Complexes of 3,4-dihydroxyphenyl derivatives. II. Complex formation processes in the nickel(II)-L-DOPA and zinc(II)-L-DOPA systems

Abstract

A pH-metric study (at 25 °C and 0.2 M ionic strength) of the nickel(II)-L-3,4-dihydroxyphenylalanine (L-DOPA) and zinc(II)-L-DOPA systems was used to determine the stoichiometric compositions and stability constants of the species formed. From the results obtained for the nickel(II) and zinc(II)- alaninate-pyrocatecholate systems, and UV and visible spectral studies of the corresponding DOPA complexes, conclusions were drawn on the mode of bonding of the complexes formed. It was found that complexes containing aminoacid-like and pyrocatechol-like bonds are formed in both systems: the latter are favoured in the case of zinc(II), and the former in the case of nickel(II). This is so marked that in the nickel(II)-DOPA system the 1:2 complex containing only (O,O) bonds is not formed at all. In both systems mixed complexes containing both (N, O) and (O,O) bonds occur to appreciable extents. It was established for some species that not only a given arrangement of donor atoms belongs to a given stoichiometric composition, but complexes with different structures are in equilibrium with one another.