Isolation and partial characterisation of a xyloglucan from the cell walls of Phaseolus coccineus

Abstract

Cell-wall material from Phaseolus coccineus was fractionated by successive extraction with aqueous inorganic solvents. From the 4 m KOH-soluble fraction, a polymer composed of l-arabinose (4.2%), l-fucose (6.0%), d-galactose (9.3%), d-xylose (34.1%), and d-glucose (46.4%) was isolated and purified by ion-exchange and cellulose column chromatography; the product was homogeneous by moving-boundary electrophoresis and ultracentrifugation. The S 20,w o and D obs were 2.92 S and 1.7 × 10 −7, respectively, and the molecular weight was ∼110,000. Methylation analysis suggested a (1→4)-linked glucan backbone with ∼3 out of 4 glucosyl residues substituted through O-6 with xylose or oligosaccharides terminating in galactose, fucose, and arabinose. Limited acetolysis gave several di- and tri-saccharide derivatives of which four [ d-Gal p-(1→2)- d-Xyl p, d-Glc p-(1→4)- d-Glc p, l-Fuc p-(1→2)- d-Gal p-(1→2)- d-Xyl p, and d-Glc p-(1→4)- d-Glc p-(1→4)- d-Glc p] were tentatively identified. Specific glycosidases were used to determine the configuration of the glycosidic linkages. The backbone was shown to be a (1→4)-β- d-glucan and the l-fucosyl groups were α. Neither α- nor β- d-galactosidase removed d-galactose. The structure of the xyloglucan is discussed in relation to other cell-wall polymers, especially cellulose.