CALCIUM-ION BINDING TO BILE-SALTS

The nature of the interactions between calcium ions and bile salt anio ns and the effect of the competition between calcium and sodium ions f or possible formation of micellar aggregates are investigated. The cry stal structures of calcium deoxycholate (CaDC) and glycodeaxycholate ( CaGDC) and quasi-elastic lightscattering measurements, carried out on sodium deoxycholate (NaDC) and glycodeoxycholate (NaGDC) aqueous solut ions 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 bot h a 1:1 (CaDC) and a 1:2 (CaGDC) stoichiometry. An assembly of wedge-s haped bilayers, cemented with CaDC anions, or an assembly of irregular trimeric units, each one containing three anions, can be recognized i n the crystal packing of CaDC. Ca2+ and Cl- ions and water molecules a re located in liquid-like regions. The crystal packing of CaGDC is cha racterized by 2(1) helices and units with a 2-fold rotation axis. The quasi-elastic light-scattering measurements show that NaDC and NaGDC a queous solutions with low concentrations of Ca2+ ions have micellar ag gregates with apparent hydrodynamic radii remarkably greater than thos e of the solutions containing only Na+ ions. The Ca2+ ions seem to hav e 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 pro posed that the Ca2+ ions act as aggregation centers of the sodium mice llar aggregates.