Structural differentiation of uronosyl substitution patterns in acidic heteroxylans by electrospray tandem mass spectrometry

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Abstract
The structures of two oligomers of acidic xylo-oligosaccharides (XOS) of the same molecular weight (634 Da), Xyl2MeGlcAHex and Xyl2GlcA2 were differentiated by electrospray tandem mass spectrometry (ESI-MS/MS). These oligomers were present in a mixture of XOS obtained by acid hydrolysis of heteroxylans extracted from Eucalyptus globulus wood (Xyl2MeGlcAHex) and Olea europaea olive fruit (Xyl2GlcA2). In the ESI-MS spectra of the XOS, ions at m/z 657 and 652 were observed and assigned to [M+Na]+ and [M+NH4]+, respectively. The ESI-MS/MS spectrum of [M+Na]+ ion of Xyl2MeGlcAHex showed the loss of Hex residue from the reducing end followed by the loss of MeGlcA moiety. Simultaneously, the loss of a Xyl residue from either the reducing or the non-reducing ends was detected. On the other hand, the fragmentation of Xyl2GlcA2 occurs mainly by the loss of one and two GlcA residues or by the loss of the GlcAXyl moiety, due to the glycosidic bond cleavage between the two Xyl residues. Loss of one and two CO2 molecules was only observed for this oligomer, where the GlcA are in vicinal Xyl residues. The ESI-MS/MS spectra of [M+NH4]+ of both oligomers showed the loss of NH3, resulting in the protonated molecule, where the presence of ions assigned as protonated molecules of aldobiuronic acid residues, [MeGlcA − Xyl+H]+ and [GlcA − Xyl+H]+, are diagnostic ions of the presence of MeGlcA and GlcA moieties in XOS. Since these structures occur in small amounts in complex acidic XOS mixtures and are very difficult, if possible, to isolate, tandem mass spectrometry revealed to be a powerful tool for the characterization of these types of substitution patterns present in heteroxylans.

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