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

Abstract

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]center dot Na+ (K-01 = 130 +/- 10 M-1) and Na+ center dot CB[7] center dot Na+ (K-02 = 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) x 10(7) M-1) and the association rate constant of (6.3 +/- 0.3) x 10(8) 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 SS 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.