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.)... 

Fig. 9.3 The structure of LLC monomers, 1 and 2, and the inverted hexagonal phase. (Reprinted with the permission of the American Chemical Society [113])...

Cyclic carbohydrates with two alkyl chains (e.g. 1,2-dialkyl (or 1,2-diacyl) glycerol 8 a (sug=Glcp, Galp) present structural similarities with glycerophospho-lipids. They form complex mesophases such as bicontinuous cubic phases, inverted hexagonal phases or myelin figures [58-61]. Other dialkyl derivatives... 

Phospholipid(s) 379, 380,382 - 387, 392. See also Specific substances bilayer diagram 391 head groups, functions of 396 inverted hexagonal phase 397 31P NMR 397 non-bilayer structures 397 Phosphomannomutase 654 Phosphomutases 526 Phosphonamidate 626s... 

Koltover, I., Salditt, T., Radler, J.O. and Safinya, C.R. (1998) An inverted hexagonal phase of cationic liposome-DNA complexes related to DNA release and delivery. Science, 281, 78-81. 

THE INVERTED HEXAGONAL PHASE OF CATIONIC LIPOSOM/DNA COMPLEXES PATHWAYS FROM LAMELLAR PHASE... 

Seddon, J.M. (1989) Structure of the inverted hexagonal phase and non-lamellar phase transitions of lipids. Biochim. Biophys. Acta., 1031, 1-69. 

Generally, lipids forming lamellar phase by themselves, form lamellar lipoplexes in most of these cases, lipids forming Hn phase by themselves tend to form Hn phase lipoplexes. Notable exceptions to this rule are the lipids forming cubic phase. Their lipoplexes do not retain the cubic symmetry and form either lamellar or inverted hexagonal phase [20, 24], The lamellar repeat period of the lipoplexes is typically 1.5 nm higher than that of the pure lipid phases, as a result of DNA intercalation between the lipid bilayers. In addition to the sharp lamellar reflections, a low-intensity diffuse peak is also present in the diffraction patterns (Fig. 23a) [81]. This peak has been ascribed to the in-plane positional correlation of the DNA strands arranged between the lipid lamellae [19, 63, 64, 82], Its position is dependent on the lipid-DNA ratio. The presence of DNA between the bilayers has been verified by the electron density profiles of the lipoplexes [16, 62-64] (Fig. 23b). 

Certain cationic lipids were found to form inverted hexagonal phase lipoplexes [21, 46, 85-87]. The Hn phase lipoplexes consist of DNA coated by lipid monolayers and arranged on a two-dimensional hexagonal lattice. This arrangement is identified by small-angle X-ray reflections in the ratio 1 3 4 (Fig. 24a). The lower intensity of the (11) and (20) lipoplex diffraction peaks relative to the Hn pattern... 

Fig. 24 Inverted hexagonal phase lipoplexes with cationic PCs forming HII phase (a) and cubic Pn3m phase (b). Lipid/DNA 4 1 w/w, 37 °C [46] (reproduced by permission of the Royal Society of Chemistry)...

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... 

Francescangeli O, Pisani M, Stanic V et al (2004) Evidence of an inverted hexagonal phase in self-assembled phospholipid-DNA-metal complexes. Europhys Lett 67 669-675... 

Caracciolo G, Caminiti R (2005) Do DC-Chol/DOPE-DNA complexes really form an inverted hexagonal phase Chem Phys Lett 411 327-332... 

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... 

For lamellar CL-DNA complexes, endosomal escape via activated fusion limits TE and strongly depends on aM, whereas the inverted hexagonal phase promotes... 

When the packing considerations are taken into account, this model practically predicts that amphiphiles with a single alkyl chain are will form micelles or bilayers, those with two alkyl chains bilayers, and those with three alkyl chains inverted hexagonal phases.[0]... 

The above-mentioned physicochemical properties of phospholipids lead to spontaneous formation of bilayers. Depending on the water-lipid ratio, on the type of phospholipids, and the temperature, the bilayer exists in different, defined mesomorphic physical organizations. These are the La high-temperature liquid crystalline form, the Lp gel form with restricted movement of the hydrocarbon chains, and an inverted hexagonal phase, Hn (see Sections 1.3.1 and 1.3.2). 

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 2. Ternary phase diagram of the system didodecyldimethylammonium bromide / water / hexene at 25°C. The nomenclature is cub cubic phase Lamj and Lam2 lamellar phases l.c. liquid crystalline, inverted hexagonal phase L2 microemulsion phase, with curvature toward water. (Courtesy of K. Fontell).

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