This is to allow for a more rapid consumption of the starting indole that decayed completely after 24 h oxidation in air. It should be noted that the conditions chosen for preparation of this melanin are quite different from those of our previous studies, in which a 10 mM concentration of the indole was used. ![]() The reaction mixture was then acidified to pH 3, and the precipitated melanin was recovered by centrifugation and subsequent lyophilization. After 24 h, a significant lowering of the absorption maximum of the starting indole at λ = 320 nm was observed, while a broad featureless absorption in the whole UV-Vis range increased ( Figure S5), typically associated with the melanization process. For comparative purposes, also, a DHICA melanin was prepared under the same conditions. Melanin from ADHICA was prepared by aerobic oxidation with the indole at 1 mM in 0.05 M sodium carbonate at a pH value of 9.0, close to the pKa of the catechol systems, to have the more oxidizable ionized form present in the mixture. Moreover, the solubility properties in selected solvents of dermo-cosmetic relevance and inhibition activity of photo- and radical-induced lipid peroxidation of this pigment were evaluated in comparison to DHICA melanin. On this basis, we report herein a complete structural characterization of the ADHICA melanin by UV-Vis, EPR, ATR-FTIR spectroscopy and MS analysis. A preliminary analysis of the oxidation behavior of DHICA carboxybutanamide (ADHICA) in air at pH 9 showed a rapid melanization of the compound with the formation of a pigment whose spectral features over the UV-Vis spectrum were typical of a melanin. To this aim, we recently developed a straightforward and general procedure for the synthesis of DHICA carboxy amides that allowed to obtain the starting materials for pigment synthesis on the gram scale. As alternative functionalization, amidation appeared a viable option, as amides are generally stable under physiological conditions, and in addition, the electron-donor character of the carboxyamide group was expected to favorably affect the chromophore of the final melanin pigment with respect to carbomethoxy group. Yet, these melanins, which on mass spectrometric analysis appeared as a collection of low molecular weight oligomers, were very light in color (pale yellow), with null absorption in the visible region, and expectedly even less effective in photoprotection than DHICA melanin. Melanins from DHICA methyl ester showed remarkable antioxidant properties in conventional chemical assays and in lipid peroxidation inhibition together with an improved solubility profile with respect to DHICA melanin and good cytocompatibility toward HaCaT cells. Overall, these results hint at this melanin, which remarkable properties are, in part, due to the electronic effects of the carboxyamide functionality as a promising functional ingredient for dermo-cosmetic formulations.īased on these considerations, we recently explored the possibility to tune the properties of DHICA melanins for dermo-cosmetic purposes through modification at the carboxylic group. The hydrogen- and/or electron-donor ability, and the iron (III) reducing power as determined by conventional assays provided evidence for marked antioxidant properties not merely ascribable to the more favorable solubility profile, while the inhibitory action of the radical- or photosensitized solar light-induced lipid peroxidation was more marked compared to that of DHICA melanin. The pigment exhibited a UVA–visible absorption even more intense than that of DHICA melanin, and a noticeable solubility in polar solvents of dermo-cosmetic relevance. Analysis of the pigment by EPR, ATR-FTIR and MALDI MS indicated a substantial structural similarity to DHICA melanin, while investigation of the early intermediates confirmed unchanged regiochemistry of the oxidative coupling. ![]() In this work, we explored the potential of a melanin obtained from the carboxybutanamide of a major eumelanin biosynthetic precursor, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), by aerobic oxidation under slightly alkaline conditions. The search for new synthetic melanin-related pigments that maintain the antioxidant and photoprotective properties of naturally occurring dark eumelanins, while overcoming their unfavorable solubility, and molecular heterogeneity is presently a very active issue for dermo-cosmetic purposes.
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