LIGNIN-FERULATE CROSS-LINKS IN GRASSES - ACTIVE INCORPORATION OF FERULATE POLYSACCHARIDE ESTERS INTO RYEGRASS LIGNINS

Active incorporation of ferulate polysaccharide esters into ryegrass l ignins has been demonstrated by NMR spectroscopy of uniformly C-13-lab eled ryegrass. Observation, in the HMBC spectrum, of products of ferul ate at its 8-position coupling with hydroxycinnamyl alcohols at the P- position (producing 8-beta'-linked structures) is proof that ferulate- lignin radical cross-coupling reactions occur in vivo. Correlations of H-alpha' (hydroxycinnamyl alcohol moiety) with guaiacyl and syringyl 1-, 2-, and 6-aromatic carbons in 8-beta' structures indicates that fe rulates couple with both coniferyl and sinapyl alcohol monomers. As no table as the presence of this and other ferulate products is the absen ce of coupling of ferulate at its 8-position with the 5- and O-4-posit ions of lignin units. Such structures were significant when ferulate w as biomimetically incorporated into a synthetic lignin. Since hydroxyc innamyl alcohols couple almost exclusively at their P-position in cros s-coupling reactions, the 8-5' and 8-O-4' structures would only be for med by coupling with higher lignin oligomers (with no side-chain conju gation). Exclusive reaction of ferulates with lignin monomers is the f irst real evidence that ferulate polysaccharide esters in grasses are acting as initiation or nucleation sites for lignification and are cri tical entities in directing cell-wall cross-linking during plant growt h and development.