title: Positive and negative electrospray ionisation tandem mass spectrometry as a tool for structural characterisation of acid released oligosaccharides from olive pulp glucuronoxylans.
abstract: 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 (Xyl(7-9)) and acidic xylo-oligosaccharides (Xyl(5-9)MeGlcA), and doubly [M+2Na](2+) charged ions of Xyl(9-13) and Xyl(7-11)MeGlcA. Ammonium adducts [M+NH(4)](+) were also observed for Xyl(5-9)MeGlcA. 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 Xyl(3-9)MeGlcA. 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 Xyl(n) and Xyl(n)MeGlcA structures. The MS/MS spectra of the [M+NH(4)](+) adducts suggests the occurrence of isomers of Xyl(5-9)MeGlcA 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 Xyl(3-9)MeGlcA 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 Xyl(7-13)- and Xyl(5-11)MeGlcA, 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.
authors.affiliation: Departamento de Química, Universidade de Aveiro, P-3810 193 Aveiro, Portugal.
authors.forename: Ana
authors.initials: A
authors.lastname: Reis
authors.forename: M Rosário M
authors.initials: MR
authors.lastname: Domingues
authors.forename: Pedro
authors.initials: P
authors.lastname: Domingues
authors.forename: A J
authors.initials: AJ
authors.lastname: Ferrer-Correia
authors.forename: Manuel A
authors.initials: MA
authors.lastname: Coimbra
mesh_terms: D002240:Carbohydrate Sequence; D006868:Hydrolysis; D008969:Molecular Sequence Data; D008970:Molecular Weight; D031658:Olea / Q000737:chemistry*; D009844:Oligosaccharides / Q000032:analysis* / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D014990:Xylans / Q000737:chemistry*; D014994:Xylose / Q000032:analysis* / Q000737:chemistry
chemical_list: D009844:Oligosaccharides; D014990:Xylans; C038910:glucuronoxylan; D014994:Xylose