FT-IR Investigation of Cell Wall Polysaccharides from Cereal Grains. Arabinoxylan Infrared Assignment

The FT-IR fingerprint of wheat endosperm arabinoxylan (AX) was investigated using a set of polysaccharides exhibiting variation of their degree of substitution and xylo-oligosaccharides comprising xylose units mono- or disubstituted by arabinose residues. Substitution of the xylose backbone by arabinose side units was more particularly studied in the 1000−800 cm-1 spectral region, by taking advantage of second-derivative enhancement. The 920−1020 cm-1 spectral region revealed two absorption bands at 984 and 958 cm-1, the intensities of which varied according to the degree of substitution. Whereas the intensity of the band at 958 cm-1 increased with the degree of substitution, that at 984 cm-1 decreased. The second-derivative spectral data of xylo-oligosaccharides indicated that these changes could be attributed to substitution of the xylan backbone by arabinose residues, and the band at 958 cm-1 was ascribed to the presence of disubstituted xylose residues. Principal component analysis of FT-IR spectra of model mixtures of AX, β-glucans, and arabinogalactans suggested that it is possible to evaluate the relative proportions of the polymers and degree of substitution of AX in complex mixtures such as the cell wall of cereal grains. Keywords: Hemicellulose; β-glucan; arabinogalactan; pentosan; wheat; principal component analysis