INTERACTIONS OF N-STEAROYL SPHINGOMYELIN WITH CHOLESTEROL AND DIPALMITOYL PHOSPHATIDYLCHOLINE IN BILAYER-MEMBRANES

Differential scanning calorimetry and x-ray diffraction have been util ized to investigate the interaction of N-stearoylsphingomyelin (C18:0- SM) with cholesterol and dipalmitoylphosphatidylcholine (DPPC). Fully hydrated C18:0-SM forms bilayers that undergo a chain-melting (gel --> liquid-crystalline) transition at 45 degrees C, Delta H = 6.7 kcal/mo l. Addition of cholesterol results in a progressive decrease in the en thalpy of the transition at 45 degrees C and the appearance of a broad transition centered at 46.3 degrees C; this latter transition progres sively broadens and is not detectable at cholesterol contents of >40 m ol%. X-ray diffraction and electron density profiles indicate that bil ayers of C18:0-SM/cholesterol (50 mol%) are essentially identical at 2 2 degrees C and 58 degrees C in terms of bilayer periodicity (d = 63-6 4 Angstrom), bilayer thickness (d(p-p) = 46-47 Angstrom), and lateral molecular packing (wide-angle reflection, similar to 1/4.8 Angstrom(-1 )). These data show that cholesterol inserts into C18:0-SM bilayers, p rogressively removing the chain-melting transition and altering the bi layer structural characteristics. In contrast, DPPC has relatively min or effects on the structure and thermotropic properties of C18:0-SM. D PPC and C18:0-SM exhibit complete miscibility in both the gel and liqu id-crystalline bilayer phases, but the pre-transition exhibited by DPP C is eliminated at >30 mol% C18:0-SM. The bilayer periodicity in both the gel and liquid-crystalline phases decreases significantly at high DPPC contents, probably reflecting differences in hydration and/or cha in tilt (gel phase) of C18:0-SM and DPPC.