Structure of the repeating units in the rhamnogalacturonic backbone of apple, beet and citrus pectins

After controlled acid hydrolysis of de-esterified apple, beet and citrus pectins, a fraction rich in galacturonic acid and rhamnose was isolated and further fractionated by ion-exchange chromatography. The main peaks, representing for each pectin > 30% of the rhamnose-rich fraction, and 50–78% of its rhamnose, were composed of rhamnose and galacturonic acid in equimolar proportions. Gel-permeation chromatography showed series of homologous oligomers with polymerisation degrees of 6 to 20. NMR data indicated a repetitive structure based on the → 4)-α-d-GalpA-(1 → 2)-α-l-Rhap-(1 → unit, where the reducing ends were rhamnose units and the non-reducing ends galacturonic acid residues. The same rhamnogalacturonan oligomers were obtained from apple, beet and citrus pectins.