Size and Structure of Bile Salt Micelles

The aggregation numbers (Ag#) of free and conjugated di- and trihydroxy bile salts were studied by equilibrium ultracentrifugation and light scattering. The effect of concentration, counterion concentration, pH, temperature, and urea are shown. All trihydroxy bile salts form very small micelles (Ag#<10) which are resistant to change in counterion concentration, temperature, and urea. Dihydroxy bile salts form small micelles at low concentrations and large micelles at higher concentrations (Ag# 12 to 100). Temperature and urea markedly decrease their aggregation number. pH has little effect on the aggregation number of the trihydroxy bile salts, but enormous aggregates form in the deoxycholate series as pH decreases towards the pKa. The small micelles are probably bonded hydrophobically. Larger micelles are formed by the aggregation of small micelles through hydrogen bonding of hydroxyl groups.