The occurrence of internal (1→5)-linked arabinofuranose and arabinopyranose residues in arabinogalactan side chains from soybean pectic substances

CDTA-extractable soybean pectic substances were subjected to enzymatic digestion with arabinogalactan degrading enzymes yielding a resistant polymeric pectic backbone and arabino-, galacto-, and arabinogalacto-oligomers. The complex digest was fractionated using size-exclusion chromatography. Monosaccharide composition analysis, HPAEC fractionation and MALDI-TOF MS analysis of the resulting fractions showed that each contained a mixture of oligosaccharides of essentially the same degree of polymerisation, composed of only arabinose and galactose. MALDI-TOF MS analysis was used for molecular mass screening of oligosaccharides in underivatised HPAEC fractions. The monosaccharide sequence and the branching pattern of oligosaccharides (degree of polymerisation from 4 to 8) were determined using linkage analysis and ES-CID tandem MS analysis of the per-O-methylated oligosaccharides in each of the HPAEC fractions. These analyses indicated the presence of common linear (1-->4)-linked galacto-oligosaccharides, and both linear and branched arabino-oligosaccharides. In addition, the results unambiguously showed the presence of oligosaccharides containing (1-->4)-linked galactose residues bearing an arabinopyranose residue as the son-reducing terminal residue, and a mixture of linear oligosaccharides constructed of (1-->4)-linked galactose residues interspersed with an internal (1-->5)-linked arabinofuranose residue. The consequences of these two new structural features of pectic arabinogalactan side chains are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.