Phase equilibria lipid mixtures

Let us assume some small number n of lipid molecules can form a relatively stable solid phase cluster when the temperature and composition of the lipid mixture is such that, according to the phase diagram, solid phase can exist in equilibrium with the fluid phase. (For example, we later assume that n 10.) Let us further assume that (1) the temperature and composition of the lipid mixture is such that X is small, X 1, and (2) all the solid phase present is in the form of clusters of n molecules each. If the clusters are randomly distributed in the plane of the membrane, then each cluster will be surrounded by a number of fluid molecules of the order of magnitude of N n/X. The area occupied by the surrounding fluid phase molecules is then NA0 where, A0 60A2. Let us now calculate lower limit on X, Xmin, such that each molecule in... 

An experimental complication is the difficulty in effecting molecular interaction between the components. The usual technique for preparing lipid-protein phases in an aqueous environment is to use components of opposite charge. This in turn means that the lipid should be added to the protein in order to obtain a homogeneous complex since a complex separates when a certain critical hydrophobicity is reached. If the precipitate is prepared in the opposite way, the composition of the complex can vary since initially the protein molecule can take up as many lipid molecules as its net charge, and this number can decrease successively with reduction in available lipid molecules. It is thus not possible to prepare lipid— protein—water mixtures, as in the case of other ternary systems, and to wait for equilibrium. Systems were prepared that consisted of lecithin-cardiolipin (L/CL) mixtures with (a) a hydrophobic protein, insulin, and with (b) a protein with high water solubility, bovine serum albumin (BSA). 

To truly control crystallization to give the desired crystalline microstructure requires an advanced knowledge of both the equilibrium phase behavior and the kinetics of nucleation and growth. The phase behavior of the particular mixture of TAG in a lipid system controls both the driving force for crystallization and the ultimate phase volume (solid fat content) of the solidified fat. The crystallization kinetics determines the number, size, polymorph, and shape of crystals that are formed as well as the network interactions among the various crystalline elements. There are numerous factors that influence both the phase behavior and the crystallization kinetics, and the effects of these parameters must be understood to control lipid crystallization. 

Besides the asymmetry between monolayers in cytomembranes, two of the more obvious differences between cubic phases and membranes are the unit cell size and the water activity. It has been argued that tire latter must control the topology of the cubic membranes [15], and hence tiiat the cubic membrane structures must be of the reversed type (in the accepted nomenclature of equilibrium phase behaviour discussed in Chapters 4 and 5 type II) rather than normal (type I). All known lipid-water and lipid-protein-water systems that exhibit phases in equilibrium with excess water are of the reversed type. Thus, water activity alone cannot determine the topology of cubic membranes. Cubic phases have recently been observed with very high water activity (75-90 wt.%), in mixtures of lipids [127], in lipid-protein systems [56], in lipid-poloxamer systems [128], and in lipid A and similar lipopolysaccharides [129,130]. 

Fig. 8 Left The phase behavior of amphiphiles as observed with the model of [114,115], is shown in the main panel, plotted as a function of rescaled temperature kgT/e and attraction width w,. ja at zero lateral tension. Each symbol corresponds to one simulation and identifies different bilayer phases. Crosses denote the gel phase, solid circles mark fluid bilayers, and vertical crosses indicate the region where bilayers are unstable. The dashed lines are merely guides to the eye. The inset shows the pair potential between tail beads (solid line) and the purely repulsive head-head and head-tail interaction (dashed line). Reprinted with permission from Ref. 114. Copyright (2005) by the American Physical Society. Right Phase separation and budding sequence for a vesicle containing a 50 50 mixture of two lipids. The vesicle is in equilibrium with a very dilute vapor of amphiphiles (i.e., the lipids seen floating in the exterior volume). From [114]...

The positive heats of mixing for lecithin-cholesterol mixtures indicate that interactions between unlike molecules are smaller than the interactions between like molecules, i.e., the hydrocarbon chain interactions with cholesterol are smaller than in each of the pure phases. If the excess heats of mixing become large enough, phase separation will occur. It may occur when the surface pressure is increased (i.e., as the films are compressed). The point at which phase separation occurs is difficult to predict, measure, or detect however, evidence of phase separation can be deduced from the following experiment. If excess amounts of two lipids are placed in water, the equilibrium surface pressure should reflect whether the surface film is a mixture. According to the phase rule (11,12, 13,14), if two bulk lipid phases are present, only one surface phase can be present at the air—water surface. Thus the composition of the equi-... 

The classic experiments were those performed by Ernest Overton and Hans Meyer at the turn of the twentieth century, where tadpoles were placed in solutions containing alcohols of increasing hydrophobicity. They found a correlation between the concentration of the alcohol required to cause cessation of movement and the concentration of the alcohol distributed into the lipid phase of a lipid-water mixture. The ratio of the concentration in the lipid phase to the concentration in the aqueous phase at equilibrium is known as the Overton-Meyer or lipid-water partition coefficient. The higher the partition coefficient, the less alcohol was needed to cause cessation of movement. 

This is an important method of preparing mono- and di-acylglycerols. The composition of the equilibrium mixture of all four components depends on the relative amount of triacylglycerol and of glycerol dissolved in the lipid phase. 

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