Synthesis of L-DOPA based pigments with different physic-chemical properties

Using air- or Fe²⁺/H₂O₂-mediated oxidation, the formation of melanin-like pigments from L-DOPA was evaluated. The effects of the various reaction conditions on the efficiency of the reaction and the physic-chemical properties of the pigments generated (UV-Vis absorbance, fluorescence emission) was evaluated. As previously observed for other phenolic and indolic compounds, the use of high concentrations of H₂O₂ resulted in melanin-like materials with a much lighter color. In addition, we observed that, unlike for air-mediated oxidation, the Fe²⁺/H₂O₂-mediated oxidation of L-DOPA can yield melanin-like pigmentation in an acidic environment. In general, our results suggest that depending on the intensity of the oxidizing reaction conditions involved, light- or dark-colored melanin-like pigments can be generated from L-DOPA. This may be an important factor when evaluating the visible outlook of histological specimens: the presence of a lighter color or the absence of a dark color may not necessarily mean the absence of melanin-like biomolecules. We discuss all our observations in the context of the importance of melanin-like pigmentation in human physiology.