Inverted micellar intermediates and the transitions between lamellar, cubic, and inverted hexagonal lipid phases. I. Mechanism of the L alpha----HII phase transitions.

A model for the thermotropic transitions between lamellar (L alpha) and inverted hexagonal (HII) phases is developed. According to this model, the first structures to form during the L alpha----HII transition are inverted micellar intermediates (IMI). The structure, formation rates, and half-lives of IMI ("lipidic particles") were described previously. IMI coalesce in the planes between apposed bilayers to form two types of HII phase precursors. The first is a monolayer-encapsulated HII tube (RMI), which forms via coalescence of IMI in pearl-string fashion. These structures have been proposed previously based on electron microscopic evidence. I show that if only RMI form, L alpha in equilibrium HII transitions cannot occur on observed time scales (faster than seconds). I propose that a second type of intermediate, a line defect (LD), forms as well. LD should form via IMI-IMI coalescence in significant numbers, and elongate rapidly into structures consisting of two apposed halves of HII tubes. Transitions via LD can occur in less than seconds, the time depending on the fraction of IMI-IMI coalescence events producing LD and the number of IMI per unit of bilayer area. Hysteresis in the phase transition temperature may be due to the difference in water content of the two phases and their low water permeabilities. The model is in qualitative agreement with morphological, NMR, and x-ray diffraction data on phospholipid systems. The results are relevant to IMI-mediated interactions between unilamellar bilayer vesicles, and to the structure of inverted cubic phases observed in some phospholipid systems. These will be discussed in subsequent publications (D. P. Siegel, manuscript in preparation).