LIGNIN-FERULATE CROSS-LINKS IN GRASSES .4. INCORPORATION OF 5-5-COUPLED DEHYDRODIFERULATE INTO SYNTHETIC LIGNIN

Ferulates and dehydrodiferulates have a significant role in cross-link ing polysaccharides to lignin in grass cell walls. Among the various f erulate dehydrodimers, the 5-5-coupled dehydrodimer ,E)-4,4'-dihydroxy -3,3'-dimethoxy-5,5'-bicinnamate is capable of the most extensive cros s-linking into lignin, forming major branch-points, However, current l iterature fails to recognise the role of radical cross-coupling reacti ons with lignin monomers/oligomers, Here we demonstrate that a synthet ic model for 5-5-coupled dehydrodiferulate polysaccharide esters in gr ass cell walls biomimetically incorporates into synthetic lignins via radical coupling mechanisms to produce a range of cross-coupled struct ures, The incorporation profile is remarkably similar to that for feru late, Importantly significant coupling at the cinnamoyl 8-position rea dily occurs, Evidence for the expected incorporation of the 5-5-couple d dehydrodiferulate into the newly discovered dibenzodioxocine structu res is readily apparent in HMQC or HSQC spectra. Since some of the str uctures that dehydrodiferulates are involved in cannot be hydrolytical ly cleaved, their current 'quantification' is a significant underestim ation of the importance of these species. What is clear is that dehydr odiferulates can have a powerful role in effecting lignin-polysaccharl de cross-linking.