Pectic substances from red beet ( Beta vulgaris L. var. conditiva). Part II. Structural characterisation of rhamnogalacturonan II

Abstract

Cell wall material from ripe red beet ( Beta vulgaris L. var. conditiva) was isolated as alcohol-insoluble residue (AIR). The cyclohexane- trans-1,2-diaminotetraacetate (CDTA) soluble extract (CDTA-soluble pectin) was fractionated by anion exchange and gel filtration chromatography. The main fraction was degraded with an endo-α-(1→4)-polygalacturonase and further fractionated by gel filtration chromatography. Methylation analysis of the intermediate-molecular weight fractions was performed after carbodiimide-activated reduction of the pectic polysaccharides with NaBD 4. Besides galacturonic acid and rhamnose, the fraction contained rare sugar residues, such as apiose, 2- O-methyl-xylose, 2- O-methyl-fucose, 3,4-linked fucose, 2,3,4-linked rhamnose, 2-linked glucuronic acid, aceric acid, 3-deoxy- d-manno-2-octulosonic acid (Kdo), and 3-deoxy- d-lyxo-2-heptulosonic acid (Dha) which are characteristic for rhamnogalacturonan II (RG-II). The constituents were present in amounts which approximately corresponded to the structural model for RG-II. The molecular weight of RG-II was determined by MALDI-TOF-MS and showed to contain two populations with molecular weights of about 4100 and 4300, respectively. Arabinopyranose residues were shown to occur only terminally linked. This result suggests that the aceric acid containing side-chain of red beet RG-II is shorter than the one from other plants. 1,3,4-linked galactose and 1,2,3,4-linked galacturonic acid were detected as well, their positions within the RG-II molecule remain to be established.