Structural Motifs of Wheat Straw Lignin Differ in Susceptibility to Degradation by the White-Rot Fungus Ceriporiopsis subvermispora

The white-rot fungus Ceriporiopsis subvermispora delignifies plant biomass extensively and selectively and, therefore, has great biotechnological potential. We previously demonstrated that after 7 weeks of fungal growth on wheat straw 70% w/w of lignin was removed and established the underlying degradation mechanisms via selectively extracted diagnostic substructures. In this work, we fractionated the residual (more intact) lignin and comprehensively characterized the obtained isolates to determine the susceptibility of wheat straw lignin's structural motifs to fungal degradation. Using C-13 IS pyrolysis gas chromatography mass spectrometry (py-GC-MS), heteronuclear single quantum coherence (HSQC) and P-31 NMR spectroscopy, and size-exclusion chromatography (SEC) analyses, it was shown that beta-O-4' ethers and the more condensed phenylcoumarans and resinols were equally susceptible to fungal breakdown. Interestingly, for beta-O-4' ether substructures, marked cleavage preferences could be observed: beta-O-4'-syringyl substructures were degraded more frequently than their beta-O-4'-guaiacyl and beta-O-4'-tricin analogues. Furthermore, diastereochemistry (threo > erythro) and gamma-acylation (gamma-OH > gamma-acyl) influenced cleavage susceptibility. These results indicate that electron density of the 4'-O-coupled ring and local steric hindrance are important determinants of oxidative beta-O-4' ether degradation. Our findings provide novel insight into the delignification mechanisms of C. subvermispora and contribute to improving the valorization of lignocellulosic biomass.