Structural characterization of underivatized arabino-xylo-oligosaccharides by negative-ion electrospray mass spectrometry.

Various arabino-xylo-oligosaccharides with known substitution patterns were assessed by negative ESI-Q-TOFMS and ESI-ITMS. The CID spectra of linear xylo-oligosaccharides and of nine isomeric mono- and disubstituted arabino-xylo-oligosaccharides established that structures differing in their substitution pattern can be differentiated by this approach. The negative-ion fragmentation spectra of the deprotonated quasi-molecular ions are mainly characterized by glycosidic cleavage ions from the C-series, which provide sequence informations, and by cross-ring cleavage (0,2)A(i) ions, which provide partial linkage information. When the collision energy increased, the cross-ring cleavage (0,2)A(i) ions underwent consecutive loss of water to produce (0,2)A(i)-18 fragment ions and glycosidic cleavage ions of the B-series are also produced besides the C(i) ions. Contrary to linear xylo-oligosaccharides, C(i) ions, which originate from C-3 monosubstituted xylosyl residues never produce the related cross-ring cleavage (0,2)A(i) ions. Disubstitution at O-2 and O-3 of xylosyl residues appears to enhance the production of the (0,2)A(i) ions compared to monosubstitution. For the differentiation of the mono- and disubstitution patterns of the penultimate xylosyl residue, the relative abundance of the glycosidic cleavage ions at m/z 263 and 299 found on Q-TOF CID spectra plays a relevant role and appears to be more informative than MS(n) spectra obtained on a ion trap instrument.