Evidence for metastability in stearoylphingomyelin bilayers

Abstract

The physical properties of aqueous stearoylsphingomyelin dispersions were investigated by differential scanning calorimetry, fluorescence spectroscopy and X-ray diffraction. The thermodynamically most stable gel state of this sphingomyelin is more highly ordered than the corresponding form of other phospholipids. This gel undergoes a transition to a liquid-crystalline conformation at 57.degree. C which is not rapidly reversible. Instead, the liquid-crystalline sphingomyelin supercools below 57.degree. C until a metastable gel is formed at 44.degree. C. The metastable gel slowly converts to the more highly ordered gel phase in an exothermic process at room temperature. The presence of various lipophilic substances enhances the stability of the metastable gel relative to the high ordered form.