Model Compounds Study for the Mechanism of Horseradish Peroxidase-Catalyzed Lignin Modification

Horseradish peroxidase (HRP) has demonstrated high activity for the modification of lignin. In this paper, several lignin model compounds with different functional groups and linkages are selected to investigate the reactivity of HRP-catalyzed lignin modification. The phenolic groups of lignin model compounds are indispensable for the HRP-catalyzed modification process. The introduction of the sulfomethylated methyl group or methoxyl group could facilitate or inhibit the modification, respectively. The oxidative coupling activity of alpha-O-4 lignin model compounds is higher than that of beta-O-4 compounds. Meanwhile, the free energy obtained by density functional theory (DFT) is used to verify the results of the experimental study, and the order of preference for linkages is beta-5 > beta-beta > beta-O-4 in most cases. In addition, electron cloud density and steric hindrance of lignin model compounds have crucial effects on the oxidation and modification processes. Finally, the mechanism of HRP-catalyzed lignin modification is proposed.