Molecular structure of cyanidin metal complexes: Al(III) versus Mg(II)

Metal complexation by anthocyanins is a very efficient mechanism for protecting plants. While Mg is an essential metal for life, typically found bound to anthocyanins, Al interferes with the metabolism of the former. Density functional theory and the polarizable continuum model are used to study cyanin (the simplest anthocyanin bearing a catechol unit) complexes with Mg(II) and Al(III), considering different metal ligand stoichiometries. Results obtained for metal-binding energies indicate that Al(III) complexes are always more stable than those of Mg(II). Furthermore, reaction energies for the metal exchange process show that free Al(III) (hexaaquo complex) is always able to displace Mg(II). This displacement is more favored when the metal ligand ratio decreases. Thus, anthocyanins are implied in suppressing Al(III) toxicity by enabling its accumulation and reducing its migration to ecosystems. The characteristics of Al(III)?Ccyanidin and Mg(II)?Ccyanidin bonds are investigated using the quantum theory of atoms in molecules. We find these complexes are more stabilized by ion?Cdipole electrostatic interactions than by electron pair sharing, as predicted by the Hard and Soft Acids Theory. Globally, two factors increase the covalent character: replacement of Mg(II) by Al(III) and replacement of water by cyanidin ligands.