Isolation and partial characterization of feruloylated oligosaccharides from maize bran

Maize bran contains phenolic acids [∼ 4% dry matter; mainly ferulic acid (Fe) and also diferulic acid], heteroxylans (∼ 50%), and cellulose (∼ 20%), but is devoid of lignin. Treatment of maize pericarp with 0.05 M trifluoroacetic acid at 100°C for 2 h released ∼90% of the heteroxylans and ∼90% of the ferulic acid and its esters. After fractionation of the products with Amberlite XAD-2 and Sephadex LH-20 three main feruloylated oligosaccharides (F3–F7) were isolated. They represented ∼ 30% of the ferulic acid, and ∼ 2% of the neutral sugars contained in the hydrolysis supernatant. The compositions of F7, F6, and F3 were Fe-Ara (1:1), Fe-Ara-Xyl (1:1:1), and Fe-Ara-Xyl-Gal (1:1:1:1), respectively. The structures of the three oligomers were determined using chemical methods (methylation, acetalation, reduction) and 13C NMR spectroscopy: F7 was 5-O-(trans-feruloyl)-d-Araf; F6 was O-β-d-Xylp-(1 → 2)-[5-O-(trans-feruloyl)-l-Araf]; and F3 was O-l-Galp-(1 → 4)-O-d-Xylp-(1 → 2)-[5-O-(trans-feruloyl)-l-Araf]. F7 has been previously isolated from other monocots especially from wheat bran and soluble arabinoxylans from wheat flour; this is the first report of feruloylated oligosaccharides F6 and F3. Our results suggest that these oligomers are side-chain constituents of heteroxylans in maize bran. Ferulic acid is probably partly responsible for the insolubility of heteroxylans by coupling polysaccharide chains through ferulic acid dimers.