Dark- or Light-Colored Melanins: Generating Pigments Using Fe2+ and H2O2

Abstract

We have studied the formation of melanin-like pigments from catechol or pyrogallol and a wide range of other phenolic compounds using Fe 2+ and H 2 O 2 . Combining UV_Vis spectroscopic measurements and size-exclusion chromatography analyses we evaluated the impact of the intensity of the oxidation reaction by varying the concentration of H 2 O 2 present in the reaction mixtures. All compounds tested, except tyrosine, reacted readily leading to mixtures that were black, brown or yellow-orange in color. For many compounds tested, the use of increasing concentrations of H 2 O 2 resulted in either precipitation of the pigment or the formation of a soluble, lighter-colored pigment. With catechol or pyrogallol as model compounds, and using different concentrations of H 2 O 2 , several materials were synthesized, purified and dried. The physic-chemical properties of these materials were compared to the properties of melanin-like pigments synthesized from the same precursors using air-oxidation in an alkaline environment. For both precursors, a distinct chemical change, as judged from FT-IR spectroscopy, was introduced in the melanin structures when using H 2 O 2 as the oxidizing agent and the relative intensity of this distinct signal strengthened with increasing concentration of H 2 O 2 used in the reaction. In general, our results suggest that depending on the precursor molecule and the intensity of the oxidizing reaction conditions involved, light-or dark-colored melanin-like pigments can be generated. This may be an important factor when evaluating the visible outlook of histological or archeological specimens: the presence of a lighter color or the absence of a dark color may not necessarily mean the absence of melanin-like biomolecules.

Synthesis of L-DOPA based pigments with different physic-chemical properties

“…A pattern of results similar to what has been observed for the Fe 2+ /H 2 O 2 -mediated generation of MN-like pigments from a multitude of different precursors. [17] In addition, the results shown in Fig. 5, panel B, indicate that the pigments generated in the presence of higher concentrations of H 2 O 2 exhibited enhanced fluorescence emissions.…”

“…[20,21] Using Fe 2+ /H 2 O 2 as the oxidizing combination, we generated MN-like pigments that in appearance could be light-or dark-colored, depending on the intensity of the oxidizing reaction conditions. [17,18] We have expanded these earlier observations by studying the synthesis of MN-like pigments from L-DOPA using air-oxidation in an alkaline environment or Fe 2+ /H 2 O 2 -mediated oxidation. The air-mediated oxidation of L-DOPA yielded darkly-colored materials when pH levels were above 6.0.…”

“…This pattern of observations is in line with what has been observed for MN-like pigments generated from other precursors. [17,18] The typical SEC profiles shown in Fig. 5, panel C, indicate peaks corresponding to high molecular mass materials (retention time about 12.8 minutes), unreacted L-DOPA (retention time about 15 minutes) and excess H 2 O 2 (retention time about 16 minutes).…”

“…[13][14][15] In previous reports we have presented our observations regarding the synthesis of MN-like pigments from a multitude of precursors using polysaccharides (PS) or a combination of Fe 2+ and H 2 O 2 . [16][17][18][19][20][21] We observed that many PS can promote the formation of MN-like pigments using different types of precursors leading to darkly colored solutions or materials. Despite the obvious presence of a darkly-colored pigment, the PS/pigment complexes tended to contain mostly PS material with very little pigment bonded to it as judged from FT-IR analyses.…”

Melanogenesis: A Search for Pheomelanin and Also, What Is Lurking Behind Those Dark Colors?

“…[28][29][30] In recent years we have presented various studies on the air-or reactive oxygen species-mediated oxidation of various catecholic compounds into dark-or light-colored pigments. [31][32][33][34][35][36][37][38] This report presents our first attempt to generate pheomelanin-like pigments by including L-cysteine into the reaction mixtures. As before, we evaluated our reactions through UV-Vis spectroscopic measurements and size exclusion chromatography (SEC) analyses.…”