Morphological transitions of brain sphingomyelin are determined by the hydration protocol: ripples re-arrange in plane, and sponge-like networks disintegrate into small vesicles

The phase transition of hydrated brain sphingomyelin occurs at around 35 de grees C, which is close to the physiological temperature. Freeze-fracture e lectron microscopy is used to characterize different gel state morphologies in terms of solid-ordered and liquid-ordered phase states, according to th e occurrence of ripples and other higher-dimensional bilayer deformations. Evidently, the natural mixed-chain sphingomyelin does not assume the flat L -beta, phase but instead the rippled P-beta, phase, with symmetric and asym metric ripples as well as macroripples and an egg-carton pattern, depending on the incubation conditions. An unexpected difference was observed betwee n samples that are hydrated above and below the phase transition temperatur e. When the lipid is hydrated at low temperature, a sponge-like network of bilayers is formed in the gel state, next to some normal lamellae. The netw ork loses its ripples during cold-incubation, which indicates the formation of a liquid-ordered (l(o)) gel phase. Ripples re-appear upon warming and t he sponge-like network disintegrates spontaneously and irreversibly into sm all vesicles above the phase transition. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.