KINETIC AND THERMODYNAMIC INVESTIGATION OF THE ALUMINUM ANTHOCYANIN COMPLEXATION IN AQUEOUS-SOLUTION

Complexation of the aluminium ion with a synthetic model of anthocyani n has been thoroughly investigated in aqueous solution. From UV-VIS sp ectroscopic data collected within the pH range 2-5, the presence of co mplexes involving not only the coloured forms but also the colourless forms of the pigment is demonstrated. A theoretical treatment is devel oped for the calculation of the corresponding stability constants, fro m 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 d ue to complexation is at a maximum has been found. From relaxation kin etics measurements (pH-jump), it is made clear that the whole process of equilibration is governed by the kinetics of complexation between A l3+ and the minor anionic quinonoidal form of the anthocyanin. Finally , H-1 NMR analysis in CD3OD in which complexation is much stronger tha n in water provides additional evidence of the anthocyanin passing fro m the red flavylium form to the deep-purple quinonoidal form upon coor dination to Al3+, a result that quantitatively demonstrates for the fi rst time the large potential of aluminium-anthocyanin complexation in colour variation.