Synthesis of L-DOPA Based Melanins with Reproducible Physic-Chemical Properties

This report expands our ongoing research efforts into the non-enzymatic synthesis of melanins. We have explored four different methods for the synthesis of L-DOPA based melanins and evaluated the reproducibility of some of their physic-chemical properties. The melanins were synthesized through the addition of NaOH, tyrosinate or Fe²⁺/H₂O₂. Two different approaches for the reactions involving Fe²⁺ and H₂O₂ were tested: a) addition of H₂O₂ spread out over multiple days or b) addition of H₂O₂ in one fraction at the start of the reaction. The physic-chemical properties of the melanins explored involved: 1) retention on size exclusion chromatography column, 2) FT-IR spectroscopy, 3) UV-Vis spectroscopy and 4) the capacity to reduce a redox dye, dichlorophenolindophenol. Overall the results obtained indicated that 1) the various synthesis methods lead to melanins with reproducible physic-chemical properties, 2) that the melanins synthesized in the presence of Fe²⁺/H₂O₂ are distinctly different from the melanins synthesized in the presence of NaOH or tyrosinate and 3) that no distinctly different melanins were generated when comparing the two different synthesis approaches involving Fe²⁺/H₂O₂. Only the melanins synthesized in the presence of Fe²⁺/H₂O₂ appeared to possess the capacity to reduce dichlorophenolindophenol.