Lignin–ferulate cross-links in grasses. Part 4.1–3 Incorporation of 5–5-coupled dehydrodiferulate into synthetic lignin

Abstract

Ferulates and dehydrodiferulates have a significant role in cross-linking polysaccharides to lignin in grass cell walls. Among the various ferulate dehydrodimers, the 5–5-coupled dehydrodimer (E,E)-4,4′-dihydroxy-3,3′-dimethoxy-5,5 ′-bicinnamate is capable of the most extensive cross-linking into lignin, forming major branch-points. However, current literature fails to recognise the role of radical cross-coupling reactions with lignin monomers/oligomers. Here we demonstrate that a synthetic model for 5–5-coupled dehydrodiferulate polysaccharide esters in grass cell walls biomimetically incorporates into synthetic lignins via radical coupling mechanisms to produce a range of cross-coupled structures. The incorporation profile is remarkably similar to that for ferulate. Importantly, significant coupling at the cinnamoyl 8-position readily occurs. Evidence for the expected incorporation of the 5–5-coupled dehydrodiferulate into the newly discovered dibenzodioxocine structures is readily apparent in HMQC or HSQC spectra. Since some of the structures that dehydrodiferulates are involved in cannot be hydrolytically cleaved, their current ‘quantification’ is a significant underestimation of the importance of these species. What is clear is that dehydrodiferulates can have a powerful role in effecting lignin–polysaccharide cross-linking.