Kinetics and Mechanism of Cation-Induced Guest Release from Cucurbit[7]uril.

The release of two organic guests from cucurbit[7]uril (CB7) was selectively monitored by the stopped-flow method in aqueous solutions of inorganic salts to reveal the mechanistic picture in detail. Two contrasting mechanisms were identified: The symmetric dicationic 2,7-dimethyldiazapyrenium shows a cation-independent complex dissociation mechanism coupled to deceleration of the ingression in the presence of alkali and alkaline earth cations (M(n+) ) due to competitive formation of CB7-M(n+) complexes. A much richer, unprecedented kinetic behaviour was observed for the ingression and egression of the monocationic and non-symmetric berberine (B(+) ). The formation of ternary complex B(+) -CB7-M(n+) was unambiguously revealed. A difference of more than two orders of magnitude was found in the equilibrium constants of M(n+) binding to B(+) -CB7 inclusion complex. Large cations, such as K(+) and Ba(2+) , also promoted B(+) expulsion from the ternary complex in a bimolecular process. This study reveals a previously hidden mechanistic picture and motivates systematic kinetic investigations of other host-guest systems.