Ferric iron complexes of dopamine and 5,6-dihydroxyindole with nta, edda, and edta as ancillary ligands

Coordination of the neurotransmitter dopamine (DA) and the metal-binding component of neuromelanin 5,6-dihydroxyindole (DHI) with ferric iron has been studied in aqueous solution in the presence of ancillary ligands containing amine nitrogen and carboxylate oxygen donor sites. With nitrilotriacetic acid (nta) and ethylenediamine diacetic acid (edda) coligands, coordination of the catecholate ligands DA and DHI is observed to be complete at physiological pH. The resulting complexes of DA have the characteristic two-component electronic spectrum observed characteristically for L₄Fe(Cat) complexes. The spectrum obtained with DHI consists of a single broad absorption in the visible region. Both DA and DHI are able to coordinate with Fe³⁺ in the presence of edta, displacing carboxylate oxygen donors at pH values just above physiological pH. These results demonstrate the strong affinity of DA and DHI for Fe³⁺, pointing to in vivo complex formation in neuronal mixtures at physiological pH.