Solution structure of copper ion-induced molecular aggregates of tyrosine melanin

Melanin, the ubiquitous biological pigment, provides photoprotection by efficient filtration of light and also by its antioxidant behavior. In solutions of synthetic melanin, both optical and antioxidant behavior are affected by the aggregation states of melanin. We have utilized small-angle x-ray and neutron scattering to determine the molecular dimensions of synthetic tyrosine melanin in its unaggregated state in D₂O and H₂O to study the structure of melanin aggregates formed in the presence of copper ions at various copper-to-melanin molar ratios. In the absence of copper ions, or at low copper ion concentrations, tyrosine melanin is present in solution as a sheet-like particle with a mean thickness of 12.5 Å and a lateral extent of ~54Å. At a copper-to-melanin molar ratio of 0.6, melanin aggregates to form long, rod-like structures with a radius of 32 Å. At a higher copper ion concentration, with a copper-to-melanin ratio of 1.0, these rod-like structures further aggregate, forming sheet-like structures with a mean thickness of 51 Å. A change in the charge of the ionizable groups induced by the addition of copper ions is proposed to account for part of the aggregation. The data also support a model for the copper-induced aggregation of melanin driven by π stacking assisted by peripheral Cu²⁺ complexation. The relationship between our results and a previous hypothesis for reduced cellular damage from bound-to-melanin redox metal ions is also discussed.