RAW AND EXTRUDED FIBER FROM PEA HULLS .2. STRUCTURAL STUDY OF THE WATER-SOLUBLE POLYSACCHARIDES

The soluble fraction (9%) obtained by aqueous extraction of an extruded pea hull fibre gave 3 peaks on DEAE-Sepharose-6B. The first population (25%) was not bound to the gel and contained xylose, arabinose and glucose (15:6:1), the second one (12%), eluted for an ionic strength of 0-08 M, contained xylose, arabinose and glucuronic acid (18:1:1), and the third one (63%), eluted for an ionic strength between 0.3 m and 0-8 m, contained placturonic acid, arabinose, xylose, rhamnose and galactose (5.3:3:1.8:1.7: 1). The structure of these fractions was studied through enzymic degradation, methylation, H-1-NMR and C-13-NMR. The first one was composed mainly of a heteroxylan having a (1 --> 4)-beta-D-xylan backbone (average dp. 103) with only very few branching points at O-2, and of a slightly branched arabinan. The second one was an acidic xylan. Both heteroxylans were acetylated. The third one was pectic in nature with a main chain consisting of (1 --> 4) linked galacturonic acid, partly substituted at the O-2 and 0-3 positions, and (1 --> 2) linked rhamnose residues which were mainly substituted at the 0-4 position. This acidic fraction was appreciably hydrolyzed by an endopolyplacturonase, indicating the presence of homogalacturonan regions in the molecules. Arabinose, galactose and xylose were present as side-chains.