STRUCTURAL CHARACTERIZATION OF RED WINE RHAMNOGALACTURONAN-II

The pectic polysaccharide rhamnogalacturonan II (RG-II), which account s for similar to 20% of the ethanol-precipitable polysaccharides in re d wine, has been isolated from wine polysaccharides by anion-exchange chromatography. Four fractions enriched with RG-II were obtained and t he RG-II then purified to homogeneity by Concanavalin A affinity and s ize-exclusion chromatographies. The glycosyl-residue compositions of t he four RG-IIs are similar, all the RG-IIs contain the monosaccharides (apiose, 2-O-methyl-L-fucose, 2-O-methyl-D-xylose, Kdo, Dha, and acer ic acid) that are diagnostic of RG-II. The glycosyl-linkages of the ne utral and acidic sugars, including aceric acid, were determined simult aneously by GC-EIMS analysis of the methylated alditol acetates genera ted from per-O-methylated acid carboxyl-reduced RG-II, Two of the RG-I Is contain boron, most likely as a berate di-ester that cross-links tw o molecules of RG-II together to form a dimer. The dimer contains 3'- and 2,3,3'-linked apiosyl residues whereas the monomer contains only 3 '-linked apiosyl residues which suggests that the berate di-ester is l ocated on at least one of the apiosyl residues of RG-II. Although the wine RG-IIs all have similar structures they are not identical since t hey differ in the length and degree of methyl-esterification of the RG -II backbone and in the presence or absence of berate di-esters. Never theless, these studies show that the major structural features of wine and primary cell wall RG-II are conserved. (C) 1996 Elsevier Science Ltd.