Laccase mediator system: Oxidation of recalcitrant lignin model structures present in residual kraft lignin

Residual kraft lignin contains a greater amount of condensed structures and β-5 intermonomeric linkages with and without phenolic groups in comparison to native lignin. Some of the nonphenolic residual lignin substructures have been previously reported to be recalcitrant to degradation by laccase mediator systems (LMS). In this work, we report on the degradation of two lignin model compounds by LMS: a β-5 and a 5-5′ lignin substructure. According to our results, the oxidation by laccase of such compounds proceeded easily in the presence of phenolic groups, whereas the corresponding fully methylated nonphenolic compounds were difficult to oxidise even in the presence of the aromatic redox mediator (HBT, ABTS). On the other hand, the degradation was accomplished when surfactants were added to the system. The type of laccase, redox mediator and surfactant affected the extent of degradation.