Sedimentation coefficients of self-associating species. II. Tests with a simulated example and with beta-lactoglobulin A.

If sedimentation equilibrium and sedimentation velocity experiments are performed on a self-associating solute under the same solution conditions, it is possible to evaluate the sedimentation coefficients (si) of the self-associating species and the usual concentration dependence parameter (g or gs). We have tested some of these methods with simulated examples. A more critical test is to use real data. Sedimentation equilibrium experiments with beta-lactoglobulin A at 20 degrees C in 0.2 M glycine buffer (pH 2.46) indicated that a nonideal monomer-dimer association was present. Sedimentation velocity experiments were performed on beta-lactoglobulin A under the same conditions. Using data from both sets of experiments we were able to evaluate s1, s2, g and gs using two different models for swa, the apparent weight average sedimentation coefficient. The empirical model for swa developed by Weirich et al. [1] gave better variance than did the model for swa developed by Gilbert and his co-workers [2-5]. Using a simulated monomer-dimer association mimicking a system having higher sedimentation coefficients than beta-lactoglobulin A did, we were able to show that one could not obtain s2 from tangents to the plot of 1/swa vs. c in the high concentration region. The methods developed here for sedimentation coefficients can be applied to other experiments in which a weight average property (or its apparent value) of a self-associating solute is measured, provided the appropriate thermodynamic experiments are done under the same solution conditions.