Studies of the length of homogalacturonic regions in pectins by acid hydrolysis

The different susceptibilities to acid hydrolysis of the glycosidic linkages in a pectin backbone were used to isolate fractions corresponding to the “smooth”, homo- d-galacturonic regions. Pectins from apple, beet, and citrus were de-esterified, and the resulting pectic acids were hydrolysed in 0.1 M HCl at 80°C for up to 72 h. The intrinsic viscosities of the hydrolysates decreased, and two stages could be distinguished. Up to 10 h, there was a fast decrease, corresponding to the cleavage of the more susceptible linkages between l-rhamnose and galacturonic acid residues, followed by a slower stage, corresponding to cleavage of the linkages between galacturonic acid residues. During the course of the reaction, some galacturonic acid and most of the neutral sugars were solubilised, giving two fractions on Sepharose CL-6B. A minor fraction, composed mostly of galacturonic acid and rhamnose, with rhamnose-galacturonic acid ratios of 1:1.5, 1:2.9, and 1:2.1 for apple, beet, and citrus, respectively, eluted at K av 0.8, and a major fraction, composed essentially of l-arabinose and d-galactose, eluted at the total volume. The acid-insoluble materials represented 84, 66, and 90% of the original pectic acids for apple, beet, and citrus, respectively. They were progressively freed of neutral sugars; after hydrolysis for 72 h, almost pure polygalacturonates (more than 98 mol% galacturonic acid), representing the homogalacturonic regions, were obtained. The molecular weights of these 72-h acid-insoluble materials from apple, beet, and citrus were similar (respectively, 21000, 19000, and 24000), corresponding to lengths of the homgalacturonic regions estimated to be a minimum of 72–100 galacturonic acid residues.