Effect of organic solvents on the kinetics of reversible inclusion of palmatine in cucurbit[7]uril

Megyesi, Mónika and Miskolczy, Zsombor and Biczók, László (2026) Effect of organic solvents on the kinetics of reversible inclusion of palmatine in cucurbit[7]uril. JOURNAL OF MOLECULAR LIQUIDS, 443. No. 129137. ISSN 0167-7322

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Abstract

The effects of the organic cosolvents methylformamide (MF), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and acetonitrile (AN) on the reversible inclusion of the natural alkaloid palmatine (Pal) in cucurbit[7]uril (CB7) were investigated using fluorescence spectroscopy, stopped-flow measurements, and isothermal titration calorimetry. Kinetic studies revealed that cosolvents not only compete with Pal for CB7 binding but also modulate both the formation and dissociation rates of Pal–CB7 complex. Cosolvent confinement in CB7 was exothermic, with dimethylformamide showing the strongest affinity due to the favorable entropic contribution to the driving force. Even small cosolvents fractions reduced the enthalpy gain of Pal–CB7 formation, which was partly compensated by entropy increases in the presence of methylformamide, acetonitrile, and dimethyl sulfoxide. Direct monitoring of the complex dissociation demonstrated cosolvent-promoted acceleration of Pal release, driven by a greater decrease in activation enthalpy than activation entropy. Dimethylformamide addition enhanced the rate of Pal association with CB7, acetonitrile exerted negligible influence, whereas dimethyl sulfoxide and methylformamide slowed the process. These results underscore the essential role of cosolvent-induced perturbations in the hydrogen-bonded water network and solvation shell in modulating both thermodynamics and kinetics of host–guest complexation.

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