Isolation and structural characterisation of rhamnogalacturonan oligomers generated by controlled acid hydrolysis of sugar-beet pulp

Abstract

Controlled acid hydrolysis was applied to a deesterified beet pulp and the resulting soluble fraction was fractionated on a Biorad AG 1X8 column eluted by ammonium acetate pH 6 from 0.05 to 2 M. Eight retained fractions were obtained, containing almost exclusively GalA and Rha. Three types of oligomers could be identified: homogalacturonans, of which mono-, di- and tri-GalA were isolated as individual components, and two series of rhamnogalacturonan (RG) oligomers. One RG oligomer, isolated after ion-exchange chromatography, was identified as α- d-GalA p-(1 → 2)-α- l-Rha p-(1 → 4)-α- d-GalA p-(1 → 2)- l-Rha p . The major peak contained oligomers of dp 6 to more than 20, of which dp 6 to 16 could be isolated on Bio-Gel P-6 + P-4. NMR of the oligomers of dp 6 to 10 showed the following structure: α- d-GalA p-(1[→ 2)-α- l-Rha p-(1 → 4)-α- d-GalA p-(1] n → 2)- l-Rha p . A second, quantitatively minor, series of RG oligomers eluted at higher ionic strength. These oligomers, which could be hydrolysed by RG-hydrolase and RG-lyase, were based on the alternating RG structure. Their non-reducing end was GalA, susceptible to hydrolysis by RG-galacturonohydrolase, and their reducing end might have more than one consecutive GalA. Most of the rhamnose present in sugar-beet pulp could be accounted for in oligomers of the structure: α- d-GalA p-(1[ → 2)-α- l-Rha p-(1 → 4)-α- d-GalA p-(1] nar2)- l-Rha p .