Isolation and characterization of a boron-rhamnogalacturonan-II complex from cell walls of sugar beet pulp

A boron (B)-polysaccharide complex was isolated from a Driselase digest of sugar beet (Beta vulgaris L.) cell walls by ion-exchange and gel-permeation chromatography. The complex contained 0.12% B (w/w). The polysaccharide moiety contained 2-O-methylfucose, rhamnose, fucose, 2-O-methylxylose, arabinose, apiose, galactose, aceric acid, galacturonic, and glucuronic acids residues, and thiobarbituric acid-assay positive sugars, presumably 3-deoxy-d-manno-2-octulosonic acid (Kdo) and 3-deoxy-d-lyxo-2-heptulosaric acid (Dha). Methylation analysis, together with glycosyl composition analysis, showed that the polysaccharide was a typical rhamnogalacturonan-II (RG-II), a structurally complex pectic polysaccharide present in the primary cell walls of plants. 11B NMR spectroscopy showed that the B was present as a tetrahedral borate-diol diester. Approximately 70% of B was released by treating the B—RG-II complex at pH 4.8 at 40 °C.