Metal binding by melanins: studies of colloidal dihydroxyindole-melanin, and its complexation by Cu(II) and Zn(II) ions

Abstract

Melanins are colloidal pigments known to have a high affinity for metal ions. In this work, the nature of the metal-binding sites are determined and the binding affinities are quantified. Initial potentiometric titrations have been performed on synthetic dihydroxyindole (DHI) melanin solutions to determine the chemical speciation of quinole/quinone subunits. Two types of acidic functionalities are assignable: catechol groups, with p K a between 9 and 13, and quinone imines (QI), with p K a of 6.3. The presence of the quinone-imine tautomer has, to our knowledge, never been assessed in polymeric melanins. Melanin solutions obtained from N-methylated DHI lack the p K a 6.3 buffer, consistent with its inability to form the quinone-imine tautomer. EPR spectroscopy of the DHI-melanin samples demonstrates that the semiquinone radical is in too low a concentration to contribute to the bulk binding of metals. Changes in the titration curves after addition of Cu(II) and Zn(II) ions were analyzed to obtain the binding constants and stoichiometry of the metal-melanin complexes, using the BEST7 program. UV–Vis spectra at neutral and high pH are used to identify absorbances due to Cu-bound quinone imine and catechol groups. The derived binding constants were used to determine speciation of the Cu(II) and Zn(II) ions coordinated to the quinone imine and catechol groups at various pH. The mixed complexes, Zn(QI)(Cat) − and Cu(QI)(Cat) − are shown to dominate at physiological pH.