Unlocking lignin valorisation: Oxyfunctionalization of lignin dimer model compounds by unspecific peroxygenases

Lignin is an abundantly available biopolymer composed of three structural units, linked by a complex network of bonds, including a high proportion of β-O-4 ether linkages. As a renewable carbon source, it can be depolymerised into a variety of small aromatic compounds such as monophenols. Enzymatic bioprocessing offers a promising alternative to traditional chemical lignin degradation strategies, potentially producing value-added compounds, such as monoaromatics. Unspecific peroxygenases (UPOs) are promising enzymes for lignin bioprocessing due to their ability to catalyse aromatic oxidation and demethylation reactions, which are critical for lignin valorisation. In this study, thirteen different UPOs were evaluated for their oxidation potential with two lignin dimer model compounds, guaiacylglycerol-β-guaiacyl ether and veratrylglycerol-β-guaiacyl ether. Both compounds were successfully processed, yielding a wide range of products, e.g., via C_α-oxidation, demethylation, and bond cleavage reactions. Notably, the cleavages frequently occurred at the C_β-O ether bond, a major linkage between the lignin monomers, being beneficial for lignin degradation and subsequent valorisation. Some of the identified products, such as vanillin, are of interest either as valuable end-products or as precursors for further conversion into specialty chemicals.