C-13 and N-15 NMR analysis of some fungal melanins in comparison with soil organic matter

A variety of fungal melanins with natural N-15 abundance are characterized by solid-state C-13 and N-15 NMR spectroscopy and are compared to solid-state C-13 and N-15 NMR spectra of organic matter from representative soils. In all solid-state N-15 NMR spectra the peptide/amide region (-220 to -285 ppm) dominates with more than 70% of the total intensity. The region between -285 and -375 ppm, assigned to amino and ammonium groups, always contains more than half of the remaining intensity. The area in the region from -30 to -220 ppm, where aromatic heterocycles would show signals, makes up less than 10% of the total intensity. These findings call into question common structural models for melanins. The solid-state C-13 NMR spectra, on the other hand, reveal large differences when the melanins are compared to each other, and to composts and soils. The concentration of the aromatic carbon varies from 5 to 40% in the melanin series. The ratio C-aro/N-tot and C-ali/N-tot were calculated, and confirm that nitrogen in these samples is bound in C-a-groups rather than in aromatic heterocyclic structures. (C) 1996 Elsevier Science Ltd