Structural study of the micellar aggregates of sodium and rubidium glyco- and taurodeoxycholate

Some salts of bile acids increase the size of their micellar aggregates in aqueous solutions by varying parameters such as pH or ionic strength and give rise to transitions: aqueous micellar solution --> gel --> fiber --> crystal. Helical fibers of sodium and rubidium salts of glycodeoxycholic and taurodeoxycholic acids (NaGDC, RbGDC, NaTDC, and RbTDC, respectively) were drawn from aqueous solutions and investigated by means of X-ray diffraction analysis to clarify the structure of their micellar aggregates and to verify whether the micellar structures of the sodium and rubidium salts were similar. In this case, the rubidium salts can be studied in place of the sodium ones by extended X-ray absorption fine structure spectroscopy. The X-ray patterns of the NaGDC, RbGDC, NaTDC, and RbTDC fibers showed a close resemblance and were interpreted by means of very similar unit cell parameters and helical structures, formed by trimers arranged in 7/1 helices. Calculations of interatomic distances provided a model of the 7/1 helix which satisfactorily packs into the unit cells of the fibers. Quasi-elastic light-scattering measurements carried out on aqueous micellar solutions up to a concentration of NaCl or RbCl of about 0.3 M supported the similarity among the structures of the NaGDC, RbGDC, NaTDC, and RbTDC micellar aggregates. Oligomers of small size were observed without NaCl or RbCl. The extent of micellar growth increased upon increasing NaCl or RbCl concentration. The micellar size, which seemed to be dependent on the peculiar cation-anion Coulombic interaction starting from a NaCl or RbCl concentration of about 0.2 M, was in the following order: NaGDC > NaTDC approximate to RbTDC > RbGDC. Electromotive force measurements accomplished on NaTDC solutions at constant ionic medium provided the distribution of micellar sizes. Most of the aggregates have aggregation numbers that are a multiple of 3. The fraction of bound Na+ ions is high in accordance with an ordered structure like that of the 7/1 helix. Further support for the 7/1 helix came from the calculation of mean hydrodynamic radii using the helical model. The agreement with quasi-elastic light-scattering measurements at low bile salt concentration and within a wide range of ionic strength was satisfactory. The helical structure of the anions seems to be similar in the micellar aggregates of NaGDC, RbGDC, NaTDC, RbTDC, and tetramethylammonium taurodeoxycholate.