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)-alpha-D-GalpA-(1 --> 2)-alpha-L-Rha p-(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.