Bile Salt Structure and Phase Equilibria in Aqueous Bile Salt and Bile Salt-Lecithin Systems

The hydrophilic-hydrophobic balance of bile salt monomers can be readily quantified by their elution sequence during reverse-phase high-performance liquid chromatography. Such studies have demonstrated that subtle variations in bile salt structure have profound effects on the hydrophilic-hydrophobic balance of this important family of detergent-like molecules. The common trihydroxy bile salt, cholate, is more hydrophilic than dihydroxy bile salts with alpha-oriented OH groups. In contrast, dihydroxy bile salts with one equatorial OH function are more hydrophilic than cholate. Hydrophilic bile salts have, in general, higher critical micellar concentrations than do hydrophobic bile salts and their primary micelles polymerize less readily to form secondary micelles either with increasing bile salt concentrations or with increases in ionic strength. Hydrophilic bile salts also disperse lecithin into mixed micelles at a slower rate than do hydrophobic bile salts. The structure of mixed bile salt-lecithin micelles is more complex than previously believed and varies with bile salt-to-lecithin ratio. These micelles are disc-like in which bile salts saturate the lecithin bilayer “core” presumably as reverse micelles, as well as coating the perimeter as a bilayered “ribbon”. The ratio of bile salt to lecithin in the bilayer and the intermicellar monomeric bile salt concentration (critical micellar concentration) determines the macroscopic phase limit. With the common bile salt species, the lecithin-to-bile salt phase limit does not correlate closely with the hydrophilic-hydrophobic balance of the bile salt monomers. However, with very hydrophilic bile salts such as the pansulfated species in physiological ionic strength, the equilibria between the large intermicellar bile salt concentrations and the bile salt molecules interacting with lecithin in mixed micelles favors the former state. As a consequence, the lecithin-to-bile salt molar ratio at the phase limit is greatly reduced. However, when the charges of the sulfate groups are electrostatically shielded (e.g., in 4 MNaCl), the lecithin-to-bile salt molar ratio at the phase limit approaches and even exceeds that of the nonsulfated bile salt species.