A COMPARISON OF NATIVE AND SYNTHETIC MUSHROOM MELANINS BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

The diffuse reflectance Fourier-transform infrared spectra of the spore of the cultivated mushroom, Agaricus bisporus, of the melanin isolated therefrom and of various synthetic melanins related to the native material were recorded. These were prepared from gamma-glutaminyl-4-hydroxybenzene (GHB; the presumed precursor of melanin in this fungus) as well as from simple phenol analogues of GHB, through auto- or enzyme-mediated oxidation and with, or without, potential non-phenolic co-reactants. The spectrum of the spore is dominated by distinct IR absorptions due to melanin, chitin and protein, while the isolated pigment is characterized mainly by the melanin absorption band at 1600 cm(-1), which can be attributed primarily to aromatic groups whose signal is enhanced by phenol functionalities and, to a lesser extent, by amido groups. In addition, pronounced alkyl and carbonyl absorption bands were observed, and there is evidence of extensive C-O and C-N bonding in the biopigment. The spectrum of the melanin made from GHB through enzyme catalysis (phenolase, EC 1.14.18.1/1.10.3.1) most closely matched that of the biopigment. Use of 4-aminocatechol (AC) instead of GHB afforded a product which was similar to the native melanin, whereas the pigment obtained by air oxidation of 4-aminophenol (AP) was largely dissimilar. Cross-linking of the reacting phenol with protein was apparent in the melanin formed enzymatically from AC, but no evidence for such reactivity was present in the AP-derived melanin. Preformed chitin did not become incorporated into either synthetic melanin. The fungus does not synthesize melanin from AP, nor directly from GHB, but from AC in a proteinaceous environment represented in part by the phenol oxidase itself.