SCARCITY OR COMPLETE LACK OF SINGLE RHAMNOSE RESIDUES INTERSPERSED WITHIN THE HOMOGALACTURONAN REGIONS OF CITRUS PECTIN

Commercial citrus pectin containing galacturonic acid and rhamnose in a ratio of similar to 40:1 was saponified and then exhaustively digest ed with endopolygalacturonase (EPG). The products were separated by ul trafiltration into low-molecular-weight (LMW) and high-molecular-weigh t (HMW) fractions. The LMW fraction accounted for 80% of the starting material, but for only 10% of the total rhamnose, The molar ratio of g alacturonic acid to rhamnose of the LMW fraction was 236, suggesting t hat very few small Rha-containing oligomers were generated by the EPG digestion. No distinct Rha-containing oligomers were found by various chromatographic analyses of the LMW fraction. The HMW fraction, which only accounted for 10% by weight of the starting pectin, contained mor e than 85% of the rhamnose. The ratio of GalA to Rha in the HMW fracti on was 1.7:1 and partial acid hydrolysis of this fraction produced a s eries of oligomers consisting of GalA-Rha repeating units, suggesting that it contained rhamnogalacturonan, which has a backbone composed of GalA-Rha disaccharide repeating units. The HMW fraction also containe d large amounts of arabinose and galactose, which probably originated from side chains linked to some of the rhamnose residues. We propose t hat commercial citrus pectin is composed of two regions: the predomina nt region consists of chains of uninterrupted 1,4-linked alpha-D-GalA residues with between 60-70% of the residues methyl esterified; and th e other region consists of rhamnogalacturonan with a backbone composed of GalA-Rha disaccharide repeating units and neutral sugar side chain s. (C) 1998 Elsevier Science Ltd. All rights reserved.