Structure of the principal non-starch polysaccharide from the cotyledons of Lupinus angustifolius (cultivar Gungurru)

Abstract Methylation analysis, selective enzymolysis ( O -glycosidases) and chemical degradation showed that the principal non-starch polysaccharide (NSP) in a 2% disodium ethylenediamine-tetraacetate (EDTA) extract (purified by ethanol precipitation) of lupin cotyledons ( Lupinus angustifolius cultivar Gungurru) has the backbone sequence of [ →4)-α- d -Gal p A -(1→2)-β- l -Rha p-(1→ ]. Side chains mainly consist of linear (1→4)-β- d -Gal p residues and shorter, highly branched (1→5)-α- l -Ara f residues are attached to the C-4 position of the rhamnosyl residues in the backbone. Some terminal β- d -Xyl p residues are linked to the galacturonosyl residues. 1 H and 13 C-NMR spectroscopy support the proposed partial structure of the in EDTA-soluble NSP, which has a sugar composition and gylcosidic linkages typical of those found in primary cell walls of dicots. The high proportion of neutral sugars, the almost equal ratio of galacturonic acid to rhamnose, with most of the latter substituted at the C-4 position, the low degree of branching of the (1→4)-linked galactan, and highly branched (1→5)-linked arabinosyl residues are the characteristic features of the lupin cotyledon pectic polysaccharides.