Radical Coupling Reactions in Lignin Synthesis: A Density Functional Theory Study

Abstract

Lignin is a complex, heterogeneous polymer in plant cell walls that provides mechanical strength to the plant stem and confers resistance to degrading microbes, enzymes, and chemicals. Lignin synthesis initiates through oxidative radical–radical coupling of monolignols, the most common of which are p-coumaryl, coniferyl, and sinapyl alcohols. Here, we use density functional theory to characterize radical–radical coupling reactions involved in monolignol dimerization. We compute reaction enthalpies for the initial self- and cross-coupling reactions of these monolignol radicals to form dimeric intermediates via six major linkages observed in natural lignin. The 8-O-4, 8-8, and 8-5 coupling are computed to be the most favorable, whereas the 5-O-4, 5-5, and 8-1 linkages are less favorable. Overall, p-coumaryl self- and cross-coupling reactions are calculated to be the most favorable. For cross-coupling reactions, in which each radical can couple via either of the two sites involved in dimer formation, the more reactive of the two radicals is found to undergo coupling at its site with the highest spin density.

Downregulation of p-Coumaroyl Quinate/Shikimate 3′-Hydroxylase (C3′H) or Cinnamate-4-hydrolylase (C4H) in Eucalyptus urophylla × Eucalyptus grandis Leads to Increased Extractability

“…These results are also supported by Density Functional Theory (DFT) simulations that calculated lower reaction enthalpies for β-5 coupling in H lignin dimers [40]. The DFT simulations also calculated lower reaction enthalpies of B (β-β resinol) subunits for S lignin dimers [40]. The work presented here shows a decrease in the β-β resinol units, consistent with the decrease in S/G ratio and the lower amount of S lignin in the C4H lines.…”

“…Although the differences were larger in alfalfa, most likely due to the higher H lignin content, the C3′H eucalyptus lines also displayed a decrease in β-O-4 and β-1 subunits and an increase in β-5 subunits. These results are also supported by Density Functional Theory (DFT) simulations that calculated lower reaction enthalpies for β-5 coupling in H lignin dimers [40]. The DFT simulations also calculated lower reaction enthalpies of B (β-β resinol) subunits for S lignin dimers [40].…”

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

“…Dimerisation of lignols was explored by DFT calculations at the M06-2X/6-311 ++G(d,p) level of theory. [181] Thef ormation of the b-O-4 linkage was predicted to be the most exothermic coupling reaction (for an H-H or G-G dimer,a pproximately 15 kcal mol À1 more favourable than the corresponding 4-O-5 linkage). This thermodynamic data concurs with the b-O-4 linkage being more abundant than 4-O-5 motifs.H owever,t his prediction should not be taken as evidence that lignification is at hermodynamically controlled process.Itisalso important to bear in mind that 4-O-5 units are formed by coupling of oligomers,w hereas lignification is mostly the result of sequential addition of monomers to the chain;the formation of b-ether units would therefore prevail over 4-O-5 counterparts,r egardless of the thermodynamics.M ost importantly,t here is convincing evidence that the processes leading to the formation of lignin linkages are instead kinetically controlled, as previously noted.…”

Recent innovations in analytical methods for the qualitative and quantitative assessment of lignin

“…Various quantum chemical methods and MD simulations have been explored in understanding structure, [199,200] lignin biosynthesis, [201] lignin pyrolysis, [202] and force-field based modeling of lignin macromolecular assembly [203,204]. Quantum chemical calculations provide insight into molecular information that is inaccessible to conventional experimental techniques.…”