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

Abstract

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°C (Δ H=3.8kcal/mol) are increasing hydration proessively lowers the transition temperature and increases the transition enthalpy, until limiting values ( T m = 45°C, Δ H = 6.7kcal/mol) are observed for hydration values > 21.4% H 2O. At low hydration levels, < 20% H 2O, an additional transition is observed at approx. 20°C. X-ray diffraction studies at temperatures below (22°C) and above (55°C) the main endothermic transition confirm that the bilayer gel (sharp 4.2Åreflection) → bilayer liquid crystal (diffuse 4.5Åreflection) transition occurs at all hydration levels with limiting bilayer hydration occurring at approx. 31.5% H 2O in the gel phase and at approx. 35% H 2O 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.