A NEW APPROACH TO POLYDISPERSITY STUDIES OF SODIUM TAUROCHOLATE AND SODIUM TAURODEOXYCHOLATE AGGREGATES USING DYNAMIC FLUORESCENCE ANISOTROPY

Sodium taurocholate (NaTC) and sodium taurodeoxycholate (NaTDC) bile salt micelles in aqueous solution were investigated using perylene as a fluorescence anisotropy probe. By the introduction of a ''reduced'' correlation time. in the anisotropy decay theory, it was possible to resolve the rotational correlation time of the smallest aggregate from these polydisperse systems. For both bile salts, comparison of the resolved correlation times with calculated values identified the smallest aggregate as the dimer. Temperature studies confirmed this result. The hydrodynamic radii of the bile salt dimers, calculated from the anisotropy results, were 7.2 +/- 0.1 angstrom for NaTC and 7.3 +/- 0.1 angstrom for NaTDC. The NaTC dimer as a percentage of all aggregates decreased from 68% to 56% at 20-degrees-C and from 82% to 67% at 40-degrees-C as the total monomer concentration of NaTC was in from 20.0 to 100.0 mM. For NaTDC at 20-degrees-C, a decrease from 43% to 35% was observed for the same increase in total concentration. The average aggregate size was found to be larger for NaTDC than NaTC. In the presence of 3.0 M NaCl, the percent dimer in NaTC solution remained relatively constant at approximately 33% as NaTC concentration was increased, although the average aggregate size increased. Addition of Tb3+ was found to significantly decrease the percent dimer in NaTC solution. A primary-secondary aggregation model is presented for NaTC in which the primary micellar unit is a dimer ''sandwich'' with perylene located between two bile salt monomers. The effects of dimer shape and the relationships between internal rotation of the probe and overall rotation of the probe-dimer complex on the experimental results are