Reactivity of p-Coumaroyl Groups in Lignin upon Laccase and Laccase/HBT Treatments

Laccase-mediator systems (LMS) are potential green tools for oxidative degradation and modification of lignin. Although LMS convert both phenolic and nonphenolic lignin structures, phenolic structures are more prone to react. Remarkably, in a previous study on laccase/HBT treatment of grasses, we observed the accumulation of p-coumaroyl moieties in residual lignin, even though such groups are free phenolic structures. To provide more insights into this apparent paradox, here, we studied the reactivity of p-coumaroyl groups in lignin and model compounds using HSQC NMR and RP-UHPLC-PDA-MSn, respectively. It was found that a p-coumaroylated model compound (VBG-pCA), in contrast to its nonacylated analogue, was rapidly converted by laccase and laccase/HBT, resulting in oxidative coupling and HBT-mediated degradation, respectively. The high reactivity of VBG-pCA was related to the phenolic character of the p-coumaroyl group. Upon laccase/HBT treatment of two grass lignin isolates, p-coumaroyl groups accumulated in residual lignin, indicating that p-coumaroyl groups in polymeric lignin display different reactivity than those in model compounds. On the basis of additional experiments, we propose that p-coumaroyl groups in lignin polymers can be oxidized by laccase/HBT but undergo HSQC-undetectable radical coupling or redox reactions rather than degradation.