Composition, Sequencing and Ion Mobility Mass Spectrometry of Heparan Sulfate-like Octasaccharide Isomers Differing in Glucuronic and Iduronic Acid Content

Here we report ion mobility mass spectrometry (IMMS) separation and tandem mass spectrometry (MS2) sequencing methods used to analyze and differentiate six synthetically produced heparin/heparan sulfate (HS)-like octasaccharide (dp8) isomeric structures. These structures are isomeric with regard to either glucuronic acid (GlcA) or iduronic acid (IdoA) residues at various positions. IMMS analysis showed that a fully GlcA structure exhibited a more compact conformation, whereas the fully IdoA structure was more extended. Interestingly, the change from IdoA to GlcA in specific locations resulted in strong conformational distortions. MS2 of the six isomers showed very different spectra with unique sets of diagnostic product ions. Analysis of MS2 product ion spectra suggests that the GlcA group correlated with the formation of a glycosidic product ion under lower energy conditions. This resulted in an earlier product ion formation and more intense product ions. Importantly, this knowledge enabled a complete sequencing of the positions of GlcA and IdoA in each of the four positions located in each unique dp8 structure.