Discrimination of Isomeric Fragment Ions Observed in Tandem Mass Spectra of Biantennary Oligosaccharides by Use of Selective Isotope Labeling

Abstract

We report herein the first mass spectrometric analyses of isotopomers of a pyridylamino (PA)-derivatized biantennary oligosaccharide, in which one of two non-reducing terminal galactose residues is selectively enriched in ¹³C by an enzymatic reaction. By use of these isotopomeric PA-oligosaccharides, individual fragment ions observed in tandem mass spectra of MALDI-QIT-TOF-MS were unambiguously assigned. It was shown that the oligosaccharide ion that lacks the N-acetyllactosamine previously linked to the Manα1 →6 antenna dominates its isomeric counterpart ion. We propose that tandem mass spectrometric analyses of oligosaccharides isotopically labeled at a selected antenna would provide us with the basis of mechanisms of formation of glycosidic linkage fragment ions.