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United States Department of Agriculture

Agricultural Research Service

Title: Lignin-Ferulate Cross-Links in Grasses. Part 3. Active Incorporation of Ferulate Polysaccharide Esters into Ryegrass Lignins

Authors
item Ralph, John
item Grabber, John
item Hatfield, Ronald

Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 13, 1995
Publication Date: N/A

Interpretive Summary: During a plant's growth, it produces two chemically distinct groups of polymers, polysaccharides and lignins. The polysaccharides comprise the fiber that is important in our diets can also be converted to simple sugars by bacteria in the rumen of ruminant animals (e.g. cows). Lignin is a polymer that plants use to bind the fibers together and confer structural rigidity to stems as well as provide other functions to the well-being of the plant. The plant can tie polysaccharides to lignin, using ferulic acid, to confer even greater stability. But we have never liked the mechanism that has been proposed and widely accepted for this cross-linking. We had demonstrated an alternative but more complex mechanism in lab experiments, but researchers have been reluctant to accept it. This manuscript provides unequivocal proof that the mechanism in which ferulic acid actively participates in the cell wall development is indeed used by plants. Since the ferulate can be irreversibly bound up in this process, the amount of ferulate and its importance in cross-linking plant polymers have always been underestimated. We are currently assessing the impact of reducing the cross-linking by ferulate on the digestibility of plants. These efforts will provide a sound basis for selecting plants that can be most efficiently utilized by ruminants.

Technical Abstract: Active incorporation of ferulate polysaccharide esters into ryegrass lignins has been demonstrated by NMR spectroscopy of uniformly 13C-labeled ryegrass. Observation, in the HMBC spectrum, of products of ferulate at its 8-position coupling with hydroxycinnamyl alcohols at the beta-position (producing 8-beta'-linked structures) is proof that ferulate-lignin radical lcross-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 ferulates couple with both coniferyl and sinapyl alcohol monomers. As notable as the presence of this and other ferulate products is the absence of coupling of ferulate at its 8-position with the 5- and O-4-positions of lignin units. Such structures were significant when ferulate was biomimetically incorporated into a synthetic lignin. Since hydroxycinnamyl alcohols couple almost exclusively at their beta-position in cross-coupling reactions, the 8-5' and 8-O-4' structures would only be formed by coupling with higher lignin oligomers (with no sidechain conjugation). Exclusive reaction of ferulates with lignin monomers is the first real evidence that ferulate polysaccharide esters are acting as initiation or nucleation sites for lignification and are critical entities in inducing cell wall cross-linking during plant growth and development.

Last Modified: 12/18/2014
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