Determination of the kinetics underlying the pKa shift for the 2-aminoanthracenium cation binding with cucurbit[7]uril

Abstract

The binding dynamics of the 2-aminoanthracenium cation (AH⁺) and 2-aminoanthracene (A) with cucurbit[7]uril (CB[7]) was studied using stopped-flow experiments. The kinetics was followed by measuring the fluorescence changes over time for AH⁺ and A, which emit at different wavelengths. The studies at various pH values showed different mechanisms for the formation of the AH⁺@CB[7] complex, with this complex formed either by the binding of AH⁺ or by the initial binding of A followed by protonation. In the latter case, it was possible to determine the protonation ((1.5 ± 0.4) × 10⁹ M⁻¹ s⁻¹) and deprotonation (89 ± 7 s⁻¹) rate constants for complexed A/AH⁺, which showed that the pK_a shift of +3.1 for A/AH⁺ in the complex is mainly due to a lower deprotonation rate constant.