Specific Buffers Affect the Stability of a Charged Cyclodextrin Complex Via Competitive Binding and Ionic Strength

The effect of 11 buffers as well as the effect of ionic strength were investigated on the binding between the bile salt taurochenodeoxycholate and the ionic sulfobutylether-β-cyclodextrin. The investigations showed that both ionic strength and competitive binding affected the stability constant. The stability constant for the sulfobutylether-β-cyclodextrin complex increased from 34,400 M ⁻¹ to 114,000 M ⁻¹ as the ionic strength of the solution increased to 0.15 M. Keeping the ionic strength constant, the stability constant for the complex depended on the buffer in the solution, with citric and succinic acid reducing the stability constant. The reduction in the stability constant by buffers was related to a competitive mechanism. The results showed that, when accounting for the variation in ionic strength between the buffers, three groupings of buffers existed. All carboxylic acid buffers decreased the stability constant of the sulfobutylether-β-cyclodextrin complex, relative to the effect observed by altering the ionic strength, whereas the other buffers only affected the stability constant in terms of the changes in ionic strength. Both buffer species and ionic strength impacted the stability of ionic cyclodextrin complexes, hence, it is important to be aware of these effects when working with, comparing and reporting stability constants.