Structural study of the micellar aggregates of sodium chenodeoxycholate and sodium deoxycholate

Quasi-elastic light-scattering data of sodium chenodeoxycholate (NaCDC), sodium deoxycholate (NaDC), and sodium taurodeoxycholate (NaTDC) show that the micellar size increases upon increasing ionic strength (NaCDC, NaDC, and NaTDC) and upon decreasing temperature and pH (NaCDC and NaDC). Their mean hydrodynamic radii (Rh) are in the following order: NaDC > NaTDC > NaCDC. The difference between the Rh values of NaDC and NaCDC is negligible without NaCl and increases upon increasing the NaCl concentration. The average intensities scattered by 0.01-0.10 M aqueous solutions are in the sequence NaTDC > NaDC > NaCDC. Thus, the growth of NaCDC is less than those of NaDC and NaTDC. The plots of the average intensity vs the square root of the mole fraction show that the experimental data are fitted by one straight line for NaCDC and two straight lines for NaDC and NaTDC, suggesting that the NaCDC or NaDC and NaTDC micellar aggregates can be represented by one or two structures, respectively. Since the R-h and the average intensity values of NaCDC and NaDC with added NaCl decrease by increasing the temperature within the range 15-85 degrees C, their micellar aggregates are stabilized mainly by polar interactions as well as the helical micellar aggregates previously proposed for NaDC and NaTDC. The decrease of pH causes a slight increase of the R-h values of NaDC and NaCDC up to a certain pH value, where a sudden increase is observed. Circular dichroism spectra of NaDC and NaCDC aqueous solutions above and below the critical micellar concentration indicate that the two salts give rise to different types of aggregation. Those of the interaction complexes between bilirubin-IXa and NaCDC or NaDC or NaTDC suggest that the micellar structures are chiral and helical. The enantioselective abilities of NaDC and NaTDC are somewhat similar, but they are different from that of NaCDC. The X-ray fiber patterns of NaCDC, NaDC, and NaTDC can be interpreted by means of different helices and support the quasi-elastic light-scattering and circular dichroism results.