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 (Mn+) due to competitive formation of CB7-Mn+ 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-Mn+ was unambiguously revealed. A difference of more than two orders of magnitude was found in the equilibrium constants of Mn+ binding to B+-CB7 inclusion complex. Large cations, such as K+ and Ba2+, 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.