Positive and negative electrospray ionisation tandem mass spectrometry as a tool for structural characterisation of acid released oligosaccharides from olive pulp glucuronoxylans Original Research Article

Xylo-oligosaccharides with degrees of polymerisation 5–13, formed by partial acid hydrolysis from an extract representative of olive pulp glucuronoxylans (GX), were analysed by electrospray ionisation mass spectrometry (ESI-MS), both in positive and negative modes. The positive spectrum showed the presence of xylo-oligosaccharides in the mass range between m/z 500 and 1500 corresponding to singly [M+Na]+ charged ions of neutral (Xyl7–9) and acidic xylo-oligosaccharides (Xyl5–9MeGlcA), and doubly [M+2Na]2+ charged ions of Xyl9–13 and Xyl7–11MeGlcA. Ammonium adducts [M+NH4]+ were also observed for Xyl5–9MeGlcA. The negative spectra showed the contribution of ions in the mass range between m/z 600 and 1400, ascribed to the deprotonated molecules [M−H]− of Xyl3–9MeGlcA. Tandem mass spectrometry (MS/MS) of the major ions observed in the MS spectra was performed. The MS/MS spectra of the [M+Na]+ adducts showed the loss of MeGlcA residues as the major fragmentation pathway and glycosidic fragment ions of Xyln and XylnMeGlcA structures. The MS/MS spectra of the [M+NH4]+ adducts suggests the occurrence of isomers of Xyl5–9MeGlcA oligosaccharides with the MeGlcA residue at the reducing end and at the non-reducing end of the molecules, although other structural isomers can also occur. Both glycosidic bond and cross-ring cleavages in the MS/MS spectra of the [M−H]− ion suggest the occurrence of Xyl3–9MeGlcA with the substituting group at the reducing end position of the xylose backbone, as the main fragmentation ions. The results obtained by ESI-MS/MS, both in positive and negative modes, of Xyl7–13- and Xyl5–11MeGlcA, allow to identify fragmentation patterns of the structural isomers with MeGlcA linked to the terminal xylosyl residues of the oligosaccharides. The occurrence of these higher molecular weight oligosaccharides with a low substitution pattern allows to infer a scatter and random distribution of MeGlcA along the xylan backbone of olive pulp.