Temperature effects on xanthone–β-cyclodextrin binding dynamics

• Published in: Canadian journal of chemistry Vol. 89; no. 3; pp. 395 - 401
• Main Authors: Pace, Tamara C.S, Bohne, Cornelia
• Format: Journal Article
• Language: English
• Published: NRC Research Press 01.03.2011
• Subjects: Chemical properties; Composition; Cyclodextrins; Molecular dynamics; Thermal properties; Canada
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/V10-140

The complexation dynamics of the triplet excited state of xanthone with β-cyclodextrin were studied at various temperatures between 10 and 50°C. Association and dissociation rate constants were determined using the laser flash photolysis quenching methodology with [Cu.sup.2+] as a quencher. The rate constants for the association and dissociation of triplet xanthone with β-cyclodextrin increased with temperature, while the equilibrium constant for the triplet excited state remained relatively constant. Equilibrium constants for the ground-state complexation of xanthone with β-cyclodextrin were determined from fluorescence studies at various temperatures. The ground-state binding efficiency decreased with temperature and was markedly greater than that of the triplet excited state at all temperatures. The enthalpy and entropy for the β-cyclodextrin complex formation of the ground and triplet excited states fall on the enthalpy-entropy compensation relationship previously established for cyclodextrin complexes. The activation enthalpies for the association and dissociation rate constants for triplet xanthone are similar. The activation entropy is favorable for the association process, whereas a negative activation entropy was measured for the dissociation process, suggesting that solvation plays a key role in the complex formation between xanthone and β-cyclodextrin.