Evidence for metastability in stearoylsphingomyelin bilayers.

The physical properties of aqueous stearoylsphingomyelin dispersions have been investigated by differential scanning calorimetry, fluorescence spectroscopy, and X-ray diffraction. These studies indicate that 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 degrees C which is not rapidly reversible. Instead, the liquid-crystalline sphingomyelin supercools below 57 degrees C until a metastable gel is formed at 44 degrees 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 highly ordered form.