Mimicking Positive and Negative Copigmentation Effects in Anthocyanin Analogues by Host-Guest Interaction with Cucurbit[7]uril and β-Cyclodextrins

Copigmentation and the anti-copigmentation effects can be mimicked by the interaction of anthocyanin's model compounds respectively with cucurbit[7]uril (CB7) and β-cyclodextrin (β-CD). The complex network of chemical reactions displayed by this family of compounds includes the colored flavylium cation, AH⁺, and quinoidal base, A, along with the colorless hemiketal, B, cis-chalcone, Cc, and trans-chalcone, Ct. Whereas AH⁺ is stable only at very acidic pH values, the remaining species are formed at slightly acidic and neutral conditions. However, under these conditions, for most of the natural and synthetic flavylium salts, the colorless species predominate (B, Cc, and Ct) at the expense of A. The host CB7 was found to improve the color of solutions in two different ways: first, it stabilizes AH⁺ at higher pH values by decreasing its acidity and, second, it enhances the mole fraction of A due to selective complexation of this species in the pH range where the flavylium cation is no longer stable. In contrast, β-CD increases the acidity of AH⁺, favoring the formation of Ct at the expense of all the other multistate species (Basĺlio et al. New J. Chem. 2013, 37, 3166-3173; Petrov et al. J. Phys. Chem. A 2013, 117, 10692-10701; Gago et al. Dyes Pigments 2014, 110, 106-112; and Lopes-Costa et al. Photochem. Photobiol. Sci. 2014, 13, 1420-1426). Although both hosts have a large effect on the hydration rate and pH domain of the flavylium cation, the effect on the degradation kinetics of anthocyanidins is modest. CB7 decreases the hydration rate and decreases the acid-base (AH⁺/A) equilibrium constant (K_a), leading respectively to a slowing and a speeding of the anthocyanin degradation rate. On the other hand, β-CD increases the hydration rate but increases K_a, both effects acting in opposite directions.