Albumin, lecithin complex

Figure 7.43 Association curves of chloramphenicol with the albumin-lecithin complex (3 % w/v) in phosphate buffer (pH = 6.8) at 25° C, in the presence of increasing amounts of sodium dodecyl sulphate (NaDS). Plots a (O)and b ( ) refer to different initial concentrations of the antibiotic, i.e.—3.1 x 10 and 6.2 x 10" m, respectively. Inset. Association curves of chloramphenicol with the albumin-lecithin complex (3% w/v) in phosphate buffer (pH 6.8) at 25° C, in the presence of increasing amounts of surfactants. Plots a and b refer to cetyltrimethylammonium bromide ( ) (CTAB) and polysorbate 80 (O) (concn in mgrnl" x 10 ), respectively. In all cases, the initial concentration of the antibiotic was 3.1 x 10 m. From Alhaique et al. [195] with permission.

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). 

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... 

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