Glucuronic acid directly linked to galacturonic acid in the rhamnogalacturonan backbone of beet pectins

Abstract

Sugar-beet pulp was de-esterified and submitted to 72 h hydrolysis by 0.1 M HCl at 80°C. Oligomers containing a single glucuronic acid (GlcA) moiety in addition to n(≥ 2) repeats of the dimer →4)-α-D-GalpA-(1→2)-α- LRhap-(1→ were isolated from the hydrolysate by ion-exchange and gel- permeation. Glycosyl linkage composition analysis and 1H NMR studies indicated that the GlcA was attached to O-3 of a galacturonic acid (GalA) residue, as shown for the two pentamers β-D-GlcpA-(1 → 3)-α-D-GalpA- (1→2)-α-L-Rhap-(1→4)-α-DGalpA-(1→2)-L-Rhap and α-D-GalpA-(1 → 2)-α-L- Rhap-(1 → 4)-[β-D-GlcpA-(1 → 3)]-α-D-GalpA-(1→2)-LRhap. Substitution by GlcA was estimated as occurring on one Gala residue out of 72 in the rhamnogalacturonan fraction of the backbone of beet pectins.