MSn of the six isomers of (GlcN)2(GlcNAc)2 aminoglucan tetrasaccharides (diacetylchitotetraoses): Rules of fragmentation for the sodiated molecules and application to sequence analysis of hetero-chitooligosaccharides

The six possible isomers of di-N-acetylchitotetraoses [AADD, ADDA, ADAD, DADA, DAAD, and DDAA, where D stands for 2-amino-2-deoxy-β-d-glucose (GlcN) and A for 2-acetamido-2-deoxy-β-d-glucose (GlcNAc)] were analyzed by ESI(+)-MSn. Collision induced dissociation via MSn experiments were performed for the sodiated molecules of m/z 769 [M+Na]+ for each isomer, and fragments were generated mainly by glycosidic bond and cross-ring cleavages. Rules of fragmentation were then established. A reducing end D residue yields the O,2A4 cross-ring [M−59+Na]+ fragment of m/z 710 as the most abundant, whereas isomers containing a reducing end A prefer to lose water to form the [M−18+Na]+ ion of m/z 751, as well as abundant O,2A4 cross-ring [M−101+Na]+ fragments of m/z 668 and B3 [M−221+Na]+ ions of m/z 548. MS3 of C- and Y-type ions shows analogous fragmentation behaviour that allows identification of the reducing end next-neighbour residue. Due to gas-phase anchimeric assistance, B-type cleavage between the glycosidic oxygen and the anomeric carbon atom is favoured when the glycon is an A residue. Relative ion abundances are generally in the order B ≫ C > Y, but may vary depending on the next neighbour towards the non-reducing end. These fragmentation rules were used for partial sequence analysis of hetero-chitooligosaccharides of the composition D2A3, D3A3, D2A4, D4A3, and D3A4.