Bile salt-hydrocarbon micelles

The structure of cholic acid helps us understand how bile salts such as sodium tauro cholate promote the transport of lipids through a water rich environment The bot tom face of the molecule bears all of the polar groups and the top face is exclusively hydrocarbon like Bile salts emulsify fats by forming micelles m which the fats are on the inside and the bile salts are on the outside The hydrophobic face of the bile salt associates with the fat that is inside the micelle the hydrophilic face is m contact with water on the outside... 

A nonpolar solubilizate such as hexane penetrates deeply into such a micelle, and is held in the nonpolar interior hydrocarbon environment, while a solubilizate such as an alcohol, which has both polar and nonpolar ends, usually penetrates less, with its polar end at or near the polar surface of the micelle. The vapor pressure of hexane in aqueous solution is diminished by the presence of sodium oleate m a manner analogous to that cited above for systems in nonpolar solvents. A 5% aqueous solution of potassium oleate dissolves more than twice the volume of propylene at a given pressure than does pure water. Dnnethylaminoazobenzene, a water-insoluble dye, is solubilized to the extent of 125 mg per liter by a 0.05 M aqueous solution of potassium myristate. Bile salts solubilize fatty acids, and this fact is considered important physiologically. Cetyl pyridinium chloride, a cationic salt, is also a solubilizing agent, and 100 ml of its A/10 solution solubilizes about 1 g of methyl ethyl-butyl either m aqueous solution. 

Bile salts carry extensive hydrophobic (hydrocarbon) portions in each molecule that attempt to reduce their contact with water (4). This is reflected in rapid, dynamic association-dissociation equilibrium to form self-aggregates or micelles as the total concentration of bile salt solute is increased (the CMC) [2-6]. Experimentally, micelles are undetectable in dilute solutions of the monomers, and are detected in increasing numbers and often size above the CMC [98]. Because bile salt micelles are often small (i.e., dimers) [5], and since self-aggregation continues to proceed in many cases with increasing concentration above the CMC [17,18,20,52,98], the detection of the lowest concentration at which the first aggregates form depends particularly upon the sensitivity of the experimental probes employed [98] and the physical-chemical conditions [3-5]. 

In an earlier review [3], mixed micelles formed by bile salts were classified into those with (i) non-polar lipids (e.g., linear or cyclic hydrocarbons) (ii) insoluble amphiphiles (e.g., cholesterol, protonated fatty acids, etc.) (iii) insoluble swelling amphiphiles (e.g., phospholipids, monoglycerides, acid soaps ) and (iv) soluble amphiphiles (e.g., mixtures of bile salts with themselves, with soaps and with detergents) and the literature up to that date (1970) was critically summarized. Much recent work has appeared in all of these areas, but the most significant is the dramatic advances that have taken place in our understanding of the structure, size, shape, equilibria, and thermodynamics of bile salt-lecithin [16,18,28,29,99-102,127, 144,218,223,231-238] and bile salt-lecithin-cholesterol [238,239] micelles which are of crucial importance to the solubihty of cholesterol in bile [1]. This section briefly surveys recent results on the above subclasses. Information on solubilization, solubilization capacities or phase equilibria of binary, ternary or quaternary systems or structures of liquid crystalline phases can be found in several excellent reviews [5,85,207,208,210,211,213,216,217] and, where relevant, have been referred to earlier. 

Although bile salts possess a polar head group, the hydrocarbon tail usually contains polar hydroxyl groups. Therefore the rigid ring system gives a tightly packed, almost solid, nonpolar phase rather than a liquid one. However, like cholesterol, bile salts can form mixed micelles with phospholipids. 

Three types of interaction mechanisms are known between the micelle and the analyte as shown in Figure 3.3 (1) incorporation of the analyte into the hydrophobic core, (2) adsorption of the analyte on the surface or on the palisade layer, and (3) incorporation of the analyte as a cosurfactant. Highly hydrophobic and nonpolar analytes such as aromatic hydrocarbons will be incorporated into the core of the micelle. The selectivity may not be very different among long alkyl-chain surfactants for this class of analyte but the distribution coefficient will be increased with longer alkyl-chain surfactants. Thus, selectivity will not be altered significantly for nonpolar hydrophobic analytes, even when different surfactants are used. However, bile salts may provide substantially different selectivity in comparison with long-alkyl chain surfactants, even for nonpolar hydrophobic analytes. 

Fig. 47. Comparison of proposed structure of primary and secondary bile salt micelles and a classical detergent micelle (43). (A, C) Longitudinal and cross sections of primary and secondary bile salt micelles, respectively. (B) Ordinary detergent micelles. Top— Orientation of molecules at oil-water interface (refer to Section V). Wavy line—Hydrocarbon chains of detergent molecules. Shaded area—Lipidsoluble cyclic hydrocarbon part of bile salt molecule. Polar head of detergent molecule. OH or ester groups. Negatively charged ionic group of bile salt.

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