Complexes of 3,4-dihydroxyphenyl derivatives. VII. Mixed ligand complexes of L-dopa and related compounds

Abstract

The stability constants of the mixed ligand complexes of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) ions with L-dopa, dopamine, L-adrenaline and L-noradrenaline as ligand A and L-alanine, L-histidine, glycylglycine and ATP as ligand B were determined pH-metrically at 25 °C and an ionic strength of 0.2 mol/dm 3 (KCl). From visible spectral studies, conclusions were drawn on the bonding properties of the ambidentate ligands in the complexes. It was found that, even in its mixed ligand complexes, the ambidentate nature of dopa permits its coordination via the amino acid side-chain (N, O) at lower pH, and via the ortho phenolic hydroxy groups (O, O) at higher pH. At the same time, the complex-forming tendency of the ethanolamine side-chain in the catecholamines plays a minor role in the mixed ligand complexes. The stabilities of the mixed ligand complexes were interpreted by consideration of the differences in the stepwise stability constants of the parent complexes, the back-coordination, the charge neutralization, and the electrostatic and hydrophobic interactions between the ligand pairs.