Hn phase

In the Hn phase and in the inverted micellar cubic phase, the water associated with the polar headgroups is trapped inside a ring structure and is not in rapid exchange with bulk water [18]. In a bicontinuous cubic phase, however, there is a continuous network of aqueous channels. 

Cationic liposomes form complexes with plasmid DNA. Also, in this case DOPE affects the architecture of the complexes. DOTAP liposomes without DOPE are able to condense DNA, while DOTAP/DOPE liposomes do not condense DNA, presumably due to Hn-phase formation [230,233]. Interestingly, multivalent cationic liposomes, such as DOGS, can condense DNA more efficiently than monovalent cationic liposomes [233]. If the complexation is carried out at high ionic strength the DNA condensation is less prominent than in plain water. 

Mab(Phyt)j/water (b),and Mab(Phyt)2/water (c) systems, respectively. The lower traces represent the schematic structure of an Hn phase of the Mal2(Phyt)2/water system, an La phase of the Mal3(Phyt)2/water and Mals(Phyt)2/water systems, respectively. 

Figure 8-12 (A) 31P NMR spectra of different phospholipid phases. Hydrated soya phosphatidylethanolamine adopts the hexagonal Hn phase at 30°C. In the presence of 50 mol% of egg phosphatidylcholine only the bilayer phase is observed. At intermediate (30%) phosphatidylcholine concentrations an isotropic component appears in the spectrum. (B) Inverted micelles proposed to explain "lipidic particles" seen in freeze fracture micrographs of bilayer mixture of phospholipids, e.g., of phosphatidylethanolanine + phosphatidylcholine + cholesterol. From de Kruijft et al.m Courtesy of B. de Kruijft.

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

Fig. 4 Elongation of the R3 phosphate ester chain of the cationic PC results in nonlamellar phase formation. Small-angle X-ray diffraction patterns recorded at 20° C show (a) lamellar La (b) cubic Pn3m (c) inverted hexagonal Hn phases formed by dioleoyl cationic PCs with ethyl, hexyl and octadecyl R3 chains, respectively, diCl8 1 -EPC [19], diC18 l-C6PC [20] and diC18 l-C18PC [21]...

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... 

ATPase reconstituted in vesicles of different lipid composition [26], Pumping efficiency (number of Ca2+ ions transported per molecule of ATP hydrolyzed) increased with the mole fraction of lipids that prefer Hn-phase formation. However, no such correlation was observed with dioleoylphosphatidylcholine or digalactosyldiglyc-eride, which do not form low-temperature H n phases. It is important to note that it is not the chemical similarity but the phase preference of the phospholipid that is decisive. 

At higher temperatures, phospholipid molecules in the neighborhood of a-tocopherol are induced to form Hn phases with a decreased radius of curvature. 

In the Hn phase a defined stoichiometry of phospholipid and a-tocopherol molecules exists [79]. 

The effects of antiviral chemotherapeutic agents such as cyclosporin A, benzyloxy-carbonyl-D-Phe-L-Phe-Gly, and amantadine on membrane properties have been studied using the combination of 31P-NMR and DSC. It was found that benzyloxycar-bonyl-D-Phe-L-Phe-Gly was most effective in raising the bilayer to Hn phase transition temperature. Cyclosporin A caused the greatest broadening of the 31P-NMR signal. It was suggested that both effects are related to the inhibitory activity on membrane fusion and possibly also to their antiviral activity [151]. 

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 5.2 The two types of hexagonal lipid-water phases. Hi and Hn- Hi consists of lipid rods in water arranged on a two-dimensional hexagonal lattice, whereas Hn has the reversed structure. The Hn phase can also be regarded as intersecting lipid bilayers (infinite in one direction) as illustrated by the corresponding Hn asymmetric unit (circled), shown enlarged to the right.

Ellens, H Bentz, J., and Szoka, F. C. (1986) Fusion of phosphatidylethanol-amine-containing liposomes and mechanism of the La-Hn phase transition. Biochemistry 25,4141—4147. 

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. 

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