Spectroscopic Study of the Common Features of Melanins Synthesized from a Diverse Range of Precursors

Melanins were synthesized using a variety of precursors, some of which are precursors of naturally occurring eumelanin or allomelanin. Purified materials were evaluated using absorbance or FT-IR spectroscopy and compared for their common physical and chemical properties. Absorbance spectra were modeled using an exponential and a novel reciprocal power equation to objectively quantify the appearance of the melanin. The application of the reciprocal power equation was demonstrated to provide an estimate of the band gap width of the melanin material. Both the absorbance and FT-IR spectra of the various melanins showed qualitative similarities despite the chemical diversity of the precursors involved. FT-IR spectroscopy was employed to evaluate the presence of carboxylic acid or carboxylate functional groups and provide evidence of possible oxidative cleavage of aromatic rings during the synthesis of melanin. Through precipitation experiments using Ca²⁺ the physical (in)stability of melanin dispersions was evaluated and their colloidal properties highlighted.