Thermodynamic analysis of ferulate complexation with α-, β- and γ-cyclodextrins

• Ferulate exhibits the highest affinity for the β-cyclodextrin.
• The β-CD cavity fits better with FER, according to the docking simulations.
• The complexation of FER with β-CD is the only one favored by entropy.
• More water molecules seem to be displaced after the complexation of FER with β-CD.

Isothermal titration calorimetry (ITC) was used to characterize the thermodynamics of the complexation processes of α-, β- and γ-cyclodextrin (CD) with ferulate (FER) in aqueous solutions. The equilibrium constants of ferulate complexation with CDs (Kc, in dm3 mol−1) at pH 9.0 and 25.0 °C were: 176.5 ± 5.0 (β-CD), 53.2 ± 3.4 (α-CD) and 19.4 ± 0.4 (γ-CD). Although FER–β-CD is the tightest complex of the three studied, its binding reaction is also the least exothermic and the only one that is entropically favored. Calculated binding enthalpies, based on the buried surface area upon complexation, are close to those determined by ITC except for the FER–β-CD complex which is more than two times more exothermic. According to these results and those obtained by molecular docking simulations, it is proposed that ferulate binds to the hydrophobic cavity of β-CD, displacing more water molecules than in the other two CD complexes.

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