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Vercruysse et al._dark or light colored melanins_Chemistry Preprint Server.pdf (1.04 MB)
Dark- or Light-Colored Melanins: Generating Pigments Using Fe2+ and H2O2
Preprints are manuscripts made publicly available before they have been submitted for formal peer review and publication. They might contain new research findings or data. Preprints can be a draft or final version of an author's research but must not have been accepted for publication at the time of submission.
submitted on 29.11.2017 and posted on 01.12.2017by Koen Vercruysse, Shelby Russell, Juan Knight, Najwa Stewart, Nicole Wilson, Nahfisa Richardson
We have studied the formation of melanin-like
pigments from catechol or pyrogallol and a wide range of other phenolic
compounds using Fe2+ and H2O2. 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 H2O2 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 H2O2 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 H2O2, 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 H2O2 as the oxidizing agent
and the relative intensity of this distinct signal strengthened with increasing
concentration of H2O2 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.