Structural analysis of acetylated hemicellulose polysaccharides from fibre flax (Linum usitatissimum L.)

Hemicellulose polysaccharides were sequentially extracted from defatted and depectinized flax bast fibres with Me2SO (20 °C), hot water (100 °C), aq 5% NaOH (20 °C), and aq 17.5% NaOH (20 °C). The ethanol-precipitated Me2SO extract was fractionated by size-exclusion chromatography, and the two main fractions (fraction II and III), covering 85% of the total, were analyzed by HPLC for constituting monosaccharides and acetyl content, and by NMR spectroscopy (1H, 13C and 2D-heteronuclear proton detected multiple-bond coherence (HMBC)). Fraction II contained a linear β-(1 → 4)-linked xylan, O-acetylated on C-2 and C-3 with an acetylation degree of 0.5. Fraction III was enriched in Man and Glc residues. NMR spectral data gave evidence for the presence of a linear β-(1 → 4)-linked glucomannan, O-acetylated on C-2 and C-3 on Man with an acetylation degree of 0.5 (based on Man). HPLC analysis and 1H and 13C NMR spectral data showed that the ethanol-precipitated hot-water extract consists of an O-acetylated rhamnogalacturonan I (RG-I) type structure. HPLC analysis of neutral monosaccharides indicates that the alkaline extracts are composed of glucomannan, xylan, and RG-I, having similar structures as those found in the Me2SO and hot-water extracts.