Structural features of cell wall polysaccharides from the cotyledons of mung bean Vigna radiata

The first analysis is reported of the cell wall of a food legume by methods that minimise the degradation of pectic polysaccharides. The cell walls of mung bean cotyledons (Vigna radiata, var. T77) were extracted in sequence with 0.05 M trans-1,2-cyclohexanediamine-N,N,N′,N′-tetraacetate, sodium salt (CDTA) at 20°C, 0.05 M Na2CO3 at 1 and 20°C, 0.5 M KOH at 1°C, 1 M KOH at 1 and 20°C, 4 M KOH at 20°C, and 4 M KOH containing 4% of boric acid at 20°C. The isolated polymers were fractionated by anion-exchange chromatography. The CDTA-solubilised pectic polysaccharides were less branched than those solubilised by Na2CO3; less than 10% of the cell wall was solubilised by these reagents. 0.5 and 1 M KOH released only 3.2% by weight of the cell wall; the extracts consisted of mixtures of xyloglucans and pectic polysaccharides. The bulk of the extractable pectic polysaccharides were only released by 4 M KOH. They were highly branched arabinorhamnogalacturonans and contained significant amounts of terminal xylopyranosyl residues. Preliminary evidence is given to show that at least some of the xylopyranosyl residues are attached to the 3-position of 1,4-linked galactopyranuronosyl residues of the backbone. Some of the pectic polysaccharides were associated with small amounts of xyloglucan even after refractionation by anion-exchange chromatography, suggesting that they may be covalently attached in a complex. Little neutral arabinan was detected. High levels of arabinans, released from the walls of other legume-cotyledons by earlier workers, were probably artefacts caused by β-elimination under their extraction conditions. In this