Complexation of common metal cations by cyanins: Binding affinity and molecular structure

The ability of diverse metal cations to form complexes with cyanin has been investigated by means of Density Functional Theory (DFT) and the Quantum Theory of Atoms in Molecules (QTAIM). The strongest preference is shown by trivalent metals which exceed that of Mg(II), indicating that ion replacement processes are suitable detoxification mechanisms for plants. Molecular structure analysis indicates that the larger the metal affinity of Cy- the longer the C2-C1' bond length and smaller rho(b) value. This is understood as upon metal complexation the Cy- ligand molecular structure is more compatible with a dienolate-like structure rather than the 4 '-keto-quinoidal-like structure. The weight of the former increases as stronger the binding. QTAIM charges indicate that the stronger the binding energy the larger the charge transfer from Cy- to the metal, reducing its positive charge below the values indicated by the corresponding Lewis structure.