Kinetic and thermodynamic investigation of the aluminium–anthocyanin complexation in aqueous solution

Complexation of the aluminium ion with a synthetic model of anthocyanin has been thoroughly investigated in aqueous solution. From UV–VIS spectroscopic data collected within the pH range 2–5, the presence of complexes involving not only the coloured forms but also the colourless forms of the pigment is demonstrated. A theoretical treatment is developed for the calculation of the corresponding stability constants, from which the percentage of the different pigment forms can be plotted as a function of pH. These results are important to plant pigmentation and, for instance, a narrow pH domain in which colour amplification due to complexation is at a maximum has been found. From relaxation kinetics measurements (pH-jump), it is made clear that the whole process of equilibration is governed by the kinetics of complexation between Al3+ and the minor anionic quinonoidal form of the anthocyanin. Finally, 1H NMR analysis in CD3OD in which complexation is much stronger than in water provides additional evidence of the anthocyanin passing from the red flavylium form to the deep-purple quinonoidal form upon coordination to Al3+, a result that quantitatively demonstrates for the first time the large potential of aluminium–anthocyanin complexation in colour variation.