Lipid-water phases

Larsson, K., Cubic lipid-water phases Structures and biomembrane aspects. J. Phys. Chem. 1989,93,7304. 

Liquid Crystalline Lipid/Water Phases with Cubic Symmetry. 36... 

FIGURE 4.2 Schematic illustration of 2 x 2 x 2 unit cells of a lipid/water phase with gyroid cubic symmetry. In reversed bicontinuous cubic phases the lipid bilayer membrane separates two intertwined water-filled subvolumes resembling 3D arrays of interconnected tunnels. Black box (right) represents an enlargement of a part of the folded liquid crystalline lipid bilayer membrane structure. 

Engblom, J. and Engstrbm, S. Azone and the formation of reversed mono-and bicontinuous lipid-water phases. International Journal of Pharmaceutics 95 173-179, 1993. 

Figure 2 Illustrative lyotropic lipid-water phase diagram for phospholipid amphiphiles with transitions between the phase ranges driven by the water content. Hatched areas indicate two-phase regions (reproduced from (52) with permission from Elsevier).

An illustration of a lipid-water phase diagram, in which the transitions are driven by water content, is shown in Fig. 2 (43). A similar phase sequence can be produced by changes in temperature as well, and phospholipid phase diagrams generally exhibit pronounced temperature dependence. A generalized phase sequence of thermotropic phase transitions for the typical membrane lipids can be defined (44) ... 

Larsson K. Aqueous dispersions of cubic lipid-water phases. Curr. 60. Opin. Colloid Interface Sci. 2000 5 64—69. 

We begin with the field of lipid-water phase behaviour. 

Numerous reviews exist that deal with characterisation of different liquid crystalline phases formed in pure lipid-water mixtures [1]. Our concern is rather with new features of lipid-water phases as revealed by thinking in terms of curvature. 

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.

Cubic lipid phases have a very much more complex architecture than lamellar and hexagonal phases. Their structural characteristics have been elucidated only very recently, and it has become clear that their subtleties are the key to a variety of biological problems. We will consider those subtleties in some detail. The three fundamental cubic minimal surfaces - the P-surface, the D-surface and the gyroid (or G-surface), introduced in Chapter 1, can all be foimd in cubic lipid-water phases. The lipid bilayer is centred on the surface with the polar heads pointing outwards. Water fills the labyrinth systems on each side of the surface. These cubic phases will be termed Cp, CD and CG/ respectively. It is likely that there are other more complex IPMS morphologies in cubic phases of lipid-water mixtures, as yet uncharacterised. 

The minimal surface description naturally reveals the infinite lipid bilayer nature of cubic phases, viz. the fact that a single bilayer with no selfintersections can separate two continuous water regions. If we consider models of these cubic lipid-water phases, the structures of the Cp (or Cd) phases look like water globules separated by bilayers and fused in four (or three) lateral directions, respectively. Such a structure is not consistent with the earlier rod description. 

It is impossible to determine the electron density distribution within the unit cell wid a resolution at the atomic scale. Hence there is no straightforward way of determining the exact structure. In summary the evidence for the zero average curvature model structure of cubic lipid-water phases then rests on ... 

Cubic phases are also unique in their ability to accommodate proteins as compared to other lipid-water phases. A wide range of globular proteins with molecular weights 5,000-150,000 are known to form cubic phases when mixed with lipids and water. So far few single ternary lipid-protein-water phase diagrams have been completely determined [7], [13] one system that has been looked at is that of monoolein-water-lysoz5one. Protein incorporation results in increased water swelling, and all three phases, Cp, Cd and CG/ occur. The protein molecules are located in the water channel systems and retain their native structure. This has been proved by thermal analysis of the phase, and measurements of enzymatic activity [7]. 

When a bicontinuous cubic lipid-water phase is mechanically fragmented in the presence of a liposomal dispersion or of certain micellar solutions e.g. bile salt solution), a dispersion can be formed with high kinetic stability. In the polarising microscope it is sometimes possible to see an outer birefringent layer with radial symmetry (showing an extinction cross like that exhibited by a liposome). However, the core of these structures is isotropic. Such dispersions are formed in ternary systems, in a region where the cubic phase coexists in equilibrium with water and the L(x phase. The dispersion is due to a localisation of the La phase outside cubic particles. The structure has been confirmed by electron microscopy by Landh and Buchheim [15], and is shown in Fig. 5.4. It is natural to term these novel structures "cubosomes". They are an example of supra self-assembly. 

The proposed model [29,30] is based on the possibility of forming sections of a cubic lipid-water phase which gives a two-dimensional minimal surface with "holes" facing alternate sides of the bilayer. If these holes through the bilayer are plugged with protein molecules, we can form a bilayer that is closely related to the "planar" bilayer conformation (cf. Fig. 5.6). As this new conformation is related to the cubic phase, we will call it whereas the "normal" conformation, which corresponds to the La phase, is called L 2. ... 

There are interesting recent reports linking membrane fusion to cubic lipid phases [9] or to reversed (or "inverted") phases [62]. So-called "inter-lamellar attachments", formed between the bilayers of liposomes on fusion, show freeze-fracture electron micrograph textures identical to those of a cubic lipid-water phase (see Fig. 5.9). The inter-lamellar attachments seem to be identical to "lipidic particles" described earlier [63]. It is also interesting to note that diacylglycerols, secondary messengers from the Pl-cycle, produce fusion in... 

Figure 1. Cross section through a number of lipid-water phases. Frozen (gel) chain phases (b, c), lamellar phases (a, b, c), and hexagonal phases (d, e) are shown.

