Phospholipids inverted hexagonal phase

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... 

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... 

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). 

The transition from to Hn can be rationalized by considering the packing parameter P (section 16.1.1). For bilayer forming carrier molecules, P is between 0.5 and 1.0, often close to 1.0 by contrast the phospholipid DOPE is cone-shaped and P is > 1, leading to a preferred formation of an inverted hexagonal phase. 

Although a wide range of phospholipids spontaneously form bilayers when dispersed in water, non-lamellaur structures such as the inverted hexagonal phase are also found under certain circum-... 

The bilayer-inverted hexagonal phase transition is taken as a model for non-bilayer transformations in this section. A calorimetric recording from an aqueous dispersion of the ether phospholipid dihexadecyl phosphatidylethanolamine is given in Fig. 2.8. 

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]. 

Under appropriafe condihons some aqueous phospholipids can exisf in non-bilayer phases, a facf fhaf may be of considerable biological imporfance. In the presence of Ca + some pure phospholipids can be converfed to fhe inverted hexagonal or Hjj phase (Fig. phase fhe phospholipid... 

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... 

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. 

The major lyotropic phases encountered with double-chain phospholipids are lamellar, inverted hexagonal, and cubic phases. Single chain lipids have surfactant properties and can also fonn spherical and cylindrical micelles. Figure 5 shows some of the possible aggregation stnictures. Phospholipids not only show lyotropic mesomorphism, i. e. different phases as a ftmetion of water content, but also thennotropic mesomorphism, i. e. transitions between different phases can be induced by varying the temperature. 

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.

Fig. 3.23 NM R spectra for macroscopic polymorphic phases of phospholipids in bilayers, hexagonal HM, and isotropic phases (small vesicles, micelles, inverted micelles, and cubic phases). Similar spectra are recorded for any one phase regardless of which...

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