Determination of binding affinity for chenodeoxycholate in equilibrium with sulfobutylether-β-cyclodextrin

A kinetic dialysis technique together with a radiolabeled chenodeoxycholate (CDC) was used to determine the existence of a relationship between the monomer concentration of CDC and the total CDC concentration in different CDC solutions containing 1 or 5 mM sulfobutylether (SBE)-β-cyclodextrin. On the basis of the nature of the relationship and a binding model with binding constants of K₁ and K₂, the binding affinity for the solutions was quantified at the best curve fitting using a least-squares technique. The very high binding affinity of K₁ and the very low (i.e., negligible) binding affinity of K₂ indicate the formation of 1:1 inclusion complexes. In addition, the values of K₁ and K₂ were reasonably interpreted. Similar analysis showed that the formation of 1:2 inclusion complexes and the self-association of the SBE-β-cyclodextrin molecules in the solutions are unlikely. The present study provides a basis for investigating the self-association, quantifying the binding affinity, and interpreting the quantified values.