Author
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QUIDEAU, STEPHANE - UNIV OF WISCONSIN-MADISON |
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Ralph, John |
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Submitted to: Journal of the Chemical Society Perkin Transactions 1
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/17/1997 Publication Date: N/A Citation: N/A Interpretive Summary: Polysaccharides provide food and energy sources for microbes and ruminant animals. However, not all of the polysaccharides in plants are available for digestion. A major mechanism preventing full utilization is the plant's ability to cross-link its polysaccharides to a much more inert substance, lignin that is broken down only with great difficulty. Compounds called diferulates, present in quite small amounts in the plants cell walls, have a remarkable ability to affect this cross-linking. In the past, the mechanisms by which it accomplished this were poorly understood. In this study, one of the diferulates that has the potential for causing the greatest amount of cross-linking was examined. It was found to be a potent cross-linker producing a range of possible structures that all have the effect of tying polysaccharides to lignin and making them inaccessible. Some of the structures were quite novel and not previously observed in plant studies. The results of this study help explain how polysaccharide digestibility of plant cell walls is being limited by certain components in the plant. Breeding plants to minimize those newly identified components, or genetic alteration by biotechnological methods, has the potential to improve the digestibility of plants that are not destined for ruminant feed. Improving plant digestibility can significantly reduce animal wastes and improve farm profitability and sustainability. Technical 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-5,5'- dimethoxy-3,3'-bicinnamate is capable of the most extensive cross-linking into lignin, forming major branch-points. However, current literature fails sto recognize 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 was remarkably similar to that for ferulate. Importantly, significant coupling at the cinnamoyl 8-position readily occurred. Evidence for the expected incorporation of the 5-5-coupled dehydrodiferulate into the newly discovered dibenzodioxocin structures was readily apparent in HMQC or HSQC spectra. Since some of the structures that dehydrodiferulates are involved in cannot be hydrolytically cleaved, their current "quantitation" 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. |
