Extraction, purification and chemical characterisation of xylogalacturonans from pea hulls

Pea hulls contained 925 mg/g sugar including 659 mg/g cellulosic glucose an d 90 mg/g uronic acid. They were de-esterified by NaOH (pH > 13 at 4 degree sC, 2 h) and treated with HCl (0.1 mol/l, 80 degreesC, 24 h). The HCl-solub le fraction represented 95 mg/g initial pea hulls. It was rich in galacturo nic acid (259 mg/g), xylose (93 mg/g) and rhamnose (91 mg/g), which co-elut ed in anion-exchange chromatography. The HCl-soluble fraction was degraded by a rhamnogalacturonan-hydrolase and the reaction products were fractionat ed by size-exclusion chromatography. Two fractions, representing together 1 8 mg/g initial pea hulls, were composed almost exclusively of galacturonic acid and xylose and could be defined as xylogalacturonans. The first fracti on exhibited a high molar mass, a molar ratio Xyl/GalA of 1 and contained a lmost 5% of rhamnose. The molar mass of the second fraction was much lower and the molar ratio Xyl/GalA was 0.6. Methylation analysis showed the prese nce in both fractions of a alpha (1 --> 4) galacturonan backbone highly sub stituted on O-3 either by terminal xylosyl residues or by short sidechains of (1 --> 2) linked xylosyl residues. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.