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

Miskolczy, Zsombor and Megyesi, Mónika and Biczók, László and Prabodh, Amrutha and Biedermann, Frank (2020) Kinetics and Mechanism of Cation-Induced Guest Release from Cucurbit[7]uril. CHEMISTRY-A EUROPEAN JOURNAL, 26 (33). pp. 7433-7441. ISSN 0947-6539 (print); 1521-3765 (online)

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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 (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.

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