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The development of innovative dip-coating technologies for surface functionalization has been a very active issue over the past decade following the discovery of the adhesion properties of polydopamine, a eumelanin-like material. New opportunities have derived from the discovery that hexamethylenediamine (HMDA) markedly enhances film deposition from a variety of catechol, including the key eumelanin precursor 5,6-dihydroxyindole (DHI). The remarkable antioxidant properties of synthetic eumelanins from the other main melanogenic precursor 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and its methyl ester (MeDHICA) have recently been described. In this work the film forming properties of MeDHICA melanin generated in the presence of HMDA or other diamines/monoamines and the antioxidant activity of the resulting films were investigated. Further to a systematic investigation, the most promising results were obtained running the aerobic oxidative polymerization of MeDHICA in aqueous buffers at pH 9.0 at 1 mM in the presence HMDA at 1:1.5 molar ratio. Under these conditions a dark yellow pigment is formed over 24 h exhibiting good film forming properties on different supports. HPLC analysis of the film solubilized in organic solvents indicated a mixture of oligomers of MeDHICA up to hexamers. Further polymerization of the film was obtained by exposure to ammonia vapors. The films showed high antioxidant activities in the 2,2-diphenyl-1-picrylhydrazyl and Ferric Reducing Antioxidant Power assays. Biocompatibility of MeDHICA/HMDA films was assessed on HaCat cells.

Antioxidant activity of film forming systems based on melanins from 5,6-dihydroxyindole derivatives