Lamellar phases inverted hexagonal transitions

The question of intermediate structures around a phase transition has caught particular attention in the case of the lamellar-to-inverted hexagonal transition of ethanolamine phospholipids, since this involves a major topological change (see Fig. 6). Inverted micellar structures were proposed as intermediates on the basis of P-NMR and electron microscopic results and also rationalized in a theoretical mechanism On the other hand, first results of time-resolved X-ray diffraction... 

Other studies of the lamellar-to-inverted hexagonal transition evidenced intermediate states. For instance, Laggner et al. showed that the L —> H2 transition in phosphatidyle-thanolamines involves the thin lamellar phase L discussed in the preceding paragraph. Robert et al. reported the formation of two transient intermediate phases, I and I, in the course of the lamellar-to-H2 transition of galactolipids. The first intermediate phase to appear, I, has a structure similar to the thin lamellar phase L . Only the intermediate phase I was evidenced in the L H2 transition of glycolipids. i The authors suggested that the intermediate phase I was not detected because it forms and disappears very rapidly. All these transitions were found to occur on the time scale of a few hundreds of milliseconds. 

Siegel DP, Epand RM (1997) The mechanism of lamellar-to-inverted hexagonal phase transitions in phosphatidylethanolamine implications for membrane fusion mechanisms. Biophys J 73 3089-3111... 

Siegel DP Membrane-membrane interactions in lamellar-to-inverted hexagonal phase transitions, in Membrane Fusion Sowers AE (ed) Plenum, New York in the press... 

Aggregation of the vesicles can also be interpreted in the light of results on lamellar-to-inverted hexagonal phase transitions in phosphatidylethanolamine, as obtained recently [12]. The initial step in the process of membrane fusion must be similar to the first step in the phase transition small connections between... 

Seddon, J. M. (1990). Structure of the inverted hexagonal (HII) phase, and non-lamellar phase transitions of lipids, Biochim. Biophys. Acta, 1031, 1-69. 

Figure 10.1 Schematic of two distinct pathways from the lamellar phase to the columnar inverted hexagonal i i phase of cationic liposome/DNA (CL/DNA) complexes. Along Pathway 1 the natural curvature C0=l/Ro of the cationic lipid monolayer is driven negative by the addition of the helper-lipid DOPE. This is shown schematically (middle top) where the cationic li DOT(4P is cylindrically shaped while DOPE is cone-like leading to the negative curvature. Along pathway II the to j transition is induced by the addition of a new class of helper-lipids consisting of mixtures...

Seddon, J.M., Cevc, G., and Marsh, D. (1983) Calorimetric studies of the gel-fluid transition (Lj) —> La) and lamellar-inverted hexagonal (La — H ) phase transition in dialkyl- and diacyl-phosphatidylethanolamines. Biochemistry 22 1280-1289. 

Siegel, D. P. (1986), Inverted micellar intermediates and the transitions between lamellar, cubic, and inverted hexagonal lipid phases. II. Implications for membrane-membrane interactions and membrane fusion, Biophys. J., 49(6), 1171-1183. 

The propensity of membranes to fuse correlates with the fraction of inverted phase-forming lipids conversely, membrane fusability is reduced with an increase of the lipid fraction that inhibits inverted phase formation. Substantial evidence suggests that the mechanism of lipid membrane fusion is related to the mechanism of lamellar/inverted phase transitions (23). The intermediates that form in membrane fusion seem to be identical to those that form during the transformations between lamellar, bicontinuous inverted cubic and inverted hexagonal lipid liquid-crystalline phases, and these transitions can be used successfully as a model for studying the lipid membrane fusion mechanism and kinetics. 

Seddon JM, Cevc G, Marsh D. Calorimetric studies of the gel-fluid (L-Beta-L-Alpha) and lamellar-inverted hexagonal (L-Alpha-HB) phase-transitions in dialkyl and diacylphosphatidylethanolamines. Biochemistry 1983 22 1280-1289. 

The lamellar phase represents the structure of cell membrane lipids under steady-state conditions. However in certain circumstances, particularly in membrane fusion events (e.g. in egg fertilization, or cell infection by some viruses), membrane lipids abandon transiently the lamellar disposition, adopting nonlamellar architectures, of which the best known is the so-called inverted hexagonal , or Hn, phase. Nonlamellar structures are at the origin of the lipid stalk , a structural intermediate that connects two bUayers in the membrane fusion process. Only certain lipids, or lipid mixtures, can undergo the Lo(-Hii thermotropic transition, and the latter can be detected by DSC. Hu, like other nonlamellar phases, has received particular attention lately because of its possible implication in important phenomena such as cell membrane fusion, or protein insertion into membranes. High-sensitivity DSC instruments allow the detection of La-Hn transitions with phospholipid suspensions of concentration 5 him or even less. 

M. Caflfey, Kinetics and mechanism of the lamellar gel lamellar liquid-crystal and lamellar inverted hexagonal phase-transition in phosphatidylethanolamine a real x-ray diffraction... 

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