A synchrotron X-ray diffraction characterization of the structure of complexes formed between sphingomyelin and cerebroside

Specific lipid-lipid interactions are believed to be responsible for lateral domain formation in the lipid bilayer matrix of cell membranes. The miscibility of glucocerebroside and sphingomyelin extracted from biological tissues has been examined by synchrotron X-ray powder diffraction methods. Fully hydrated binary mixtures of egg-sphingomyelin codispersed with glucosylceramide rich in saturated C22 and C24 N-acyl fatty acids were subjected to heating scans between 20 and 90 degrees C at 2 degrees C.min(-1). X-ray scattering intensity profiles were recorded at 1 degrees C intervals simultaneously in both small-angle and wide-angle scattering regions. A gel phase characterized by a single symmetric peak in the wide-angle scattering region was transformed in all mixtures examined to a fluid phase at about 40 degrees C, similar to dispersions of pure egg-sphingomyelin. A coexisting lamellar structure was identified at temperatures up to about 75 degrees C which was characterized by a broad Bragg reflection. The scattering intensity of this structure increased relative to the structure assigned as bilayers of pure sphingomyelin with increasing proportions of glucosylceramide in the mixture. The relationship between the scattering intensities of the two peaks and the relative mass fractions of the two lipids showed that the bilayers assigned to a glucosylceramide-rich structure were composed of sphingomyelin and glucosylceramide in molar ratios of 1 : 1 and 2 : 1, respectively, at temperatures below and above the order-disorder phase transition temperature of the sphingomyelin (40 degrees C).