Measurement of bile salt aggregation equilibria using kinetic dialysis and spreadsheet modeling

Bile salts initially self-associate into small aggregates such as dimers and trimers before forming micelles at higher concentrations. The very size of these small aggregates hampers quantitation of their formation. We have devised a method using kinetic dialysis measurements to quantitatively examine aggregation of bile salt at concentrations below 10 mM. Data for rate of dialysis versus concentration were fitted to hypothetical models of aggregation using a personal computer spreadsheet. For sodium taurocholate these data best fit a model of initial dimer and trimer formation with stepwise association constants of 100 and 160, respectively. Addition of larger aggregates to the model did not improve the fit. For sodium taurodeoxycholate the data best fit a model which included not only dimers and trimers, but also tetramers or larger aggregates up to 10-mers with stepwise association constants of 40, 400, and 1700 for dimers, trimers, and tetramers, respectively. These data agree reasonably well with existing literature and suggest that kinetic dialysis with spreadsheet modeling is a useful technique for the study of bile salt aggregation at relatively low concentrations.