Lipids water cubic phases

Portmann, M Landau, E. M., and Luisi, P. L. (1991). Spectroscopic and rheological studies of enzymes in rigid lipidic matrices the case of a-chymotrypsin in a lysolecithin/water cubic phase. J. Phys. Chem., 95, 8437 0. 

Figure 6. Cubic structures in lipid-water systems based on space-filling polyhedra. The data from the monoglyceride-water cubic phases fit with the body-centered structure to the right.

Among the various lipids that form cubic phases, monoglycerides have been studied most. Phosphatidylcholines exhibit cubic phases only at very low water content, whereas phosphatidylethanolamines form cubic phases in similar regions of the lipid-water system as monoglycerides e.g. GMO). Lysophospholipids (single-chain lipids) exhibit cubic phases of quite different kinds, liiey will not be discussed here as they are not considered to be relevant in the formation of self-assembled structures of biological... 

Figure 6.35 Structure of glyceryl monooleate-water cubic phase with inset showing the lipid bilayer.

Luzzati V, Tardieu A, Gulik-Krzywicki T, Rivas E and Reiss-Flusson F 1968 Structure of the cubic phases of lipid-water systems Nature 220 485-8... 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

Another phase which has attracted recent interest is the gyroid phase, a bicontinuous ordered phase with cubic symmetry (space group Ia3d, cf. Fig. 2 (d) [10]). It consists of two interwoven but unconnected bicontinuous networks. The amphiphile sheets have a mean curvature which is close to constant and intermediate between that of the usually neighboring lamellar and hexagonal phases. The gyroid phase was first identified in lipid/ water mixtures [11], and has been found in many related systems since then, among other, in copolymer blends [12]. 

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

In this work we will focus on the use of the cubic phase as a delivery system for oligopeptides - Desmopressin, Lysine Vasopressin, Somatostatin and the Renin inhibitor H214/03. The amino acid sequences of these peptides are given in Table I. The work focuses on the cubic phase as a subcutaneous or intramuscular depot for extended release of peptide drugs, and as a vehicle for peptide uptake in the Gl-tract. Several examples of how the peptide drugs interact with this lipid-water system will be given in terms of phase behaviour, peptide self-diffusion, in vitro and in vivo release kinetics, and the ability of the cubic phase to protect peptides from enzymatic degradation in vitro. Part of this work has been described elsewhere (4-6). 

The Desmopressin diffusion coefficient in the cubic phase at 40 C (D=0.24 x 10-10 m2s-l) is about a factor 9 smaller than in 2H20-solution at 25 C (D=2.25 x 10-10 m s" ), a difference which is larger than what is expected from pure obstruction effects a reduction factor of three is expected from the inclusion of a solute in the water channels of the cubic phase (13). Thus, the results indicate an interaction between the peptide and the lipid matrix and/or membrane surface, especially since the peptide and lipid diffusion coefficients are very similar in the cubic phase (Table... 

It should be pointed out that cubic phases, such as the one discussed in this work, frequently occur in lipid-water systems (77), and the concept of using cubic phases as drug vehicles is therefore not limited to the use of monoolein only. From a toxicological stand-point, it is tempting to try to use membrane lipids, such as phospholipids, instead of monoolein for parenteral depot preparations (18-20). 

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. 

The lipidic cubic phase has recently been demonstrated as a new system in which to crystallize membrane proteins [143, 144], and several examples [143, 145, 146] have been reported. The molecular mechanism for such crystallization is not yet clear, but the interfacial water and transport are believed to play an important role in nucleation and crystal growth [146, 147], Using a related model system of reverse micelles, drastic differences in water behavior were observed both experimentally [112, 127, 128, 133-135] and theoretically [117, 148, 149]. In contrast to the ultrafast motions of bulk water that occurs in less than several picoseconds, significantly slower water dynamics were observed in hundreds of picoseconds, which indicates a well-ordered water structure in these confinements. 

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

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. 

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