Possible Models for the Micellar Aggregates of Glycocholate and Taurocholate Salts from Crystal Structures, QELS, and CD Measurements

Abstract

The structures of crystals of rubidium glycocholate (monoclinic) and rubidium taurocholate (monoclinic and tetragonal) were solved by X-ray diffraction analysis. These structures, together with those of the sodium salts formerly determined, were used to infer models for the corresponding micellar aggregates in aqueous solutions. The crystal packings of the sodium and rubidium salts were characterized by similar structural units. These were bilayers, 21 helices, distorted 21 helices, and units with a two-fold rotation axis. On the other hand, circular dichroism spectra of the sodium and rubidium salts with bilirubin-IXα in aqueous solution indicated the formation of very similar interaction complexes and micellar aggregates. This hypothesis was supported by quasi-elastic light scattering measurements carried out on aqueous micellar solutions as a function of the ionic strength, concentration, and temperature. Low values of the apparent hydrodynamic radius, corresponding to oligomers of small size, were obtained for all the salts. The micellar growth was practically independent of the concentration and temperature, within the range 20−60 °C, at low ionic strength, and slightly increased upon increasing the ionic strength. We studied the rubidium salts in place of the sodium ones by extended X-ray absorption fine structure spectroscopy when their structures are similar. The measurements at the Rb+ K-edge, previously accomplished on the crystal and the aqueous micellar solution of rubidium glycocholate, were in agreement with the model of the 21 helix and allowed us to discard the bilayer. Circular dichroism spectra, recorded as a function of the ionic strength, pointed out that bilirubin-IXα gives rise to a different enantioselective complexation with dihydroxy or trihydroxy salts. The different structure of the corresponding micellar aggregates was also supported by quasi-elastic light scattering measurements accomplished on dihydroxy and trihydroxy salts. The calculation of the hydrodynamic radius for the 21 helix, observed in the monoclinic crystal of sodium taurocholate, showed that there is a possible satisfactory agreement with the quasi-elastic light scattering data.