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

Abstract

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-2-carboxylic acid (DHICA) and its DHICA 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 1 week or to solar simulator over 3 hr. Different from DHICA melanin, MeDHICA melanin was proved to be fairly soluble in different water-miscible organic solvents, suggesting its use in dermocosmetic applications.

Keywords: (photo)stability; 5,6-dihydroxyindole-2-carboxylic acid methyl ester; EPR spectroscopy; MALDI MS analysis; antioxidant; melanin; solubility.