Cross-linking of arabinoxylans via 8-8-coupled diferulates as demonstrated by isolation and identification of diarabinosyl 8-8(cyclic)-dehydrodiferulate from maize bran

Dehydrodiferulates are likely the most important arabinoxylan cross-links in cereals and grasses in general. However, association of dehydrodiferulates and arabinoxylans 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 β-d-xylopyranosyl-(1→2)-5-O-trans-feruloyl-l-arabinofuranosyl sidechains are involved in the formation of 8-O-4-dehydrodiferulates.