Micellar aggregates of sodium glycocholate and sodium taurocholate and their interaction complexes with bilirubin-IXα. Structural models and crystal structure

Abstract

Sodium glycocholate (NaGC) and taurocholate (NaTC) have been studied by means of X-ray and circular dichroism (CD) measurements, using bilirubin-IXα(BR) as probe molecule, together with potential-energy calculations. Helical models for the micellar aggregates of NaGC and NaTC were inferred from crystal structures solved by X-ray analysis. Since it is known that chiral molecules, micellar aggregates and macromolecules select preferentially or exclusively one of the two enantiomeric conformers of BR, CD spectra of BR in submicellar and micellar aqueous solutions of NaGC and NaTC were recorded as a function of pH and BR concentration in order to verify these helical models and the enantioselective ability of the bile salt monomers and micellar aggregates. Potential-energy calculations supported the CD experimental results and provided reasonable bile salt–BR interaction models. The behaviour of NaGC and NaTC is compared with that of sodium deoxycholate (NaDC), previously studied. The CD spectra of the bile salt–BR systems seem to allow characterisation of the typical structure of the bile salt micellar aggregates.