The nature of lipidic particles and their roles in polymorphic transitions

The structural transition between bilayer (L alpha), inverted hexagonal (HII) and inverted cubic (CII) phases in mixtures of unsaturated phosphatidylethanolamines (PE) and phosphatidylcholines (PC) were investigated. Freeze fracture electron micrographs of intermediate stages of phase transitions showed that CII was a stable intermediate form between the L alpha--HII transition. The electron microscopic observation was supported by X-ray diffraction and 31P-NMR results. Detailed morphology revealed that during the L alpha--CII transition, interlamellae attachment points (conical lipidic particles) connect adjacent bilayers to form arrays of entrapped water pockets (inverted micelles). These water-containing spherical units were packed in a cubic lattice. In the CII to HII transition, these spherical units were linearly connected to form tubes. During the L alpha--HII transition, a ripple pattern was observed across the otherwise smooth lamellar. The troughs of the ripples were transformed into linear connections between adjacent bilayers, thereby converting multilayer structures into parallel tubes. No lipidic particles were involved in this type of transition. We show that there are different mechanisms involved in the L alpha, HII, CII polymorphic transitions, and that different types of 'lipidic particles' representing different molecular organizations may be involved in each case. Models of these transitions are proposed.