title: 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 H(3)O(+) 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(+) (K(01) = 130 ± 10 M(-1)) and Na(+)·CB[7]·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) × 10(7) M(-1)), and the association rate constant of (6.3 ± 0.3) × 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 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.
authors.affiliation: Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6.
authors.forename: Hao
authors.initials: H
authors.lastname: Tang
authors.forename: Denis
authors.initials: D
authors.lastname: Fuentealba
authors.forename: Young Ho
authors.initials: YH
authors.lastname: Ko
authors.forename: Narayanan
authors.initials: N
authors.lastname: Selvapalam
authors.forename: Kimoon
authors.initials: K
authors.lastname: Kim
authors.forename: Cornelia
authors.initials: C
authors.lastname: Bohne