Comparative study of water-soluble and alkali-soluble hemicelluloses from perennial ryegrass leaves ( Lolium peree)

Abstract

One water-soluble and four alkali-soluble hemicellulosic preparations from perennial ryegrass leaves were comparatively studied, and their chemical composition, structural features, and physicochemical properties were examined. The relative monosaccharide compositions of the hemicelluloses were determined by high performance anion exchange chromatography after acid hydroloysis. 13C NMR and FT-IR spectroscopic methods gave details of the anomeric linkage configuration and confirmed the structure of the hemicelluloses. The results showed that the water-soluble hemicelluloses probably consist of galactoarabinoxylans, β-glucan, and noticeable amounts of pectic polysaccharides, which had a lower molecular weight (19,360 g mol −1), while the four alkali-soluble hemicellulosic preparations are presumably composed of galactoarabinoxylans, l-arabino (4- O-methyl- d-glucurono)xylans, and β-glucan, which had higher molecular weights between 31,060 and 36,140 g mol −1. Galactoarabinoxylans constitute a major fraction of both water- and two alkali-soluble hemicelluloses H 2 and H 3, isolated with a higher concentration of alkali for a shorter period (18% NaOH or 18% KOH at 22 °C for 2 h). They consist of a linear β-(1 → 4) linked xylan backbone to which α- l-arabinofuranose units and/or short chains of sugar residues containing arabinose and galactose are attached as side residues via α-(1 → 3) and/or α-(1 → 2) linkages. On the other hand, the other two alkali-soluble hemicellulosic preparations H 4 and H 5, extracted with a relatively lower concentration of alkali for a longer period (10% NaOH or 10% KOH at 22 °C for 16 h), mainly comprised l-arabino (4- O-methyl- d-glucurono)xylans, in which the β-(1 → 4) linked xylan backbone is substituted in O-3 and/or O-2 by signal α- l-arabinose residue and α- d-glucuronic acid or 4- O-methyl-glucuronic acid. β-Glucan, both in water- and alkali-soluble hemicelluloses, was characterized as linear homopolymers consisting of both β-(1 → 3) and β-(1 → 4) glucosidic linkages.