Unexpected impact of esterification on the antioxidant activity and (photo)stability of a eumelanin from 5,6-dihydroxyindole-2-carboxylic acid

Summary

To inquire into the role of the carboxyl group as determinant of the properties of 5,6-dihydroxyindole melanins, melanins from aerial oxidation of 5,6-dihydroxyindole (DHICA) and its methyl ester MeDHICA were comparatively tested for their antioxidant activity. MALDI MS spectrometry analysis of MeDHICA melanin provided evidence for a collection of intact oligomers. EPR analysis showed g-values almost identical and signal amplitudes (ΔB) comparable to those of DHICA melanin, but spin density was one order of magnitude higher, with a different response to pH changes. Antioxidant assays were performed and a model of lipid peroxidation was used to compare the protective effects of the melanins. In all cases MeDHICA melanin performed better than DHICA melanin. This capacity was substantially maintained following exposure to air in aqueous buffer over one week or to solar simulator over 3 h. Differently from DHICA melanin, MeDHICA melanin proved fairly soluble in different water miscible organic solvents, suggesting its use in dermocosmetic applications.

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