Diffusion coefficients of single bile salt and bile salt-mixed lipid micelles in aqueous solution measured by quasielastic laser light scattering.

Diffusion coefficients of single bile salt micelles and mixed binary micelles consisting of single bile salts and phosphatidylcholine in the molar ratio 3 : 1 have been measured using quasielastic laser light scattering spectroscopy. Among single systems, marked concentration dependence of the diffusion coefficient was observed only in those containing sodium taurodeoxycholate. Micellar size correlates with bile salt molecular structure in a manner similar to that observed by earlier workers with some minor differences to towards a more consistent ordering. Phosphatidylcholine produces small increases in micellar size in most systems depending on bile salt structure, and seems most plausibly interpreted as simple swelling of the single species micelles. In some systems, when phosphatidylcholine is added, size or shape changes are indicated. Diffusion coefficients were also studied in the ternary systems bile salt-phosphatidylcholine-cholesterol which were saturated and metastably super-saturated with respect to cholesterol. The Stokes radii in both systems were the same within experimental error, 35 A. Our results indicate that intermicellar interactions significantly contributed to the measured diffusion coefficients in the ternary systems as well as in the single sodium taurodeoxycholate and binary sodium taurodeoxycholate-phosphatidylcholine systems in the concentration range studied.