STRUCTURAL CHARACTERIZATION OF THE PECTIC POLYSACCHARIDE, RHAMNOGALACTURONAN-II

An octasaccharide was released from sycamore cell wall rhamnogalacturo nan-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 beta-L-arabinofuranosyl residue link ed to position 2 of the alpha-L-rhamnopyranosyl residue of the aceric acid-containing heptasaccharide 2 that had been previously isolated fr om RG-II [M.W. Spellman et al. Carbohydr. Res., 122 (1983) 131-153]. H eptasaccharide 2 and octasaccharide 1 were found to be mono- or di-O-a cetylated. The O-acetyl groups were located, by ESMSMS, on the termina l nonreducing 2-O-methyl-alpha-L-fucosyl residue and/or on the 2-linke d beta-L-aceryl acid residue. Octasaccharide 1 and heptasaccharide 2 h ave the following structures: [GRAPHICS] Six other oligosaccharides re lated 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 wa s shown, using a combination of exo- and endo-polygalacturonase digest ions and partial acid hydrolysis, to be composed of up to eleven linea r 1,4-linked alpha-D-galactosyluronic acid residues. These results pro vide additional information about the structure of RG-II.