Differentiation of isomeric Lewis blood groups by positive ion electrospray tandem mass spectrometry

Lewis histo-blood group antigens are one of the major classes of biologically active oligosaccharides. In this work, underivatized Lewis blood groups were studied by electrospray tandem mass spectrometry (ESI-MSn) in the positive mode with three different mass analyzers: Q-TOF (quadrupole time-of-flight), QqQ (triple quadrupole), and LIT (linear ion trap). It was observed that, under collision-induced fragmentations, type 1 Lewis antigens (Lea and Leb) could be distinguished from type 2 (Lex and Ley) on the basis of specific fragmentations of the GlcNAc unit. Whereas O-4-linked sugars of the GlcNAc are lost as residues, the O-3-linked sugars undergo fragmentation both as sugar units and as sugar residues (unit −18 Da). Type 2 Lewis antigens also showed a characteristic cross-ring cleavage 0,2A2 of the GlcNAc. As a result, the product ions at m/z 388 and 305, characteristic of Lex, and m/z 372, characteristic of Lea, are proposed to distinguish the trisaccharide isomers Lex/Lea. Also, the product ions at m/z 534 and 305, characteristic of Ley, and m/z 372, characteristic of Leb, are proposed to distinguish the tetrasaccharide isomers Leb/Ley. These diagnostic fragment ions were further applied in the identification of Lewis type 2 antigens (Lex and Ley) in the lipopolysaccharide of the human gastric pathogen, Helicobacter pylori.