Mechanistic insights into the formation and deconstruction of phenyl glycoside linkages in lignocellulosic biomass

Abstract

Covalent linkages between lignin and cellulose/hemicellulose, referred to as lignin carbohydrate complexes (LCCs), have been identified to significantly contribute to the refractory nature of lignocellulosic biomass. However, experimental resolution of LCCs is limited, leading to a very limited knowledge of the chemical and structural details of LCCs. As a result, the present work uses first-principles based computational methods to quantify the reaction mechanisms, kinetics and thermodynamics associated with the formation and deconstruction of the prominent phenyl glycoside (PG) LCC linkage in biomass. The two previously proposed formation mechanisms, hemi-acetal and transglycosylation, are associated with significant activation barriers, suggesting these pathways are kinetically limited. A new mechanism is proposed, the electrophilic addition of hemicellulose to a lignin quinone methide (QM) intermediate, that possesses facile kinetics and is exergonic, suggesting it could be the pathway responsible for the significant fraction of PG linkages observed in the native biomass. Moreover, PG formation showed a composition dependence, suggesting that xylans will have higher fractions of PG linkages compared to mannans, explaining why softwoods and hardwoods have different reported types of LCCs. Additionally, the reaction mechanisms, kinetics and thermodynamics associated with the deconstruction of the PG LCC linkages in biomass under acidic conditions are investigated. The chemical degradation of the hemicellulose moieties is the primary competing reaction; however, the deconstruction energetics demonstrate that breaking PG linkages is kinetically and thermodynamically favored in acid catalyzed deconstruction. This indicates that PG linkages may not significantly contribute to the biomass recalcitrance.