Structural characterization of allomelanin from black oat

Abstract

The brown to black coloration found in plants is due to the melanins, which have been relatively poorly investigated among the plant pigments. The aim of this work was to study the dark pigment extracted from the black oat hull with respect to composition and structure. Ultraviolet-visible (UV-Vis) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) and Fourier transform infrared (FT-IR) spectroscopy were applied for the characterization of the pigment. UV-Vis spectroscopy revealed that the extracted material displayed the absorption profile typical of melanins. MALDI-TOF MS measurements demonstrated that oat melanin is a homopolymer built up from p-coumaric acid and consists mainly of low molecular weight (500-900 Da) oligomers of between 3 and 9 monomer units. The tetramer oligomer proved to be dominant. The results of the FT-IR analysis indicated that oat melanin is a fully conjugated aromatic system containing tetrasubstituted aromatic rings linked by C-C coupling. The in vitro preparation of melanin from p-coumaric acid by horseradish peroxidase was performed for comparison. The resulting polymer consisted of oligomers of 4 to 9 monomer units similarly to those in oat melanin. However the building blocks proved to be connected to each other via C-O-C linkages in contrast with the C-C linkages in oat melanin. This is the first demonstration of the structure of melanin originating from plants.

Spectroscopic Characterization of Natural Melanin from a Streptomyces cyaneofuscatus Strain and Comparison with Melanin Enzymatically Synthesized by Tyrosinase and Laccase

“…In fungi and other microorganisms, melanins are classified on the basis of their precursor molecules: 3,4-dihydroxyphenlyalanine (dopa), dihydroxynaphatalene (DHN), and homogentisic acid (pyomelanins). This group of polymers is classified as allomelanins, and differently from pheomelanins, they contain neither sulphur nor nitrogen [ 17 , 18 ]. Some species of bacteria and fungi secrete pyomelanins, produced by a different phenylalanine and tyrosine degradation pathway.…”

Melanin formation in barley grain occurs within plastids of pericarp and husk cells

“…The pheomelanins, found only in certain yellow, orange or reddish hair and feathers, are also formed from tyrosine, but contain sulfur 6 . Plant and fungal melanins, classified as allomelanins 2 , are the least well understood, as well as the most heterogeneous group: their precursors are particularly varied 3,7 . In plants, melanins protect against damage from excessive light, but also give mechanical strength to the testa, thereby protecting the developing embryo [8][9][10][11] .…”

“…It is even uncertain as to whether the melanin formed in Compositae species is chemically similar to what is formed during PPO-induced tissue browning 13,14 . Melanins have been demonstrated as contributing to the dark pigmentation developed by the testa of sunflower, watermelon 1 , tomato 11 , morning glory 15 , oat 7 and garlic 16 .…”

Unprecedented high catecholamine production causing hair pigmentation after urinary excretion in red deer

“…The oxidation of DOPEG likely occurs when the urine of male red deer contacts the air, while the spread of the urine through the ventral fur probably leads to the polymerization of the oxidized intermediates and to the formation of relatively stable pigments after generating complexes with the keratin of hairs. These pigments can thus be categorized into the group of allomelanins, a heterogeneous class of nitrogen-free pigments derived from catechols and other polyphenols present in plants and fungi [24].…”