Determining binding constants for 1:1 and 1:2 inclusion complexes of ester betulin derivatives with (2‐hydroxypropyl)‐β‐cyclodextrin by affinity capillary electrophoresis
Viktoria V. Sursyakova, Vladimir A. Levdansky, Anatoly I. Rubaylo
Abstract
Abstract The complexation of ester betulin derivatives with (2‐hydroxypropyl)‐β‐cyclodextrin (HP‐β‐CD) was studied by mobility shift affinity CE. Electrophoretic mobility for triangular peaks was calculated using the parameter a 1 of the Haarhoff–Van der Linde function instead of the peak top time. Dependences of the viscosity corrected electrophoretic mobility on HP‐β‐CD concentration were not described on the basis of only complexes with 1:1 stoichiometry due to the fact that these binding curves did not reach a plateau. However, the dependences were well described taking into account both 1:1 and 1:2 complexes. The presence of higher order equilibria was also revealed by x ‐reciprocal plots. The values of apparent binding constant logarithm, obtained for the first time, for 1:1 and 1:2 HP‐β‐CD complexes of betulin 3,28‐diphthalate and betulin 3,28‐disuccinate with 95% confidence interval limits in brackets are the same within error and are equal to 4.85 (4.73–4.95), 8.56 (7.75–8.82), 4.92 (4.86–4.97), and 8.54 (8.23–8.72) at 25°C, respectively. These values for 1:1 and 1:2 HP‐β‐CD complexes of betulin 3,28‐disulfate at 25°C are 4.61 (4.57–4.64) and 7.11 (6.57–7.34), respectively. The binding constants for betulin 3,28‐disulfate agree with the previously obtained results from the separation in the thermostatted capillary segment.