Membranes inverted hexagonal phases

FIG. 7 Structures of various liquid-crystalline phases of membrane lipids. (A) Normal hexagonal phase (Hi) (B) lamellar phase (C) inverted hexagonal phase (Hu). Cubic phases consisting of (D) spherical, (E) rod-shaped, and (F) lamellar units. The hydrocarbon regions are shaded and the hydrophilic regions are white. (Reprinted by permission from Ref. 11, copyright 1984, Kluwer Academic Publishers.)... 

Due to its ability to form inverted hexagonal phase, DOPE is believed to impart fusogenicity to lipoplexes, thus facilitating fusion followed by destabilization of the endosomal membrane, lipoplex escape from the endosomes, and eventually the DNA release. Indeed, inclusion of DOPE into lipoplexes was shown to enhance considerably the transfection activity of some of the cationic lipid carriers [35,120, 121]. For example, formulations of oxypropyl quaternary ammonium cationic lipids with 50 mol% DOPE have been reported to exhibit 2-5 times higher transfection activity in COS7 cells than formulations with pure cationic lipid (Fig. 29) [35]. Recently, a triple-bond dialkynoyl analog of DOPE has been... 

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... 

CL-DNA complexes form spontaneously when solutions of cationic liposomes (typically containing both a cationic lipid and a neutral helper lipid) are combined. We have discovered several distinct nanoscale structures of CL-DNA complexes by synchrotron X-ray diffraction, three of which are schematically shown in Fig. 1. These are the prevalent lamellar phase with DNA sandwiched between cationic membranes (Lo,c) [22], the inverted hexagonal phase with DNA encapsulated within inverse lipid tubes (Hnc) [23], and the more recently discovered Hj0 phase with hexagonally arranged rod-like micelles surrounded by DNA chains forming a continuous substructure with honeycomb symmetry [24]. Both the neutral lipid and the cationic lipid can drive the formation of specific structures of CL-DNA complexes. The inverse cone shape of DOPE favors formation of the... 

When temperature is raised, the membrane bilayer not only becomes increasingly fluid due to enhanced motions of acyl chain, but it also tends to shift increasingly towards forming lipid aggregates in the inverted hexagonal phase (.hexagonal II, Hn, phase) (Hazel, 1995). The temperature at which this type of phase change... 

Figure 13.2. Inverted hexagonal phase model of nonclassical transport. The vector-cargo complex initially binds to the cell surface via electrostatic interactions. This induces an inverted hexagonal phase within the lipid bilayer, permitting the complex to become encapsulated within an inverted micelle and subsequently shuttled to the opposite side of the membrane. (Potential use of non-classical pathways for the transport of macromolecular drugs. Kueltzo, L.A. and Middaugh, C.R. 2001. Exp Opin Invest Drugs 9(9) 2039-2050. Used with permission.)...

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

Apart from the protein matrix, where it is possible for protons to move effectively in keeping with a mechanism somewhat similar to that proposed for the motion of protons in ice there is another path for protons through hydrophobic barrier of which the membrane is an example. This transition is based on the observations of phase transitions in a bilayer by X-ray diffraction methods. In this mechanism developed for mitochondria the main role falls to cardiolipin, which accounts for 33% of the total amount of lipids in the mitochondrial membrane.146 The protonation of the head groups of cardiolipin contained in the membranes of mitochondria caused a phase transition of the bilayer into an inverted hexagonal phase.147 This process is promoted by calcium ions whose reactions with the head groups of lipids favor neutralization of the membrane charge and effective dehydration of the polar heads. 

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... 

Siegel, D.P. (1999) The modified stalk mechanism of lamellar/inverted hexagonal phase transitions and its implications for membrane fusion. Biophysical Journal, 76, 291-313. 

INTERACTIONS BETWEEN LAMELLAR AND INVERTED HEXAGONAL Hn PHASE OF CL/DNA COMPLEXES AND ANIONIC GIANT LIPOSOMES MIMICKING THE CELL PLASMA MEMBRANE... 

As stated, biological membranes are normally arranged as bilayers. It has, however, been observed that some lipid components of biological membranes spontaneously form non-lamellar phases, including the inverted hexagonal form (Figure 1.9) and cubic phases [101]. The tendency to form such non-lamellar phases is influenced by the type of phospholipid as well as by inserted proteins and peptides. An example of this is the formation of non-lamellar inverted phases by the polypeptide antibiotic Nisin in unsaturated phosphatidylethanolamines [102]. Non-lamellar inverted phase formation can affect the stability of membranes, pore formation, and fusion processes. So-called lipid polymorphism and protein-lipid interactions have been discussed in detail by Epand [103]. 

Other functional liposomes are mainly stimuli-responsive liposomes. The pH-sensitive liposomes contain pH-sensitive lipids such as l,2-dioleoyl-vn-3-phosphatidylethanolamine (DOPE) showing an inverted hexagonal configuration in a low-pH environment and release entrapped drugs in the low-pH environment of tumor tissues due to liposomal membrane destabilization [89], Temperature-sensitive liposomes are prepared from special lipids such as DPPC whose phase transition temperature (Tc = 41°C) is proper to perform clinical anticancer therapy. When up to Tc, the fluidity of liposomal membranes increases sharply, followed by... 

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. 

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