QUASI-ELASTIC LIGHT-SCATTERING STUDIES OF AQUEOUS BILIARY LIPID SYSTEMS - SIZE, SHAPE, AND THERMODYNAMICS OF BILE-SALT MICELLES

From measurements of the autocorrelation function and the time-averaged intensity of light scattered from aqueous bile salt solutions we have deduced the mean hydrodynamic radius (Rh), shape, aggregation number (n), and polydispersity of bile salt micelles as a function of bile salt species, concentration, temperature, and NaCl and urea concentrations. Our data confirm that the trihydroxy bile salt taurocholate (TC) forms smaller micelles than the dihydroxy bile salts taurodeoxycholate (TDC), tauroursodeoxycholate (TUDC), and taurochenodeoxycholate (TCDC) and also show that the latter species vary in size in the following order: TDC > TCDC > TUDC. In 0.15 M NaCl the Rh and n values are weakly dependent on bile salt concentration, whereas in 0.6 M NaCl the micelles grow appreciably with increasing concentration. The extent of micellar growth increases as the temperature is lowered from 60 to 20 °C but is diminished by the addition of urea (0-6 M). The large micelles are found to be rodlike and quite polydisperse. These data are shown to be quantitatively consistent with a two-stage model of bile salt aggregation in which bile salt monomers first associate hydrophobically at the cmc to form small globular primary micelles. At higher bile salt concentrations the primary micelles then polymerize into rodlike secondary micelles. We have deduced the polymerization constant (AT) for this process as a function of each physical-chemical variable. From a thermodynamic analysis of the K values, it is concluded that the driving force for secondary micelle formation is also an unfavorable hydrophobic interaction between the surface of the primary micelle and the aqueous solvent which is largely enthalpic in origin. It is shown that the NaCl dependence of K can be predicted from the Verwey-Overbeek theory of double layer repulsion and that the influence of urea on secondary micelle formation is consistent with experimental data for the free energy of transferring nonpolar amino acid side groups from water to urea solutions. © 1979, American Chemical Society. All rights reserved.