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 ∼40:1 was saponified and then exhaustively digested with endopolygalacturonase (EPG). The products were separated by ultrafiltration into low-molecular-weight (LMW) and high-molecular-weight (HMW) fractions. The LMW fraction accounted for 80% of the starting material, but for only 10% of the total rhamnose. The molar ratio of galacturonic acid to rhamnose of the LMW fraction was 236, suggesting that 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 more than 85% of the rhamnose. The ratio of GalA to Rha in the HMW fraction was 1.7:1 and partial acid hydrolysis of this fraction produced a series 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 contained large amounts of arabinose and galactose, which probably originated from side chains linked to some of the rhamnose residues. We propose that commercial citrus pectin is composed of two regions: the predominant region consists of chains of uninterrupted 1,4-linked α-d-GalA residues with between 60–70% of the residues methyl esterified; and the other region consists of rhamnogalacturonan with a backbone composed of GalA–Rha disaccharide repeating units and neutral sugar side chains.