Disorder vs. order: the common features in the spectroscopic analyses of diverse melanin materials

Melanin materials were synthesized using a variety of precursors. Crude reaction mixtures and select purified materials were evaluated using absorbance spectroscopy from the UV to the visible and near-infrared regions of the electromagnetic spectrum. Combining new and previous experimental results, 968 different reactions were evaluated objectively for their type of appearance as eumelanin- or pheomelanin-like. In addition, two mathematical models were compared to evaluate the intensity of the color associated with the melanin material. One based upon the commonly used exponential equation, and another based on a reciprocal power equation. The results obtained from both approaches yield similar results: objective, numerical parameters, combining concentration- and material-specific factors, to compare different melanin materials built from varying types of precursors. FT-IR spectroscopic analyses of select purified materials highlight the common features of the melanin materials, independent of the precursor employed. In addition, FT-IR spectroscopy was used to evaluate the presence of carboxylic acid or carboxylate functional groups. Such features may be present due to the precursor employed and/or due to the oxidative cleavage of aromatic rings that may occur during the synthesis of the material. Depending on the purification process involved, the melanin material may contain carboxylates (dialysis) or carboxylic acids (HCl-induced precipitation or acidification followed by dialysis).