The role of sterol rings and side chain on the structure and phase behaviour of sphingomyelin bilayers.

The role of the side chain of sterols and the sterol ring structure on the formation of ordered phases of the type observed in membrane rafts has been examined in aqueous dispersions of binary mixtures of sphingomyelin and androsterol. Comparisons have been made with binary systems of cholesterol, stigmasterol, beta-sitosterol, and ergosterol with either sphingomyelin or dipalmitoylphosphatidylcholine. Thermotropic phase behaviour and structure of the mixed aqueous dispersions were characterized by differential scanning calorimetry, synchrotron X-ray diffraction, freeze-fracture electron microscopy, and Fourier-transform infrared spectroscopy. We show that: (i) Androsterol is less efficient in promoting the formation of liquid-ordered phase than other naturally occurring sterols which possess a side chain, (ii) cholesterol is the most efficient sterol of those investigated in forming liquid-ordered phase, (iii) the molecular stoichiometry of egg sphingomyelin and androsterol in the liquid-ordered phase is about 2:1, and (iv) sphingomyelin can form more stable liquid-ordered phase than glycerophospholipid in binary systems containing androsterol.