Hydrated multibilayers of N-stearoyl sphingomyelin were investigated as a function of hydration using differential scanning calorimetry (DSC) and X-ray diffraction. Anhydrous N-stearoyl sphingomyelin exhibits an endothermic transition at 75 degrees C (delta H = 3.8 kcal/mol); increasing hydration progressively lowers the transition temperature and increases the transition enthalpy, until limiting values (Tm = 45 degrees C, delta H = 6.7 kcal/mol) are observed for hydration values greater than 21.4% H2O. At low hydration levels, less than 20% H2O, an additional transition is observed at approx. 20 degrees C. X-ray diffraction studies at temperatures below (22 degrees C) and above (55 degrees C) the main endothermic transition confirm that the bilayer gel (sharp 4.2 A reflection)----bilayer liquid crystal (diffuse 4.5 A reflection) transition occurs at all hydration levels with limiting bilayer hydration occurring at approx. 31.5% H2O in the gel phase and at approx. 35% H2O in the liquid crystal phase. The thermotropic properties and metastability of this partial synthetic N-stearoyl sphingomyelin differ in some respects from that of the previously studied synthetic DL-erythro-N-stearoyl sphingomyelin (Estep, T.N., Calhoun, W.I., Barenholz, Y., Biltonen, R.L., Shipley, G.G. and Thompson, T.E. (1980) Biochemistry 19, 20-24), suggesting an influential role of the interfacial molecular conformation.