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

Abstract

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.

Fast and Selective Production of Aromatics via Efficient Lignin Depolymerization: Critical Factors and Mechanism Studies

“…Pine lignin showed a significant difference in structure, whereas G units dominated the aromatic region, and almost no signals of S units were observed in the NMR spectrum. Recent research reported that p -coumaric acid ( p CA) and ferulic acid (FA) moieties in cell walls of fast-growing grass biomass, functioning as cross-linking agents between lignin and carbohydrate moieties, could be etherified to the isolated lignin. − The incorporation of p CA and FA moieties in corn cob lignin were noticeable, which could introduce readily cleavable ester bonds into the lignin structure, permitting easier depolymerization to produce monolignol p -coumarate conjugates . However, these two moieties were not evident in wood lignin, in particular, no signals were detected in pine lignin.…”

Laccase-mediated synthesis of bioactive natural products and their analogues

“…For this reason, Hilgers et al investigated in detail the reactivity of p -coumaroyl derivatives in lignin and model compounds in presence of laccase or a laccase-mediator system. 157 The use of (HBT) and N -hydroxyacetanilide (HPI), both being –N–OH type mediators, were found to be incorporated to lignin in presence of TvL or Pleurotus ostreatus laccase. 158 The grafting of these mediators may induce changes in the properties of lignin.…”

Isothermal titration calorimetric assessment of lignin conversion by laccases

“…Laccase possesses moderately high redox potentials of the active site T1 Cu center (0.42–0.79 V) and can oxidize, in principle, only phenolic residues of lignin directly. Thus, in most studies of laccase‐catalyzed lignin conversions, mediators have been used to mitigate the challenges associated with the large size of lignin and the high redox potential of the non‐phenolic reactions (Cañas & Camarero, 2010 ; Christopher et al, 2014 ; Hilgers et al, 2018 ; Hilgers, Kabel, et al, 2020 ; Hilgers, van Erven, et al, 2020 ; Rich et al, 2016 ; Zhu et al, 2020 ). Laccase catalyzes the one‐electron oxidation of the mediator to cation radicals which subsequently react with the high redox potential groups within bulky lignin.…”