Submitted to: Journal of Cereal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2006
Publication Date: 1/1/2008
Citation: Bunzel, M., Allerdings, E., Ralph, J., Steinhart, H. 2008. Cross-linking of arabinoxylans via 8-8-coupled diferulates as demonstrated by isolation and identification of diarabinosyl 8-8(cyclic)-dehydrodiferulate from maize bran. Journal of Cereal Science. 47(1):29-40. Interpretive Summary: Plant cell walls are rich in polysaccharides that provide considerable nutrition to ruminant animals and are part of the indigestible fiber component valuable in the human diet. Very small amounts of cross-linking agents in the walls tie polysaccharides together, reducing their digestibility in ruminants, but possibly providing health benefits to humans and animals. To date, a natural cross-linking agent, ferulate, has been well implicated to operate by cross-linking in a variety of ways. The way in which it cross-link polysaccharides needs to be elucidated by isolating that component still tethered to small pieces of the polysaccharide after most of the large polysaccharide has been selectively hydrolyzed away. The isolation turns out to be extremely difficult, but collaborators at the University of Hamburg succeeded in isolating compounds which we showed by NMR to be the “di-5-O-L-arabinosyl ester of 8-8(cyclic)-dehydrodiferulic acid.” Its existence provides the required evidence that these other cross-linking mechanisms are operating in cell walls. Such cross-linking has profound effects on the structure of plant cell walls, influencing the physical and possibly physiological properties of cereal grain fibers in general. In the paper we speculate further on the nature of the coupling of polysaccharide chains mediated by ferulate.
Technical Abstract: Dehydrodiferulates are likely the most important arabinoxylan cross-links in cereals and grasses in general. However, association of dehydrodiferulates and arabinoxylan has only been authenticated for 5-5- and 8-O-4-dehydrodiferulates to date. In the present study, a saccharide ester of 8-8(cyclic)-dehydrodiferulate was isolated from maize bran insoluble fibre following mild acidic hydrolysis by using Sephadex LH-20 chromatography, gel chromatography on Bio-Gel P-2, and RP-HPLC. Mass spectrometry, one- and two-dimensional NMR and analysis of the carbohydrate and phenolic constituents following further hydrolysis identified the isolated compound as the di-5-O-L-arabinosyl ester of 8-8(cyclic)-dehydrodiferulic acid. From this finding it is apparent that 8-8(cyclic)-dehydrodiferulate exists as such in the plant cell wall and acts as an arabinoxylan cross-link. In addition, a fraction was isolated that contained two saccharide esters of 8-O-4-dehydrodiferulates. This fraction was comprised of two compounds, both built from 8-O-4-dehydrodiferulate, a 5-linked arabinofuranose and a 5-linked xylopyranosyl-(1>2)-arabinofuranose unit. These compounds show that, in addition to the 5-O-(trans-feruloyl)-L-arabinofuranosyl sidechain, the more complex beta-D-xylopyranosyl-(1>2)-5-O-trans-feruloyl-L-arabinofuranosyl sidechains are involved in the formation of 8-O-4-dehydrodiferulates.