Enzymatic demethylation of Kraft lignin for lignin-based phenol-formaldehyde resin applications

Abstract

Wood-rot fungi from the Boreal Forest of Canada were studied for their ability to demethylate Kraft lignin (KL). Demethylation by the action of enzymes (O-demethylases) removed the O-methyl/methoxyl groups of lignin liberating methanol, and produced a demethylated KL enriched in vicinal-hydroxyl groups with potential to serve as lignin-based phenol-formaldehyde polymers. Screening experiments identified the liberation of methanol (measured by selected-ion flow-tube mass spectrometry), lignin-demethylating enzymes, alcohol oxidase, and other ligninolytic enzymes. Highest amounts of headspace methanol (parts-per-billion) were detected in the genus Aspergillus, Ctenomyces, Cunninghamella, Penicillium, and Sporobolomyces. Methanol generated from lignin demethylation induced alcohol oxidase activity, but which was higher in Aspergillus, Ctenomyces, Entoloma, and non-sporulating fungi. Among the fungi tested, three brown-rot, i.e., Fomitopsis pinicola and Galerina autumnalis and a mitosporic Aspergillus sp.3 BRI 270, were cultured solely on KL, and lignin model compounds (LMCs) to determine lignin demethylation. Various carbohydrate supplements added to nutrient media containing KL significantly influenced demethylating activity. Aspergillus sp.3 BRI 270 showed the highest degree of lignin demethylation (30.1%) which no evidence presented for this comment occurred when cultivated on KL media supplemented with birchwood xylan as analyzed by 1H NMR following O-acetylation of modified KLs. All fungi demonstrated considerable demethylating activity by utilizing softwood KL, but OrganoSolv lignins (poplar, willow, wheat straw, and mixed agricultural wastes), because of their harsh chemical treatments of extraction, affected the microbial and enzymatic demethylation. Extracellular demethylating enzymes from Aspergillus sp.3 BRI 270 generated high vicinal-diol content (measured by the pyrocatechol titanium(III)-nitrilotriacetate method) in LMCs: 4-hydroxy-3-methoxy cinnamaldehyde (326.00 μmol/mL), syringaldehyde (102.67 μmol/mL), and Kraft lignin (397.46 μmol/mL) as analyzed by 1H NMR analysis.