Combined calorimetric, spectroscopic and microscopic investigation on the inclusion complex from cyclocurcumin and sulfobutylether-β-cyclodextrin in aqueous solution and Kinetics of thermal cis-trans isomerization

The complexation reaction between cyclocurcumin (CyCur), a natural curcuminoid with bioactive effects, and sulfobutylether-β-cyclodextrin (SBE-βCD), one of the more versatile and tolerate β-cyclodextrin derivates, was investigated in aqueous solution at 298K. The UV-vis spectral changes of CyCur in the presence of SBE-βCD indicate the formation of the host-guest inclusion complex according to a 1:1 stoichiometry. The binding constant (Kb) determined by applying the Benesi-Hildebrand model was comparable to the value obtained by Isothermal Titration Calorimetry (ITC) measurements. Moreover, the thermodynamic parameters of the SBE-βCD/CyCur interactions demonstrate that the complexation process is enthalpically driven. Interestingly, microscopy analysis highlights the tendency of cyclocurcumin to aggregate into spherical fluorescent structures able to change their aspect and morphology in the presence of SBE-βCD. Finally, the thermal cis-trans isomerization rates of CyCur in the temperature range 294-314K and the energetic parameters calculated by Arrhenius and Eyring plots were spectroscopically determined in SBE-βCD aqueous solution.