POLYMORPHISM OF GANGLIOSIDE-WATER SYSTEMS - A NEW CLASS OF MICELLAR CUBIC PHASES - FREEZE-FRACTURE ELECTRON-MICROSCOPY AND X-RAY-SCATTERING STUDIES

The (T, c)-dependent phase diagrams of two gangliosides, GM1 and GM1(acetyl), have been explored, in spite of the frequent occurrence of metastable states. In GM1 two lamellar, one hexagonal and two cubic (aspects # 5 and 13) phases were identified, in addition to the isotropic micellar solution, In GM1(acetyl) two of the phases are cubic (aspects # 8 and 13), one is the fluid isotropic solution. The structure of the lamellar and the hexagonal phases are trivial. The structure of the cubic phases was determined using a combination of freeze-fracture electron microscope and X-ray scattering experiments. The three cubic phases consist of lipid micelles of type I (oil-in-water); in two of the phases (Q(225) and Q(229)) the micelles are all identical and of quasi-spherical shape. Phase Q(223) was previously known to contain two types of micelles, one quasi-spherical, the other slightly flattened. The radii of the micelles determined from the dimensions of the electron density troughs were consistent with the chemical data. In keeping with what is known of the micellar solutions, the size of the micelles of the cubic phases of GM1(acetyl) is compatible with a spherical shape, whereas the micelles of GM1 seem to be somewhat too large to be compatible with the length of the molecules and with a spherical shape. Such a wealth of micellar cubic phases is unusual in lipid-water systems.