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

• Published in: Chemistry : a European journal 2020-06, Vol.26 (33), p.7433-7441
• Main Authors: Miskolczy, Zsombor, Megyesi, Mónika, Biczók, László, Prabodh, Amrutha, Biedermann, Frank
• Format: Article
• Language: English
• Subjects: host–guest systems; kinetics; reaction mechanisms; salt effect; self-assembly
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201905633

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. Contrasting mechanisms: Stopped‐flow studies on the release of two organic guests from cucurbit[7]uril (CB7) in aqueous solutions of inorganic salts revealed two contrasting mechanisms. Whereas metal cations Mn+ promote the release of berberine (B+) from CB7 in a multistep process, the expulsion of dicationic dye 2,7‐dimethyldiazapyrenium has a simpler mechanism.