Sequential inclusion of two berberine cations in cucurbit[8]uril cavity: kinetic and thermodynamic studies

Miskolczy, Zsombor and Biczók, László (2014) Sequential inclusion of two berberine cations in cucurbit[8]uril cavity: kinetic and thermodynamic studies. Physical Chemistry Chemical Physics, 16 (37). pp. 20147-20156. ISSN 1463-9076

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Abstract

A combination of absorption and fluorescence spectroscopic studies with isothermal calorimetric titrations and stopped-flow measurements is a powerful way to reveal the thermodynamics and kinetics of inclusion complex formation with cucurbit[8]uril (CB8). The unique photophysical characteristics of berberine (B), a pharmaceutically important natural alkaloid, was exploited to distinguish the consecutive encapsulation processes, and to examine the confinement in CB8 cavity. The highly environment sensitive fluorescence lifetime of B permitted of the selective detection of various cucurbituril complexes, and indicated to what extent the embedded guest is available for interaction with water. Highly stable 1:1 and 2:1 B-CB8 complexes were produced due to the release of the high energy water molecules from the CB8 interior, and the second binding step proved to be almost 3 times more exothermic. The favorable entropy change appreciably contributed to the driving force of 1:1 encapsulation. Contrarily, the embedment of the second B in CB8 led to substantial entropy diminution. The kinetics of encapsulation was followed in real time by recording the fluorescence intensity change after rapid mixing of B and CB8. No evidence was found for intermediates. The rate constants of (64 ± 9)×10^6, and (5.0 ± 0.5)×10^6 M-1 s-1 were found for 1:1 and 2:1 associations, whereas 3.8 ± 0.6, and 0.6 ± 0.1 s-1 were obtained for the rate constants of the reverse processes at 283 K, respectively.

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