The yellow and the black of synthetic melanins

Abstract

The observations reported here are a continuation and expansion of observations described earlier: the typical conditions to synthesize melanin-like materials from catecholic precursors (air-oxidation in an alkaline environment) leads to the generation of a light-colored substance in addition to the typical dark substances. This light-colored substance appears to be associated with the dark-colored materials through non-covalent interactions. We employed a novel co-precipitation process that allows for a simple separation of this light-colored substance from the dark-colored materials. We studied some of the physic-chemical properties (color, fluorescence, FT-IR absorbance) of some of the fractions we obtained and discussed these properties in the context of what has been discussed for eumelanin, pheomelanin or neuromelanin. Overall, the current observations strengthen our hypothesis that synthetic melanins may be built from at least two types of substances: 1) a soluble, yellow-to-orange colored component, possibly rich in unoxidized precursors and 2) an insoluble, dark component. Our observations do suggest that when studying the chemistry or physiology of melanins one should look beyond the dark colors typically displayed by melanin-like materials.

Appearing as eumelanin or as pheomelanin and the push towards eumelanin by cysteine.

“…[15][16][17][18][19] Our current report is a combination of results obtained from new experiments and through an evaluation of experimental data collected over the past three years; some of which have been detailed elsewhere. [20][21][22][23][24][25] In these previous reports we detailed and discussed observations that this current report expands on and integrates into one overarching theme about the balance between EuMN and PhMN:…”

“…24 3) The post-synthesis precipitation of EuMN-like materials yielded supernatants that contained materials that in appearance and in other properties were PhMN-like; although no C was present during the synthesis of these materials. 23 4) Our previous observations prompted us to hypothesize that the synthesis of EuMN leads to a mixture of at least two different sets of materials. In addition, we hypothesized that the parameters which affect the kinetics of the reaction in part determine the balance between these two sets of materials and affect the ultimate appearance of the reaction mixture: light-or dark-colored.…”

“…In addition, we hypothesized that the parameters which affect the kinetics of the reaction in part determine the balance between these two sets of materials and affect the ultimate appearance of the reaction mixture: light-or dark-colored. [22][23][24] 5) Many of the results and the accompanying discussions revolve around the subjective description of the synthesized materials; particularly regarding their appearance. Thus, we developed an objective mathematical process to evaluate and compare the colors of the synthesized materials.…”

“…The mathematical process extracts important parameters from the experimental data: a) values of A650/A500, b) values of the decay constant k and c) calculations of the areaunder-the-curve (AUC) of the absorbance profiles as a measure of the darkness of the material. 21,23 Darkcolored materials tend to have higher values of A650/A500, lower absolute values of k and higher values of AUC.…”

“…The EuMN/CaCO3 coprecipitate can then be washed with HCl to dissolve CaCO3 resulting in an easy isolation of the remaining EuMN. 23 Figure 1 shows the chemical structures of the precursors involved in this report: L-DOPA, dopamine, epinephrine, norepinephrine, caffeic acid, chlorogenic acid, catechol, pyrogallol and 3,4-dihydroxybenzoic acid. The reactions involving chlorogenic acid (and to a lesser extent caffeic acid) and AA lead to a different set of reaction products and some of our observations on this have been reported and discussed elsewhere.…”