Molecular arrangements in sphingolipids. Thermotropic phase behaviour of tetracosanoylphytosphingosine

The phase behaviour of a ceramide species containing C18-phytosphingosine and C24-fatty acid was studied by X-ray diffraction methods, in order to elucidate the packing principles of lipids with unequally long hydrocarbon chains. Six solid phases were observed. In five of them, the ceramide molecules have an extended, V-shaped conformation and pack in single layer arrangements with the sphingosine and fatty acid chains forming separate matrices. Differences between these phases are mainly due to thermotropic changes in packing efficiency and thus in tilt of the hydrocarbon chains. The chain packing undergoes transitions from triclinic (T|) to monoclinic (M|) and hexagonal, and between orthorhombic (O⊥) and hexagonal subcell arrangements, respectively. Only one case was observed, in which the molecules pack with their two chains parallelly stacked in a double layer arrangement in which the long fatty acid tails deeply interdigitate between the two opposite layer halves. In a natural membrane containing different lipids, however, long fatty acid tails probably arrange randomly and contribute to the formation of a liquid hydrocarbon zone in the center of the bilayer.