Coupling of Flavonoid Initiation Sites with Monolignols Studied by Density Functional Theory

Lignin recalcitrance presents a challenge for the development of a bioeconomy that employs lignocellulosic feedstocks. The efficiency of lignin deconstruction is improved by a reduction of molecular weight, and given the discovery that flavones serve as initiation sites in lignin biosynthesis, these molecular weight reductions could potentially be achieved with plant metabolic engineering to over-express flavonoids. Upon increasing the flavonoid content in lignin, the bond strengths and properties of flavonoid-monolignol linkages become increasingly important. To that end, the current work applies density functional theory calculations to elucidate the bond dissociation enthalpies (BDEs) of flavonoid-monolignol linkages, including dimers with oxidized monolignols. Specifically, the dimer bond strengths and monomer hydrogen abstraction energies for the flavonoids tricin, chrysoeriol, luteolin, apigenin, catechin, epicatechin, epigallocatechin, and epigallocatechin gallate are calculated, when coupled to seven natural and engineered monolignols. Results indicate that 4′-O-β linkage strengths between flavonoids and monolignols are of comparable strength to inter-monolignol β-O-4 linkages, with average flavonoid-monolignol BDEs of 70.7 kcal/mol relative to ∼69.3 kcal/mol in analogous canonical monolignols. Epigallocatechin yielded the lowest 4′-O-β bond strength of 52.3 kcal/mol when coupled to an oxidized monolignol, while the flavones overall produced lower average BDEs, relative to the flavanols. Substituents at the 3′-C and 5′-C positions on flavonoids affected the dimer linkage strengths to a greater extent than glycosylation or substituents further from the linkage. Erythro and threo stereochemistry across the flavonoid-monolignol linkage library exhibited only small energetic differences and no pronounced correlations. Taken together, the predictions from this work support the concept that higher concentrations of flavonoid initiation sites in lignin may afford linkage properties conducive to more facile lignin depolymerization.