Morphology of the intermediate stages in the lamellar to hexagonal lipid phase transition

Abstract

The addition of calcium to suspensions of egg phosphatidylcholine and cardiolipin converts multiwalled liposomes to the hexagonal (HII) phase (Rand, R.P., Sengupta, S. (1972) Biochim. Biophys. Acta 255:484-492). We have studied this lamellar to hexagonal phase transition by freeze-fracture, thin-section electron microscopy, and X-ray diffraction and have morphologically characterized the intermediate stages. The first step in the transition involves the invagination and fusion of bilayers, marked by the appearance of lipidic intramembrane particles and "crater-like" indentations, as the large liposomes are converted to smaller flattened and elongated vesicles. The next step is the formation of tightly packed hexagonal arrays of tubules, each tubule being about 11 to 15 nm in diameter. These tubules are filled with fluid and a lipid bilayer forms the wall of each cylinder. Finally this tubular bilayer phase is converted to the hexagonal (HII) phase, where the distance between tubes is 5.5 to 7.5 nm.

Lipidic intramembranous particles

“…From freeze-fracturing one could interpret them as larger Hjl type cylinders. Other authors have interpreted these 14.0 nm tubes as the complex H H phase, in which the cylinders are in fact bilayers [34]. This interpretation is based on their appearance in thin-section micrographs.…”

“…Finally, one can find three-dimensional arrays of particles (Fig. 5J, K) in a variety of systems either in the presence or the absence of cryoprotectants [34,38]. The repeat distance can vary from 8-9 nm to 13-16 nm and is dependent upon the type of lipid, water content, temperature and presence of cryoprotectants (see subsection IIIC.…”

“…The appearance of lipidic particles is also influenced by the presence of cryoprotectants [38,39] and time [34,40], as well as environmental factors such as temperature [3,9], divalent cations [3,9], pH [8] and ionic strength [56]. The presence of cryoprotectants can decrease the number of particles [34,[38][39][40] and has been shown to change the diameter of the particles in a certain lipid mixture [38].…”

“…The presence of cryoprotectants can decrease the number of particles [34,[38][39][40] and has been shown to change the diameter of the particles in a certain lipid mixture [38]. The reason for this effect is likely a change in the balance of forces existing between the lipid molecules, in particular, hydrogen bonding between lipid and water, and water-water interactions.…”

“…In multilamellar systems, where one has multiplepoint fusion, the life-time of the lipidic particles may be increased. It has also been found that the appearance of lipidic particles is increased by the presence of cryoprotectants which stabilize these intermediate structures [34,[38][39][40].…”