Isolation and structural identification of complex feruloylated heteroxylan side-chains from maize bran

Abstract

Three complex heteroxylan side-chains acylated with ferulate and one arabinosyl ester of p-coumaric acid have been isolated from maize bran insoluble fibre after acidic hydrolysis and fractionation by gel permeation chromatography and semi-preparative RP-HPLC. The complete structural elucidation of all isolated compounds was achieved by 1D/2D NMR spectroscopy and HPLC-MS in combination with methylation analysis. The absolute configuration of the carbohydrate constituents was determined by chiral GC after acidic hydrolysis and trifluoroacetylation. The identified feruloylated tetrasaccharides α- d-xylopyranosyl-(1 → 3)-α- l-galactopyranosyl-(1 → 2)-β- d-xylopyranosyl-(1 → 2)-5- O- trans-feruloyl- l-arabinofuranose (FAXGX) and α- d-galactopyranosyl-(1 → 3)-α- l-galactopyranosyl-(1 → 2)-β- d-xylopyranosyl-(1 → 2)-5- O- trans-feruloyl- l-arabinofuranose (FAXGG) are the most complex heteroxylan side-chains from maize bran that have been isolated to date. The isolated trisaccharide α- l-galactopyranosyl-(1 → 2)-β- d-xylopyranosyl-(1 → 2)-5- O- trans-feruloyl- l-arabinofuranose (FAXG) contributes to the complexity of heteroxylan side-chains from maize bran and 5- O- trans- p-coumaroyl- l-arabinofuranose represents the first p-coumaroylated heteroxylan side-chain isolated from cereal grains. Complex feruloylated heteroxylan side-chains are possibly, like ferulate cross-linking of the heteroxylans and binding of heteroxylans to lignin, a factor contributing to limited enzymatic degradation of fibre.