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) sh ow that the micellar size increases upon increasing ionic strength (Na CDC, NaDC, and NaTDC) and upon decreasing temperature and pH (NaCDC an d NaDC). Their mean hydrodynamic radii (Rh) are in the following order : NaDC > NaTDC > NaCDC. The difference between the Rh values of NaDC a nd NaCDC is negligible without NaCl and increases upon increasing the NaCl concentration. The average intensities scattered by 0.01-0.10 M a queous solutions are in the sequence NaTDC > NaDC > NaCDC. Thus, the g rowth 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 th e experimental data are fitted by one straight line for NaCDC and two straight lines for NaDC and NaTDC, suggesting that the NaCDC or NaDC a nd NaTDC micellar aggregates can be represented by one or two structur es, respectively. Since the R-h and the average intensity values of Na CDC and NaDC with added NaCl decrease by increasing the temperature wi thin the range 15-85 degrees C, their micellar aggregates are stabiliz ed mainly by polar interactions as well as the helical micellar aggreg ates 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 spe ctra of NaDC and NaCDC aqueous solutions above and below the critical micellar concentration indicate that the two salts give rise to differ ent types of aggregation. Those of the interaction complexes between b ilirubin-IXa and NaCDC or NaDC or NaTDC suggest that the micellar stru ctures are chiral and helical. The enantioselective abilities of NaDC and NaTDC are somewhat similar, but they are different from that of Na CDC. The X-ray fiber patterns of NaCDC, NaDC, and NaTDC can be interpr eted by means of different helices and support the quasi-elastic light -scattering and circular dichroism results.