Relaxation Times, Paramagnetic Effects, and N.Q.R. Studies.—A study of the relaxation times of phosphoryl compounds at two magnetic fields, and of the dependence of spin rotation and dipolar interactions upon viscosity and temperature, led to the approximate separation of dipole-dipole, anisotropy, and spin-rotation interactions, and indicated that second-order paramagnetic shielding was dominant. The P relaxation times 7i and Tz were determined for several lipid-water phases. Comparisons of changes of Tg which occur at the transition temperature for dipalmitoyl-lecithin indicated that the relaxation times reflect the mobility of the lipid head-group. ... 

Engstrom, S., Drug delivery from cubic and other lipid-water phases. Lipid Technol, 2, 42-45 (1990). 

Two other structures, which together with the lamellar phase are the most important liquid-crystalline phases in lipid-water systems, are shown in Fig. 8.9. It should be pointed out that the classification of lipids into polar and non-polar is best defined from their interaction with water. Lipids which do not give lipid-water phases are thus non-polar whereas those forming aqueous phases are classified as polar lipids. 

Several cubic lipid-water phases have been observed. Usually they are formed in the temperature and existence range between Hu and L , L and Hi, or between Hi and the micellar solution. Their structure has been the subject of numerous studies and the last word has certainly not yet been said. 

This rod-system structure was considered to be the general structure for cubic lipid-water phases (Luzzati et al., 1968). Thus anhydrous strontium myristate, lecithin and galactolipids from maize chloroplasts with low water content were thought to have the structure shown in Fig. 8.10 with the polar groups arranged in two network systems of rods. The inverse structure, i.e. with hydrocarbon chains forming the rod systems and water forming the outer medium, was proposed to exist in aqueous systems of potassium soaps of lauric, myristic and palmitic acid and in aqueous systems of lauroyl- and palmitoyl-trimethylammonium bromide. 

The concept of IPMS should be considered in the future in connection with structural discussions on cubic lipid-water phases. It may be expected that all these three fundamental cubic IPMS structures really occur through the need to account for the wide variations in the shape of lipid molecules. Although the structures with the methyl end group gap located in the IPMS appears to be the most probable type, the reversed alternatives, i.e. bilayers with the gap between the polar head-groups forming the IPMS, should also be considered. 

There are two types of lipid-water phase diagrams. The first type, discussed above, is obtained from polar lipids, which are insoluble in water (i.e. the solubility is quite small, monolaurin for example has a solubility of about 10 m). Fig. 8.12 illustrates the principles of phase equilibria in this type of lipid-water system. The second type of binary system is obtained when the lipid is soluble as micelles in water. Examples of such lipids are fatty acid salts and lysolecithin. An aqueous soap system is illustrated in Fig. 8.13. When the lipid concentration in the micellar solution is increased, the spherical micelles are transformed into rod-shaped micelles. At still higher lipid concentrations the lipid cylinders are hexagonally arranged and the liquid-crystalline phase Hi is formed. The lamellar liquid-crystalline phase is usually formed in the region between Hi and the anhydrous lipid. Excellent reviews of the association behaviour of amphiphiles of this type have been published (Wennerstrom and Lindman, 1979 Lindman and Wennerstrom, 1980). 

The gel phase consists of crystalline lipid bilayers alternating with water layers. When the L -phase is cooled through the hydrocarbon chain crystallization temperature, a gel phase can be formed which is usually metastable. There are even amphiphile-water systems which exhibit thermodynamically stable gel phases as the only type of lipid-water phase. One example is the tetradecylamine-water system (Larsson and Al-Mamun, 1973) shown in Fig. 8.17. Other lipids which only give gel phases in their aqueous systems are cholesterol sulphate and cholesterol phosphate (Abrahamsson et al., 1977). The gel phase of tetradecylamine consists of bilayers with vertical chains in the orthorhombic chain packing. At low water content this structure swells to a water layer thickness of 14 A. At very high water concentrations, however, another gel phase with the same lipid bilayers but with a water layer several hundred A thick is formed. The reason for this seems... 

These close relationships existing between different monolayer phases and three-dimensional polymorphic forms occurring in non-polar lipids are considered here as conclusive evidence for the occurrence of the same structures. In the case of polar lipids, however, the interaction with water means that a relationship between a monolayer phase and a bulk phase can only be expected when the monolayer phases are compared with lipid-water phases, such as L - and gel phases. 

The dielectric properties of lipid-water phases have recently been analysed by Fontell and Lundstr6m/(1977). The measured dielectric constant of the lamellar liquid-crystalline phase is much lower than what should be expected for a lipid-water mixture. The difference was explained by local anisotropy. 

Phase diagrams of monoglyceride-water systems have been reported by Lutton (1966) (C12-C22), Larsson (1967) (C6-C10) and Krog and Larsson (1968) (industrially distilled monoglycerides). The main features of the different monoglyceride-water systems were given in Section 8.2 as an illustration of lipid-water phase equilibria (Fig. 8.12). Only additional comments to this earlier work will be given here. 

Liquid-crystalline lipid-water phases can under certain conditions incorporate proteins. On the basis of studies of liquid-crystalline phases in various lipid-protein-water systems (Gulik-Krzywicki etal.y 1969 Rand and SenGupta, 1972), it was concluded that a prerequisite for the formation of composite liquid-crystalline phases is that the lipid is charged (Rand, 1976). Recently, however, a system with a neutral lipid was examined which appeared to form a lipid-... 

Larsson, K. (1989). Cubie lipid-water phases structures and biomembrane aspects. The Journal of Physical Chemistry, 93(21), 7304-14. 

Keywords lipid-water phases liquid crystals swelling sponge phase small-angle X-ray scattering SANS micelles glycerol monooleate octyl glucoside... 

Larssrai K.J., Colloidal dispersions of ordered lipid-water phases, Journal of Dispersion Scitaice and Technology, 20, 27-34 (1999)... 

---