Isolation and structural determination of two 5-5'-diferuloyl oligosaccharides indicate that maize heteroxylans are covalently cross-linked by oxidatively coupled ferulates

Large amounts of ferulic acid (∼2.6% w/w) and dehydrodiferulic acids (∼1.3% total, w/w) were detected in saponified extracts of maize bran. After treatment of maize bran by mild acid hydrolysis and fractionation of the solubilised products by liquid chromatography, two dehydrodiferuloylated oligosaccharides (F8 and F9) were isolated. The compositions of F8 and F9 were shown to be 1:2:1 5,5′-diferulic acid (5,5′-diFA)–arabinose–xylose and 1:2 5,5′-diFA–arabinose, respectively. Their structure was determined using chromatography and LCMS and confirmed by 1H and 13C NMR spectroscopy. Fraction F9 was composed of 5,5′-diFA esterified at both carboxylic groups by a 5-O-l-arabinofuranoside moiety, while F8 was similar but with one of the arabinofuranosyl residues further substituted with a (1→2)-linked xylopyranosyl residue. These results provide direct evidence that the heteroxylans in maize cell walls are covalently cross-linked through dehydrodiferulates.