Decomposition of Lignin by White-rot Fungi. II. Characterization of Heavily Degraded Lignins from Decayed Spruce

Abstract

Heavily degraded spruce lignins isolated from wood decayed by Coriolus versicolor and from wood decayed by Polyporus anceps were compared with three sound lignins from spruce on the basis of functional group analyses, spectroscopic characterizations and chemical degradation studies. Results did not reveal any differences in the degradation of lignin by the two fungi. The fungus-degraded lignins had much lower phenolic hydroxyl contents than sound lignins, were lower in aliphatic hydroxyl content, and had about 0.2 alpha-carbonyl group (alpha-to aromatic rings) and 0.5 carboxyl group per C9-unit (monomer unit). The carboxyl groups were conjugated, approximately one-third being aromatic-conjugated, and the remainder being alpha,beta-unsaturated. The degraded lignins were about 25per thousand deficient in methoxyl content, but this loss of methoxyl groups was not accompanied by formation of persisting new phenolic hydroxyl groups. Phenolic hydroxyl groups resulting from aromatic hydroxylation also were absent. On oxidative degradation of the methylated lignins, the yields of methoxylated benzoic acids from the fungus-degraded lignins were substantially different than the yields from sound lignins. Similarly, acid hydrolysis gave markedly different yields of principal single-ring products from the degraded lignins. Most of the differences between sound and degraded lignins in the UV, IR and PMR spectroscopic characteristics reflect the higher contents of alpha-carbonyl groups and the conjugated carboxyl groups in the degraded material. A substantially decreased number of aromatic rings in the degraded lignins is indicated by the IR spectra. The data indicate that the polymeric degraded lignins contain both oxidized side chains and the aliphatic residues left after oxidative cleavage of aromatic rings.