No abstract

In this work the effect of aggregation and oxidation on the optical absorption of eumelanin oligomeric sheets is investigated by applying quantum mechanics and atomistic simulation studies to a simplified eumelanin structural model that includes 1-3 sheets of hexameric oligomer sheets. The oligomeric hypothesis is supported by AFM characterizations of synthetic eumelanins, formed by auto-oxidation or electrochemical oxidation of dihydroxyindole (DHI). Comparison of calculated absorption spectra to experimental spectra demonstrates a red shift in absorption with oxidation and stacking of the eumelanin and validates the theoretical results.

Spectroscopic simulations of a leading structural model for melanin, the pigment responsible for coloration and photoprotection in humans and animals, were done. We performed density functional theory (DFT) calculations using both the local density approximation (LDA) and the generalized gradient approximation (GGA) on a recent structural model for eumelanin based on higher oligomers of the monomer of neutral 5,6-indolequinone and its reduced forms, semiquinone and hydroquinone. This paper reports on our semiempirical spectroscopic simulations for the monomer and dimer energy-minimized structures. Our second study (part II) extends this approach to higher oligomers (tetramers through hexamers) and points out that the known optical spectra of eumelanins can be adequately explained on this basis.

Spectroscopic simulations of a leading structural model for melanin, which is the pigment responsible for coloration and photo protection in humans and animals, were conducted. In direct continuation of an earlier study on possible monomer and dimer subunits of eumelanin, we have performed density functional theory (DFT) calculations on a recent structural model for eumelanin based on higher oligomers of neutral 5,6-indolequinone. This paper further reports on our semiempirical spectroscopic simulations for the higher oligomer (tetramers through hexamers) energy-minimized structures. A linear combination of the oligomeric spectra reproduces several features of the experimental spectrum. This result strongly supports the assumption that melanin is constituted of substructures such as those considered in this study.

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 D(2)O and H(2)O 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 A and a lateral extent of approximately 54 A. At a copper-to-melanin molar ratio of 0.6, melanin aggregates to form long, rod-like structures with a radius of 32 A. 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 A. 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 pi stacking assisted by peripheral Cu(2+) complexation. The relationship between our results and a previous hypothesis for reduced cellular damage from bound-to-melanin redox metal ions is also discussed.