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

A variety of fungal melanins with natural ¹⁵N abundance are characterized by solid-state ¹³C and ¹⁵N NMR spectroscopy and are compared to solid-state ¹³C and ¹⁵N NMR spectra of organic matter from representative soils. In all solid-state ¹⁵N 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 ¹³C 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.