Comparisons of IR-spectra from melanins of microscopic soil fungi, humic acids and model phenol polymers

Abstract

Microscopic soil fungi, such as Epicoccum nigrum, Stachybotrys chartarum, Hendersonula toruloidea and others when cultivated on glucose form high molecular weight melanins in the culture media and in the cells. The melanins resemble humic acids with respect to certain chemical properties and the IR-spectra also indicate similarities in functional groups. However, bands at 2900, 1460 and 1380 cm −1 suggest a higher content of aliphatic CH-components, and a pronounced shoulder at 1540 cm −1 suggests peptide-like material. A shoulder at 1720 cm −1 and a broad band at 1630–1650 cm −1 form evidence of fewer free carboxyl groups than in most soil humic-acid preparations. Upon hydrolysis with 6 N HCl methylation or a short heat treatment, distinct adsorptions at 1720 and in the 1600 cm −1 region appeared in the residues, indicating C=O vibrations of free carboxyl groups and aromatic C=C vibration. Comparisons with the variability of the carboxyl band in model polymers from phenols of various structures and prepared either by phenolase or autoxidative polymerization in the presence or absence of amino-acid compounds, suggested that the carboxyl group in fungal melanins may be bonded by H-bridges which shift the C=O absorption into lower frequences. Addition of clay minerals to the culture media of fungi accelerated or increased biomass and melanin formation. Comparison of band locations and shape of the spectra from the melanins with or without clays indicated that no basic alteration occurred in the composition of the melanins from clay culture. However, a higher ash content and the presence of specific bands from the added clays, together with sharpening and a shifting of the broad 1650 cm −1 band from the controls into higher frequencies, suggest chemical bonding of the melanins with clays through participation of the carboxyl groups. The model polymers from specific mixtures of phenols together with peptones and prepared either by oxidation with phenolase or by autoxidation, closely resembled the fungal melanins. This represents strong evidence for a similarity in the structural units and their linkages in both the melanins and the model polymers. The similarity of fungal melanins to some humic acids from recent lake and marine sediments and soil humic-acid fractions is discussed.