Bile salts as atypical surfactants and solubilizers

Recent research has suggested that self-association of bile salts does not follow the micellar pattern of self-association exhibited by typical flexible chain surfactants and detergents. A working model for the self-association of bile salts is proposed. It includes a mild degree of cooperativity in the early stages of the growth of aggregates and coexistence of a number of aggregates of different aggregation numbers. The polydispersity implies an increase in the average aggregation number with increasing concentration of the bile salt. Bile salts can be purified by foam fractionation. Surface tension data for sodium cholate are in agreement with the above qualitative model of self-association. An isoextraction method is useful for estimating monomer activities. Results for sodium deoxycholate suggest little self-association in dilute solutions and a mildly cooperative self-association at higher concentrations. A comparative study of the interactions of the fluorescent probe, 2-p-toluidinyl-naphthalene-6-sulfonate, with sodium alkyl sulfates and sodium deoxycholate indicates that bile salts may differ significantly from classical micellar systems in their solubilization characteristics also. The evidence suggests strongly that a specific adduct formation with an optimum number of bile salt anions may be important in solubilization brought about by the rigidity and the complex shape of the bile salt anions.