Structural characterization of the pectic polysaccharide, rhamnogalacturonan-II

An octasaccharide was released from sycamore cell wall rhamnogalacturonan-II (RG-II) by selective acid hydrolysis of the glycosidic linkages of apiosyl residues and purified to homogeneity by gel-permeation and high-performance anion-exchange chromatographies. The octasaccharide 1 contains a terminal nonreducing β-l-arabinofuranosyl residue linked to position 2 of the α-l-rhamnopyranosyl residue of the aceric acid-containing heptasaccharide 2 that had been previously isolated from RG-II [M.W. Spellman et al. Carbohydr. Res., 122 (1983) 131–153]. Heptasaccharide 2 and octasaccharide 1 were found to be mono- or di-O-acetylated. The O-acetyl groups were located, by ESMSMS, on the terminal nonreducing 2-O-methyl-α-l-fucosyl residue and/or on the 2-linked β-l-acetyl acid residue. Octasaccharide 1 and heptasaccharide 2 have the following structures: Six other oligosaccharides related to 1 and 2 were also generated by the acid hydrolysis of RG-II, and their partial structures were determined. The backbone of RG-II was shown, using a combination of exo- and endo-polygalacturonase digestions and partial acid hydrolysis, to be composed of up to eleven linear 1,4-linked α-d-galactosyluronic acid residues. These results provide additional information about the structure of RG-II.