Effect of cyclodextrins on the thermodynamic and kinetic properties of cyanidin-3-O-glucoside

In this study, the effect of alpha and beta-cyclodextrin on cyanidin-3-O-glucoside color was investigated by UV-visible absorption techniques. The equilibrium and kinetic constants of the network of chemical reactions taking place in cyanidin-3-O-glucoside were also studied in water at 25 degrees C by UV-visible absorption techniques. The results showed that the addition of beta-cyclodextrin resulted in the fading of anthocyanin solution, and this fading effect was greater at higher pH. This anti-copigmentation effect is caused by the selective inclusion and stabilization of the anthocyanin colorless forms into the beta-cyclodextrin cavity. Oppositely, no changes were observed in the cyanidin-3-O-glucoside absorption spectra with the addition of alpha-cyclodextrin. Direct pH jump, from thermally equilibrated solutions at pH=1.0 (flavylium cation, AH(+)), shows three kinetic processes: formation of the base A, hydration reaction to form the hemiketal B and the chalcone cis-trans isomerization (Cc-Ct). The results obtained clearly indicated that the equilibrium and kinetic constants of the network of chemical reactions taking place in cyanidin-3-0-glucoside were affected by the presence of beta-cyclodextrin. Molecular inclusion in the beta-cyclodextrin cavity resulted in the increase of the isomerization observed rate constant (k(obs)) at pH 5.3 and in the increase of the hydration equilibrium constant K-h which is in agreement with the fading of the anthocyanin solution. For the macrocycle alpha-cyclodextrin, no significant changes were observed on the equilibrium and kinetic constants, which suggests that the inclusion of cyanidin-3-glucoside in the alpha-cyclodextrin's cavity is not favored. (C) 2013 Elsevier Ltd. All rights reserved.