Sphingomyelin--lecithin bilayers and their interaction with cholesterol.

Utilizing X-ray diffraction and differential scanning calorimetry (DSC), we have studied (1) the structure and thermotropic properties of hydrated N-palmitoylsphingomyelin, (2) the interaction of N-palmitoylsphingomyelin with dimyristoyllecithin, and (3) the interaction of cholesterol with N-palmitoylsphingomyelin and dimyristoyllecithin, both individually and in a 50:50 (mol/mol) mixture. N-Palmitoylsphingomyelin forms bilayers which undergo a thermotropic order--disorder (gel--liquid crystalline) transition at 40.5 degrees C (delta H = 5.8 kcal/mol). The bilayer repeat distance is 66.8 A at 10 degrees C and 61.6 A at 50 degrees C. N-Palmitoylsphingomyelin exhibits miscibility with dimyristoylecithin in both the gel and liquid-crystalline phases, and no lateral phase separation occurs. Scanning calorimetry indicates that interaction with cholesterol is similar for both N-palmitoylsphingomyelin and dimyristoyllecithin and that in a 50:50 (mol/mol) mixture cholesterol shows no preferential affinity for either phospholipid.