Computational Evidence for Kinetically Controlled Radical Coupling During Lignification

Lignin is an alkyl-aromatic biopolymer that, despite its abundance, is underutilized as a renewable feedstock because of its highly complex structure. An approach to overcome this challenge that has gained prominence in recent years leverages the plasticity and malleability of lignin biosynthesis to tune lignin structure in planta through genetic approaches. An improved understanding of lignin biosynthesis can thus provide fundamental insights critical for the development of effective tailoring and valorization strategies. Although it is widely accepted that lignin monomers and growing chains are oxidized enzymatically into radicals that then undergo kinetically controlled coupling in planta, direct experimental evidence has been scarce because of the difficulty of exactly replicating in planta lignification conditions. Here, we computationally investigate a set of radical reactions representative of lignin biosynthesis. We show that, contrary to the notion that radical coupling reactions are usually barrierless and dynamically controlled, the computed activation energies can be qualitatively consistent with key structural observations made empirically for native lignin in a variety of biomass types. We also rationalize the origins of regioselectivity in coupling reactions through structural and activation strain analyses. Our findings lay the groundwork for first-principles lignin structural models and more detailed multiscale simulations of the lignification process.