Evaluation of the effect of background electrolyte composition and independence of parameters in determining binding constants of betulin derivatives to β- and dimethyl-β-cyclodextrins by affinity capillary electrophoresis.

Abstract

The values of the apparent binding constants for β-cyclodextrin complexes of betulin derivatives determined by mobility shift affinity capillary electrophoresis were found to be independent of the composition of the two background electrolytes used (tetraborate buffer, pH 9.18, and phosphate buffer, pH 8.00, both of them with 20 mM ionic strength). It has been found that if there is not a constant plateau on the binding curve then four independent parameters can be determined: binding constants (also referred to as stability, association, or formation constants) and ionic mobilities of 1:1 and 1:2 complexes. However, at least 10-12 data points in the binding curve should be used to reliably estimate the parameters. For the first time, the apparent binding constants for complexes of ester betulin derivatives with dimethyl-β-cyclodextrin have been determined by mobility shift affinity capillary electrophoresis. The logarithms of the constants for 1:1 and 1:2 complexes at 25°C for betulin 3,28-diphthalate with a 95% confidence interval are 4.98 (4.95-5.01) and 7.52 (7.26-7.68); for betulin 3,28-disulfate, the values are 4.97 (4.89-5.03) and 8.24 (6.82-8.52). It has been found that betulin 3,28-disuccinate forms only a 1:1 complex and the binding constant logarithm is 5.25 ± 0.02.