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

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Sodium and rubidium deoxycholate (NaDC and RbDC, respectively) fibers have been drawn near the gelation point by lowering the pH of aqueous micellar solutions. Their X-ray diffraction patterns show a very close resemblance and can be interpreted by means of similar unit cell parameters and helical structures, formed by trimers arranged in 8/1 helices. Because the structures of the resulting fibers are connected with those observed in crystals, the helix of the RbDC crystal has been chosen to construct the 8/1 helix of the fibers. Calculations of interatomic distances support the 8/1 helix that can be used as a structural model of the NaDC micellar aggregates, especially near the gelation point. The lowering of pH within a narrow range in NaDC aqueous solutions causes a remarkable increase of the apparent hydrodynamic radius (R-h) and of the average scattered intensity. The intensity strongly changes within 2 degrees C around the temperature of the sol-gel transition. A bimodal R-h distribution, corresponding to very small and very big aggregates, is observed. The dominant mechanism causing the decay of the intensity autocorrelation function seems to be the translational motion of the center of mass of the aggregates. The absence of an appreciable rotational contribution is ascribed to the formation of roughly isotropic aggregates with spheroidal shape. It is proposed that the isotropic big aggregate is formed by helices randomly coordinated around a solvated hydrogen ion or a cluster containing hydrogen ions and water molecules, and that a network of cross-links, due to polar forces, connects the big aggregates and is responsible for the gel formation

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