Kinetics and Mechanism of Cation‐Induced Guest Release from Cucurbit[7]uril
Zsombor Miskolczy, Mónika Megyesi, László Biczók, Amrutha Prabodh, Frank Biedermann
Abstract
Abstract 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.