(111h) Investigating Thermodynamics and Kinetics of the Formation of Lignin-Carbohydrate Complex Linkages in Lignocellulosic Biomass Using Ab Initio Methods

The recalcitrant nature of biomass poses challenges for its economical conversion to fuels and chemicals. This recalcitrance is proposed to be a consequence of lignin being covalently bonded to other biopolymers forming lignin carbohydrate complexes (LCC). However, controversy surrounds these linkages due to the harsh experimental techniques implemented to characterize them; therefore, LCCs existence in biomass is usually inferred and remains questionable. Hence, in this work, the thermodynamic and kinetic feasibility of LCC formation is evaluated using ab initio methods.

The predominant intramolecular linkage in all lignin is the β-O-4 linkage, which requires the re-aromatization of a quinone methide intermediate via nucleophilic addition, where the nucleophile is typically assumed to be water. However, theoretically other biopolymers could also act as a nucleophile. Here, an individual monolignol containing a quinone methide structure is taken to be the reactant along with the known hemicellulose sugars. Conformational sampling of the reactants was performed by systematically rotating key torsion angles. Car-Parinello molecular dynamics (CPMD) and metadynamics was employed for efficient conformation sampling. The lowest energy structures were subsequently optimized with the hybrid-meta M06-2X functional with the appropriate and benchmarked basis set. Additionally, the proton catalyzed re-aromatization of the quinone methide intermediate via the nucleophilic addition of water and hemicellulose mechanisms are elucidated using Density Functional Theory (DFT) calculations.

The nucleophilic addition of hemicellulose to the quinone methide intermediate (to form LCC linkages) demonstrated to be thermodynamically more favorable compared to the nucleophilic addition of water. Both pathways are kinetically facile (activation barriers < 15 kcal/mol). The present investigation provides direct computational evidence supporting frequent LCC formation in native biomass. Future work includes extending this study to polymer systems to assess the optimal deconstruction route for LCC linkages.