Influence of Acylation by Hydroxycinnamic Acids on the Reversible and Irreversible Processes of Anthocyanins in Acidic to Basic Aqueous Solution

In this work, the thermodynamics and kinetics of the reversible and irreversible processes of cyanidin 3,5-<i>O</i>-diglucoside and cyanidin 3-<i>O</i>-(2-<i>O</i>-glucosyl, 6-<i>O</i>-sinapoyl)glucoside-2-<i>O</i>-glucoside, 5-<i>O</i>-glucoside were studied by covering all pH range (holistic approach). The acylation (i) decreases the mole fraction of the colorless hemiketal in acidic medium and increases that of the colored quinoidal base, (ii) expands the pH domain of the flavylium cation, and (iii) moderately decreases the rate of tautomerization and isomerization of the neutral and monoanionic species. Degradation of cyanidin-3,5-<i>O</i>-diglucoside in a basic medium occurs in two distinct stages. After the formation of the anionic quinoidal bases (double proton loss from flavylium cation), a fast kinetic step, only observed by stopped flow, gives rise to the B₄^<i>n</i>- hydroxide adducts (<i>n</i> = 2,3) in equilibrium with the respective quinoidal bases A^<i>n</i>- (<i>n</i> = 1,2), leading to a first transient state. The quinoidal bases and B₄^<i>n</i>- adducts disappear completely from the first to a second transient state by means of two parallel reactions: (i) one reversible, giving the anionic <i>cis-</i> and <i>trans</i>-chalcones, (ii) the other irreversible, giving degradation products and exhibiting a pH-dependent bell-shaped mole fraction with maximum around pH = 12. From the second transient state, the anionic chalcones disappear completely in a few days. Acylation prevents formation of the first transient state. All of these effects are compatible with anthocyanidin-acyl π-stacking interactions (intramolecular copigmentation).