Guest Binding Dynamics with Cucurbit[7]uril in the Presence of Cations

The binding dynamics of R-(+)-2-naphthyl-1-ethylammonium cation (NpH+) with cucurbit[7]uril (CB[7]) was investigated. Competitive binding with Na+ or H3O+ cations enabled the reaction to be slowed down sufficiently for the kinetics to be studied by fluorescence stopped-flow experiments. The binding of two Na+ cations to CB[7], i.e., CB[7]·Na+ (K01 = 130 ± 10 M–1) and Na+·CB[7]·Na+ (K02 = 21 ± 2 M–1), was derived from the analysis of binding isotherms and the kinetic studies. NpH+ binds only to free CB[7] ((1.06 ± 0.05) × 107 M–1), and the association rate constant of (6.3 ± 0.3) × 108 M–1 s–1 is 1 order of magnitude lower than that for a diffusion-controlled process and much higher than the association rate constant previously determined for other CB[n] systems. The high equilibrium constant for the NpH+@CB[7] complex is a consequence of the slow dissociation rate constant of 55 s–1. The kinetics results showed that formation of a complex between a positively charged guest with CB[n] can occur at a rate close to the diffusion-controlled limit with no detection of a stable exclusion complex.