Controlling the Competition: Boosting Laccase/HBT-Catalyzed Cleavage of a β-O-4′ Linked Lignin Model

Over the past years, laccase/mediator systems (LMS) have received a lot of attention as potential sustainable tools for biocatalytic lignin degradation. Nevertheless, it has often been reported that Cα-oxidation, rather than ether bond cleavage, is the main result of LMS treatments, which limits the overall efficiency and effectiveness. Remarkably few studies have attempted to influence this product profile and thereby enhance the effectivity of LMS-catalyzed lignin degradation. Here, we studied the influence of buffer properties on the product profile of a β-O-4′ linked lignin model dimer upon conversion by a laccase/hydroxybenzotriazole system. We show that the ratio between β-O-4′ ether cleavage and Cα-oxidation can be substantially increased by using unconventional buffer properties (i.e., highly concentrated buffers at near-neutral pH). Whereas <10% ether cleavage was obtained in conventional buffer (i.e., weak buffer at pH 4), as much as 80% ether cleavage was obtained in highly concentrated buffers at pH 6. In addition, this alteration of buffer properties was found to improve the stability of both laccase and mediator. The underlying reactions were further studied by using experimental and computational (density functional theory, DFT) approaches. Based on the outcomes, we propose detailed reaction mechanisms for the reactions underlying ether cleavage and Cα-oxidation. We propose that increasing buffer pH or increasing buffer strength enhances H-bonding between the lignin model and buffer anions, which drives the overall reaction outcome toward ether cleavage. These insights may pave the way for more efficient and effective biocatalytic lignin degradation.