Lamellar-to-inverted hexagonal transitions

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. 

Lo(-Hn transitions have been observed mainly in PE or related phospholipids. Egg PE at pH 5.0 exhibits one such transition at 30 °C that is observable by DSC. DEPE at neutral pH has a La-Hn transition at 65 °C, and A-monomethyl dioleoylpho-sphatidylethanolamine (DOPE-Me) has the corresponding transition at - 63 °C. 

CH4 DIFFERENTIAL SCANNING CALORIMETRY IN THE STUDY OE LIPID STRUCTURES 

---

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... 

Figure 4.6 Lamellar-to-inverted hexagonal transition of DEPE-ceramide mixtures. Ceramide was A/-hexadecanoyL sphingosine (Cerl6). Data from J. Sot (unpublished)...

Membrane proteins have a profound influence on the thermotropic properties of lipid bilayers. Moreover, the effects of intrinsic (or integral) proteins are very different from those of extrinsic (or peripheral) proteins, reflecting their different modes of interaction, respectively, from the hydrophobic and hydrophyllic moieties of lipids. Some proteins can also influence, even at very low proteindipid ratios, the lamellar to inverted hexagonal transitions of certain lipids. 

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... 

The first studies involving transitions from and to cubic phases lamellar-to-cubic, cubic-to-cubic, cubic-to-inverted hexagonal, were performed using a slow T-jump and monitoring the transitions by TR-SAXS. AH transitions except the cubic-to-cubic ones were completed in less than 3 s. The body-centered cubic-to-primitive cubic phase transitions were much slower and required between 30 and 1800 s. An unidentified stable intermediate phase was evidenced in the transition L Qf. ... 

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...

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. 

Siegel, D. P. (1986) Inverted micellar intermediates and the transitions between lamellar, cubic and inverted hexagonal lipid phases. I. Mechanism of the La to Hn phase transitions. Biophys. J. 49,1155-1170. 

The rich lyotropic phase behavior exhibited by membrane lipids is well known. The lyotropic phase behavior of membrane lipids whose structure can be described as diacylglucosylglycerols can be classified as sugar fatty acid esters, and have been studied by Mannock et al. [111]. These types of surfactants often exhibit lamellar phases at low temperature, and a transition to a different inverted nonlamellar mesophase, often reverse hexagonal (Hn) or reverse micellar cubic (Qn) phase. In this particular study acyl chains with different terminus, based on stearic and palmitic acid, were studied. Only the shorter chained derivatives tended to form a Qn phase the remainder formed Hn phases over a range of temperatures above 70 °C. 

---