Computational Evidence for Kinetically Controlled Radical Coupling During Lignification

Gregg Beckham, Terry Gani, Michael Orella, Eric Anderson, Michael Stone, Fikile Brushett, Yuriy Roman-Leshkov

Research output: Contribution to journalArticlepeer-review

21 Scopus Citations

Abstract

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.

Original languageAmerican English
Pages (from-to)13270-13277
Number of pages8
JournalACS Sustainable Chemistry and Engineering
Volume7
Issue number15
DOIs
StatePublished - 2019

Bibliographical note

Publisher Copyright:
© 2019 American Chemical Society.

NREL Publication Number

  • NREL/JA-2A00-74856

Keywords

  • Density functional theory
  • Lignin biosynthesis
  • Lignin structure
  • Polymerization
  • Radical coupling

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