Studies on wheat lignin degradation by Pleurotus species using analytical pyrolysis

Abstract

The aromatic-ring substitution patterns of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) lignin-units are conserved after pyrolytic breakdown enabling analysis of the composition of lignins with different origins. Analytical pyrolysis has been used to investigate lignin degradation by several white-rot fungi including Pleurotus eryngii, a species characterised by the ability to degrade lignin selectively. Using Py-GC/MS it was found that the composition of wheat lignin was modified by P. eryngii causing a decrease of S/G ratio in the residual lignin. Simultaneously, degradation products were incorporated into lignin-polysaccharide complexes. In order to get additional insight on wheat lignin modification by these fungi, the biodegraded samples were permethylated before Py-GC/MS. In this way it was shown that the free phenolic lignin-units (around 50% of the total H and G units and only 6% of S units) were preferentially removed as compared with the etherified ones. Moreover, it was possible to demonstrate that the lignin side-chains were attacked in situ resulting in the increased amounts of ether-linked vanillic and syringic acids, identified as the corresponding methyl esters by Py-GC/MS of permethylated samples. When 50% of wheat lignin was degraded by P. eryngii, the relative abundance of aromatic acids was over 15% of the total lignin derived products. These results validate some of the mechanisms postulated for lignin degradation by white-rot fungi, which were based on simple model compounds.