Laccase-induced modification of lignin nanoparticles: Mechanisms and fractional analysis

The adjustment of the surface chemistry of lignin nanoparticles (LNPs) can be effectively achieved through laccase-induced biocatalysis. However, the mechanistic and fractional aspects of this process remain largely unknown. This study investigated the reaction mechanisms induced by recombinant Coprinopsis cinereus laccase (CcLcc9) in LNPs and phenolic acid model compounds and evaluated the impact of pairing saccharides (naturally present in industrial biomass extracts) on the reactions. Furthermore, the composition of lignocellulosic moieties released in pellet (nanoparticles) and supernatant (to date discarded) fractions was systematically compared. The results revealed that laccase induced oxidative coupling, side-chain oxidation, and cleavage of phenolic molecules. Cleavage reactions led to in-situ formation of low molecular weight redox mediators. The presence of these molecules led to the formation of a laccase-mediator system, which favored oxidative coupling and enabled side-chain oxidation reactions. Oxidative coupling polymerization, which was also influenced by the composition of lignin and saccharides present in the medium, resulted in the formation of larger lignin molecules with denser morphology. The adsorption of pairing saccharides on LNPs was particularly favored by the treatment with laccase. The fractionation of lignocellulosic compounds between the supernatant and the pellet followed a bidirectional pathway, with the lignin crosslinked by laccase being efficiently retained in the nanoparticle.