Lecithin systems

Figure 5. Mixed monolayers of cholesterol-1,2-dimyristoyl-3-lecithin system at 23°C., pH 6, and 5 dynes per cm.

Type II is represented by line LOM in Figure 12, with a sharp change in slope at point O. Either this behavior or Type III behavior can be found when the pure expanded component ejdiibits a fully expanded tt-A curve. The cholesterol-dimyristoyl lecithin system, illustrated in Figure 5, is an example of Type II behavior. 

Naylor et al. (1993) studied the ability of lecithin to modify the rate and mechanism of dissolution of hydrocortisone in the presence of sodium taurocholate (NaTC) solutions. They found that in the presence of lecithin, the CMC of NaTC dropped owing to the more effective solubilization capacity of the mixed micelle. Furthermore, the CMC value dropped more on saturation with hydrocortisone, implying some interaction between hydrocortisone and the NaTC/lecithin micelles. These results indicated that in the NaTC-only system, wetting effects predominated dissolution, whereas in the NaTC/lecithin system, the dissolution rate of hydrocortisone was enhanced mainly through solubilization. 

Trauble H, Sackmann E. Studies of crystalline-liquid crystalline phase-transihon of lipid model membranes.3. Structure of a steroid-lecithin system below and above lipid-phase transition. J. Am. Chem. Soc. 1972 94 4499-4510. 

Carey MC. Bile-Salt Structure and Phase-Equilibria in Aqueous Bile-Salt and Bile Salt-Lecithin Systems. Hepatology 1984 4 S138-S150. 

When looking at Region b of the lecithin system, it was discovered that it is most quahfied, from the five characterization tests, to be a bicontinuous phase. This implication was confirmed by wavelike patterns observed via cryoTEM. When analyzed from a broader perspective, it was concluded that this bicontinuous phase could be the transition phase, which coexists between the oil-in-water and other phases. 

Ekwall and Baltcheffsky [265] have discussed the formation of cholesterol mesomorphous phases in the presence of protein-surfactant complexes. In some cases when cholesterol is added to these solutions a mesomorphous phase forms, e.g. in serum albumin-sodium dodecyl sulphate systems, but this does not occur in serum albumin-sodium taurocholate solutions [266]. Cholesterol solubility in bile salt solutions is increased by the addition of lecithin [236]. The bile salt micelle is said to be swollen by the lecithin until the micellar structure breaks down and lamellar aggregates form in solution the solution is anisotropic. Bile salt-cholesterol-lecithin systems have been studied in detail by Small and coworkers [267-269]. The system sodium cholate-lecithin-water studied by these workers gives three paracrystalline phases I, II, and III shown in Fig. 4.37. Phase I is equivalent to a neat-soap phase, phase II is isotropic and is probably made up of dodecahedrally shaped lecithin micelles and bile salts. Phase III is of middle soap form. The isotropic micellar solution is represented by phase IV. The addition of cholesterol in increasing quantities reduces the extent of the isotropic... 

The ternary diagrams in Fig. 4.37 indicate the complexity of the bile salt-lecithin systems especially in the presence of cholesterol which limits the extent of the isotropic micellar phase (IV) in particular. Intense interest in these systems has been generated in the search for the cause of cholelithiasis. Of great importance in understanding gallstone formation is how cholesterol, which is a major... 

Similar results were obtained in reconstitution experiments with Upophilin and proteolipid apoprotein-lecithin systems, sarcoplasmic reticulum Ca +, Mg +-ATPase, rhodopsin, and glycophorin. In all these cases deuterium NMR revealed only one lipid population while the epr spectra (as far as available) showed two components. The results further show that proteins either disorder orhave little effect on hydrocarbon chain order in membranes above the gel-to-liquid crystal phase transition temperature, Tc, of the pure lipids. 

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