Calcium Ion Binding to Bile Salts

The nature of the interactions between calcium ions and bile salt anions and the effect of the competition between calcium and sodium ions for possible formation of micellar aggregates are investigated. The crystal structures of calcium deoxycholate (CaDC) and glycodeaxycholate (CaGDC) and quasi-elastic lightscattering measurements, carried out on sodium deoxycholate (NaDC) and glycodeoxycholate (NaGDC) aqueous solutions containing CaCl2, are discussed. The calcium ions are coordinated to oxygen atoms of carboxylate groups and water molecules by means of ion-ion and ion-dipole interactions. The hydroxyl groups of the bile salt anions are not involved. The Ca2+-bile salt complexes fulfill both a 1:1 (CaDC) and a 1:2 (CaGDC) stoichiometry. An assembly of wedge-shaped bilayers, cemented with CaDC anions, or an assembly of irregular trimeric units, each one containing three anions, can be recognized in the crystal packing of CaDC. Ca2+ and Cl- ions and water molecules are located in liquid-like regions. The crystal packing of CaGDC is characterized by 2(1) helices and units with a 2-fold rotation axis. The quasi-elastic light-scattering measurements show that NaDC and NaGDC aqueous solutions with low concentrations of Ca2+ ions have micellar aggregates with apparent hydrodynamic radii remarkably greater than those of the solutions containing only Na+ ions. The Ca2+ ions seem to have a greater affinity for the deoxycholate and glycodeoxycholate anions than the Na+ ions, and the micellar aggregates with Na+ and Ca2+ ions seem to be more stable than those containing only Na+ ions. It is proposed that the Ca2+ ions act as aggregation centers of the sodium micellar aggregates.