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

Abstract

<p>We investigated the synthesis of melanin-like materials from DOPA, dopamine, norepinephrine and epinephrine in the presence of L-cysteine. We observed that L-cysteine delayed the formation of pigment from these catecholamines and that the presence of L-cysteine yielded darker-colored reaction mixtures. No reddish pigment was observed that would indicate the synthesis of pheomelanin-like material. The reactions were performed in the presence of Na<sub>2</sub>CO<sub>3</sub> and through the addition of CaCl<sub>2</sub> at the end of the reaction; the black, eumelanin-like material was co-precipitated with CaCO<sub>3</sub>. The remaining supernatant solutions were observed to be light-yellow to rusty-orange in color depending on the catecholamine used in the reaction. Size exclusion chromatography (SEC) analyses indicated that the removal of the black pigment left behind an oligomeric material that exhibited a strong absorbance band around 280nm. Our experimental and analytical observations prompt us to raise a number of points of discussion or hypotheses. 1) The presence of L-cysteine during the air-mediated oxidation of catecholamines leads to darker-colored pigments; not reddish or lighter-colored pigments that would visually resemble pheomelanin-like pigments, 2) SEC analyses suggested that the black pigment generated during the air-mediated oxidation of catecholamines is not necessarily the main reaction product, 3) The pre-formed, dark-colored pigments obtained through the air-mediated oxidative melanogenesis process can readily be deposited on insoluble mineral surfaces using an <i>in situ</i> co-precipitation procedure, 4) The air-mediated oxidation of catecholamines leads to a binary product that contains an insoluble, melanin-like substance and a soluble, oligo- or polymeric substance containing unoxidized precursor units, 5) The melanogenesis process leads to a binary product involving a non-covalently bonded combination of dark-colored pigment and a lighter-colored or colorless substance; the latter being understudied or ignored in the <i>in vitro</i> or <i>in vivo</i> studies of the melanogenesis process, 6) The kinetics of the melanogenesis process may determine the balance between insoluble and soluble components of the binary product generated; the slower the reaction the more dark-colored, insoluble pigment generated, 7) One should consider the possibility of intermolecularly, N-to-C, bonded units of catecholamines when evaluating the structure of melanins, polydopamines, etc. and 8) There is a need for a systematic study of the effect of amino acids (beyond just L-cysteine) and amines in general on the melanogenesis process.</p>

The yellow and the black of synthetic melanins

“…Overall, the current observations strengthen our hypothesis that MN materials are built from at least two types of substances: 1) a soluble, yellow-to-orange colored component, rich in unoxidized precursors and 2) an insoluble, dark component. In addition, our earlier experiments [2][3] and other reports 15, 19-20, 32, 40-42 indicate that the ratio of the various reaction products generated, and/or their physic-chemical properties, may depend on the reaction conditions applied.…”

“…The observations discussed in this report confirm and expand, by the inclusion of catechol and pyrogallol, our earlier reports on the possibility to precipitate MN-like materials on mineral substrates like CaCO3 post synthesis. 2 Similar co-precipitations can be achieved on other minerals like Ca3(PO4)2, Ca(OH)2 or the corresponding magnesium salts. Biocompatible materials like PDA for their potential effect on the polymorphism and physical dimensions of CaCO3 materials.…”

“…The current observations are in accordance with our earlier observations. 2 The typical conditions to synthesize MNs, air-oxidation in an alkaline environment, leads to the generation of a light-colored substance in addition to the typical dark substances. This lighter-colored substance appears to be associated with the darker-colored materials through non-covalent interactions and, judging from the experiments associated with Figure 10, can not be removed through dialysis.…”

“…Building upon earlier observations and the hypotheses derived from them, we explored further the possibility that melanins (MNs), synthesized through air-oxidation in an alkaline environment, consist of at least two distinctly different types of materials. [1][2][3] We observed that from within the dark-brown to black material typically generated during the MN synthesis process, one can extract substances that possess a much lighter (orange to yellow) color. We have expanded these observations in part with the inclusion of catechol and pyrogallol as precursors for MN-like materials.…”

The yellow and the black of synthetic melanins

“…The observations discussed in this report confirm and expand, by the inclusion of catechol and pyrogallol, our earlier reports on the possibility to precipitate MN-like materials on mineral substrates like CaCO3 post synthesis. 2 Similar co-precipitations can be achieved on other minerals like Ca3(PO4)2, Ca(OH)2 or the corresponding magnesium salts. * Biocompatible materials like PDA have been studied for their potential effect on the polymorphism and physical dimensions of CaCO3 materials.…”