Analysis of the hexagonal II phase and its relations to lipidic particles and the lamellar phase. A freeze-fracture study.

Model systems of phosphatidylethanolamine (PE) and cardiolipin (DPG), as pure components and in binary mixtures with phosphatidylcholine (PC) have been morphologically analysed. The relation between the hexagonalII (HII) phase and lipidic particles as well as between the HII phase and the lamellar phase has been studied. Moreover, the periodicity of the various HII tubes was determined. (1) The periodicity of the HII phase of cardiolipin is dependent on the cation involved. DPG-Ca exhibits the smallest tube to tube distance when compared to Mg2+ and Mn2+. Moreover, the DPG-Ca tubes are quite straight, in contrast to the Mg2+ and Mn2+ tubes, which appear to be frequently curved. (2) HII tubes with two distinct diameters have been observed in HII phase containing lipid mixtures. The thickness of the HII tube is related to the composition of the tube. In the cardiolipin-lecithin system, structural separation of the pure cardiolipin HII phase has been suggested with Mg2+ and Mn2+, but not with Ca2+. (3) Models for the HII to lamellar phase transition and for the HII phase to the lipidic particles are presented. (4) Lipidic particles are exclusively found in lipid model systems, which contain HII phase favouring lipids. Morphological evidence is presented which suggests these lipidic particles represent inverted micells. These observations include: (i) there is a strong topological and quantitative relation between HII tubes and lipidic particles, (ii) lipidic particles occur densely packed in conglomerates without the presence of a smooth layer.