Occurrence of cellobiose residues directly linked to galacturonic acid in pectic polysaccharides

The study carried out in this work concerns the structural characterization of pectic polysaccharides from plum (Prunus domestica L.) and pear (Pyrus communis L.) cell walls and commercial pectic polysaccharides, obtained from Citrus. The α-(1→4)-d-galacturonic acid backbone was submitted to a selective hydrolysis with endo-polygalacturonase (EPG) and the fractions with low molecular weight (<1kDa) obtained by size-exclusion chromatography were analysed by mass spectrometry using electrospray ionisation (ESI-MS). The ESI-MS spectra obtained revealed the presence of several [M+Na]⁺ ions of pectic oligosaccharides identified as belonging to different series, including oligosaccharides constituted only by galacturonic acid residues (GalA_n, n=1-5) and galacturonic acid residues substituted by pentose residues (GalA₃Pent_n, n=1-2). Surprisingly, it was also observed the occurrence of galacturonic acid residues substituted by hexose residues (GalA_nHex_m, n=2-4, m=1-2). The fragmentation of the observed [M+Na]⁺ ions, obtained under ESI-MS/MS and MSⁿ allowed to confirm the proposed structures constituent of these pectic oligosaccharides. Furthermore, the ESI-MSⁿ spectra of the ions that could be identified as GalA_nHex_m (n=2-4, m=1-2) confirmed the presence of Hex or Hex₂ residues linked to a GalA residue. Methylation analysis showed the presence, in all EPG treated samples, of terminally linked arabinose, terminally and 4-linked xylose, and terminally and 4-linked glucose. The occurrence of GalA substituted by Glc, and Glc-β-(1→4)-Glc are structural features that, as far as we know, have never been reported to occur in pectic polysaccharides.