Structural features of the pectic polysaccharides isolated from retted hemp bast fibres

Pectic polysaccharides were solubilized from retted hemp bast fibre bundles, by sequential extraction with water and ammonium oxalate at 100 °C. The polysaccharides isolated from the extracts were de-esterified and fractionated by anion exchange chromatography, yielding five fractions from boiling water extract and five fractions from oxalate extract. Five of these fractions were characterized by chemicals methods, 1H and 13C NMR, as polysaccharides containing galacturonic acid, rhamnose and galactose units in variable ratios. Their chemical structure comprises a disaccharide repeating unit → 2)-α-l-Rhap-(1 → 4)-α-GalpA-(1 → backbone, with short side chains attached to the rhamnosyl residues. The β-d-Galactose residues are attached to O-4 of the rhamnosyl residues, but the amount of l-rhamnosyl residues that are 2,4-linked can vary from 36 to 75%, from one fraction to another. A fraction corresponding to a mixture of oligosaccharides with an average dp of 13, with ten galacturonic acid, two rhamnose and one galactose units was also isolated.