Structural characterization of oligosaccharides isolated from the pectic polysaccharide rhamnogalacturonan II

Abstract

Rhamnogalacturonan II (RG-II) is a structurally complex pectic ( d-galactosyl-uronic acid-rich) polysaccharide that is present in the primary (growing) cell-walls of higher plants. RG-II is composed of ∼60 glycosyl residues. The isolation and structural characterization of 23 oligosaccharide fragments of the residue of RG-II that remained after removal of hepta- and di-saccharides by partial hydrolysis with acid are reported. In order to obtain the oligosaccharide fragments characterized herein, the carboxyl groups of RG-II were dideuterio-reduced, and the carboxyl-reduced polysaccharide was per- O-methylated. The per- O-methylated polysaccharide was fragmented by partial hydrolysis with acid, producing partially O-methylated oligosaccharides. These derivatized oligosaccharides were reduced, to afford a mixture of partially O-methylated oligoglycosyl-alditols, which was then per- O-methylated. The structures of the resulting per- O-methylated oligoglycosylalditols were determined by chemical-ionization mass spectrometry, electron-impact mass spectrometry, fast-atom-bombardment mass spectrometry, 1H-n.m.r. spectroscopy, and analysis of corresponding, partially O-acetylated, partially O-methylated alditols. Seventeen of the oligosaccharides isolated from RG-II were parts of a single heptasaccharide, namely.