A borate-containing pectin was solubilized from sugar beet (Beta vulgaris L. ) cell walls by treatment with 0.5 M imidazole, pH 7. The molecular weight of the pectin was reduced when the borate ester was hydrolyzed by treatment with 1 N HCl. Treatment of the acid-treated pectin with boric acid in the presence of Pb(2+) gave a product whose molecular weight distribution was similar to the imidazole-soluble pectin. The imidazole-soluble pectin was saponified and then digested with endo- and exo-polygalacturonases. These treatments shifted the boron peak at the high molecular weight region to the low molecular weight (10 kDa), which corresponds to rhamnogalacturonan II-borate ester cross-linked dimer (dRG-II-B). The treatment also generated rhamnogalacturonan I (RG-I), dRG-II-B, monomeric rhamnogalacturonan II and galacturonic acid. These results show that imidazole solubilizes a high molecular weight borate-containing pectic complex composed of homogalacturonan-rhamnogalacturonan II and RG-I. Our data suggest that borate esters formed between rhamnogalacturonan II molecules cross-link the macromolecular pectin.