Comparison of laccase-catalyzed cross-linking of organosolv lignin and lignosulfonates

Lignin, an underutilized by-product from chemical pulping of wood, can be modified enzymatically through oxidation by laccase. However, little is known about the molecular details surrounding the cross-linking which is a result of the oxidation. To reduce this lack of knowledge, we used oxygen consumption rate data, phenolic content data and molecular weight data together with data from NMR and FTIR spectroscopy to characterize laccase-catalyzed cross-linking of the industrial lignin preparations organosolv lignin and lignosulfonate. The organosolv lignin preparation had a Mn of 780 g/mol, a Mw of 5200 g/mol, and a phenolic content of 1.8 mmol/g. The lignosulfonate preparation had a Mn of 6000 g/mol, a Mw of 19800 g/mol, and a phenolic content of 1.1 mmol/g. Laccase-catalyzed oxidation of organosolv lignin was characterized by a relatively slow increase in molecular weight, decreased intensities for aromatic signals and p-hydroxycinnamyl groups, and increased intensity for β-O-4′ signals, whereas oxidation of lignosulfonates resulted in a very rapid increase in molecular weight, and strongly decreased intensities for aromatic signals. The data suggest that lignosulfonates cross-linked by couplings to the aromatic ring (e.g. 5-5′ and 4-O-5′), whereas β-O-4′ coupling characterized cross-linking of organosolv lignin, probably involving cinnamyl alcohol end-groups.