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.