Influence of Acylation\nby Hydroxycinnamic Acids on\nthe Reversible and Irreversible Processes of Anthocyanins in Acidic\nto Basic Aqueous Solution

In\nthis work, the thermodynamics and kinetics of the\nreversible\nand irreversible processes of cyanidin 3,5-<i>O</i>-diglucoside\nand cyanidin 3-<i>O</i>-(2-<i>O</i>-glucosyl,\n6-<i>O</i>-sinapoyl)glucoside-2-<i>O</i>-glucoside,\n5-<i>O</i>-glucoside were studied by covering all pH range\n(holistic approach). The acylation (i) decreases the mole fraction\nof the colorless hemiketal in acidic medium and increases that of\nthe colored quinoidal base, (ii) expands the pH domain of the flavylium\ncation, and (iii) moderately decreases the rate of tautomerization\nand isomerization of the neutral and monoanionic species. Degradation\nof cyanidin-3,5-<i>O</i>-diglucoside in a basic medium occurs\nin two distinct stages. After the formation of the anionic quinoidal\nbases (double proton loss from flavylium cation), a fast kinetic step,\nonly observed by stopped flow, gives rise to the B<sub>4</sub><sup><i>n</i>–</sup> hydroxide adducts (<i>n</i> = 2,3) in equilibrium with the respective quinoidal bases A<sup><i>n</i>–</sup> (<i>n</i> = 1,2), leading\nto a first transient state. The quinoidal bases and B<sub>4</sub><sup><i>n</i>–</sup> adducts disappear completely from\nthe first to a second transient state by means of two parallel reactions:\n(i) one reversible, giving the anionic <i>cis-</i> and <i>trans</i>-chalcones, (ii) the other irreversible, giving degradation\nproducts and exhibiting a pH-dependent bell-shaped mole fraction with\nmaximum around pH = 12. From the second transient state, the anionic\nchalcones disappear completely in a few days. Acylation prevents formation\nof the first transient state. All of these effects are compatible\nwith anthocyanidin–acyl π-stacking interactions (intramolecular\ncopigmentation).