The influence of bile salt structure on self-association in aqueous solutions

The relationship between chemical structure and the concentration at which self-association occurs in water or in 0.15 M Na+ ion was examined for more than 50 bile salts and bile salt analogues varying in substituents on the steroid nucleus or in the structure of the side chain. Nuclear substituents varied in type (alpha- or beta-hydroxy, or oxo group) and number (1, 2, or 3); side chain structure varied in the nature of the ionic group (unconjugated, glycine- or taurine-conjugated, or zwitterion) or length of the side chain (5-, 4-, or 3-carbon atoms). The midpoint of the concentration range over which aggregation occurred was called the critical micellar concentration (CMC), even though bile salt aggregation is known to be more gradual than that of most typical ionic detergents. CMC values were obtained by surface tension measurements using an improved maximum bubble-pressure method, as well as by dye solubilization. Results obtained by the two methods agreed well. The CMC values varied from about 1 to greater than 250 mM. For a given bile salt, the addition of a hydroxy or oxo group increased the CMC; and for a given number of substituents, the changing of a hydroxy group to an oxo group increased the CMC values as well. The orientation of hydroxy substituents also influenced the CMC values: the changing of a hydroxy substituent from an alpha- to a beta-configuration increased the CMC values, as bile salts possessing alpha- and beta-hydroxy substituents had higher CMC values than bile salts with only alpha-hydroxy substituents. Inspection of space-filling models suggested the hypothesis that the greater contiguous hydrophobic area of the molecule, the lower the CMC value. The CMC value also increased exponentially as the side chain was shortened from C5 to C4 to C3. Conjugation of the side chain carboxylic group with glycine or taurine, although increasing the length of the side chain, caused little change in the CMC values. The addition of Na+ ion to a total concentration of 0.15 M lowered the CMC in a predictable manner for all anionic bile salts. The results indicate that the concentration at which bile salt aggregation occurs varies widely and is determined not only by the number, type, and orientation of nuclear substituents, but also by side chain structure.