Isolation and structural characterization of β- d-glucosyluronic acid and 4- O-methyl β- d-glucosyluronic acid-containing oligosaccharides from the cell-wall pectic polysaccharide, rhamnogalacturonan I

Abstract

Rhamnogalacturonan I (RG-I), a pectic polysaccharide isolated from the walls of suspension-cultured sycamore cells, was shown by glycosyl-residue composition analysis to contain d-glucosyluronic acid (Glc pA) residues (1 mol%) and 4- O-methyl- d-glucosyluronic acid (4- O-Me-Glc pA) residues (0.5 mol%). These monosaccharides were shown, by glycosyl-linkage analysis, to be present in RG-I as terminal nonreducing residues. The glycosyl sequences containing Glc pA and 4- O-Me-Glc pA were determined by structurally characterizing the acidic oligosaccharides released by partial acid hydrolysis of RG-I. Six acidic oligosaccharides were purified by semipreparative high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and characterized by glycosyl-residue and glycosyl-linkage composition analyses, GLC-CIMS, GLC-EIMS, electrospray MS (ESMS), and 1H NMR spectroscopy. We propose that three of the acidic oligosaccharides characterized, 4- O-Me-β- d-Glc pA-(1 → 6)- d-Gal, β- d-Glc pA-(1 → 6- d-Gal, and β- d-Glc pA-(1 → 4)- d-Gal, originate from the galactosyl-containing side chains of RG-I. The three other acidic oligosaccharides characterized, α- d-Gal pA-(1 → 2)- l-Rha, α- d-Gal pA-(1 → 2)-α l-Rha p-(1 → 4)-α- d-Gal pA-(1 → 2)-α- l-Rha, and α- d-Gal pA-(1 → 2)-α- l-Rha p-(1 → 4)-α- d-Gal pA-(1 → 2)-α- l-Rha p-(1 → 4)-α- d-Gal pA-(1 → 2 )-α- l-Rha, were generated by partial hydrolysis of the RG-I backbone. No evidence was obtained for the presence of galactosyluronic acid in the side chains of RG-I. To our knowledge this is the first report that rhamnogalacturonan I contains glycosyluronic acid or 4- O-methyl glucosyluronic acid residues.