Binary lipid phase diagrams

In binary mixtures of water, surfactants, or lipids the most common structure is the gyroid one, G, existing usually on the phase diagram between the hexagonal and lamellar mesophases. This structure has been observed in a very large number of surfactant systems [13-16,24—27] and in the computer simulations of surfactant systems [28], The G phase is found at rather high surfactant concentrations, usually much above 50% by weight. 

Figure 2 shows a spin-label-derived phase diagram for binary mixtures of (II) and (IV), dipalmitoylphosphatidylcholine and dielaidoylphosphatidylcholine. It will be seen that the diagram describes miscibility of these two lipids in both the solid and solution phases. (Other binary mixtures of lipids show immiscibility in the solid as well as the fluid phases.45,54)... 

Although studies of the thermotropic phase behavior of singlecomponent multilamellar phospholipid vesicles are necessary and valuable, these systems are not realistic models for biological membranes that normally contain at least several different types of phospholipids and a variety of fatty acyl chains. As a first step toward understanding the interactions of both the polar and apolar portions of different lipids in mixtures, DSC studies of various binary and ternary phospholipid systems have been carried out. Phase diagrams can be constructed by specifying the onset and completion temperatures for the phase transition of a series of mixtures and by an inspection of the shapes of the calorimetric traces. A comparison of the observed transition curves with the theoretical curves supports... 

The properties of lipid bilayers formed from mixtures of lipids are very relevant to the understanding of the lipid bilayers that form the basis of biological membranes. Detailed studies have been performed on bilayers formed from binary lipid mixtures, and some reports in the recent literature describe phase diagrams of lipid bilayers prepared from ternary mixtures that include cholesterol. Figure 4 (24-31) shows some phase diagrams of lipid bilayers formed from binary and ternary mixtures of lipids. The general observation is that lipids in a bilayer are not very... 

Figure 4 Some phase diagrams for lipid bilayers in excess water prepared from binary and ternary lipid mixtures, a) Multibilayer lipid vesicles prepared from binary mixtures of DMPC and DPPC (24) b) Multibilayer lipid vesicles prepared from binary mixtures of DMPC and DSPC [adapted by Reference (25) from data for perdeuterated lipids published by Knoll et al. (26)] c) Multibilayer lipid vesicles prepared from binary mixtures of diCi/.QPC and C22 oCi2 oPC (27) d) Multibilayer lipid vesicles prepared from binary mixtures of DMPC and cholesterol (28) e) Multibilayer lipid vesicles prepared from ternary mixtures of palmitoyl sphingomyelin, POPC, and cholesterol [adapted by Reference (29), from data published by De Almeida et al. (30)] Lipid bilayers prepared from ternary mixtures of DSPC, DOPC, and cholesterol (31).

Fig. 15. T -phase diagram of the equimolar binary lipid mixtures DMPC/DPPC and DMPC/DSPC in excess water as a function of pressure.

To describe the behavior of a lipid in water, it is customary to use a phase diagram in which the physical state or molecular arrangement of the lipid or the system is shown in relation to the composition of the mixture (Fig. 1). Composition is usually expressed in percentage by weight (abscissa), and the ordinate is temperature. In two-component systems, called binary... 

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). 

Monolinolien (MLO) is among the surfactant-like lipids with propensity to form inverted type non-lamellar phases. The binary MLO-water phase diagram is shown in Figure 1 [83]. 

Figure 1. Left Phase diagram of the binary MLO-water system. Right Phase sequence in a binary or ternary lipid system that is displayed upon increasing temperature and/or solubilizing oil. The phases are the following (A) a fluid lamellar (L(t) phase, (B) three bicontinuous cubic (V2) phases, (C) a H2 phase, (D) a diseontinuous cubic Fd3m phase, (E) and an inverted-type water-in-oil (W/O) microemulsion system (the L2 phase) (the figures have been taken with permission from reference [83]).

Systematic studies of the phase behavior of lipid-surfactant systems have been performed particularly on mixtures of surfactants of the Ci2EOn type and phosphatidylcholines with different chain lengths and saturation [109-113]. The phase diagrams of these pseudo-binary mixtures in excess water are complicated and become even more so when the water content is varied. 

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