Complexation of aluminum(III) by anthocyanins and synthetic flavylium salts: a source for blue and purple color

The single crystal structure of two synthetic flavylium salts possessing a catechol unit in position 7 and 8, (7,8-dihydroxy-4-methylflavylium,7,8- dihydroxyflavylium) is shown. The formation of aluminium complexes with these ligands as well as with the natural anthocyanin, cyanin, and the deoxyanthocyanidin, luteolinidin, both bearing a cathecol unit in position 3′ and 4′, is described. Complexation with Al³⁺ stabilizes the blue ionised quinoidal base avoiding its degradation by reaction with oxygen. The simple model developed to describe the complexation, indicates a metal ligand stoichiometry of 1:1. The complexation constants lie between 5.5 × 10⁶ < K_cp < 1.3 × 10⁹.