“Internal Residue Loss”: Rearrangements Occurring during the Fragmentation of Carbohydrates Derivatized at the Reducing Terminus

Rearrangement reactions involving migration of fucose and, occasionally, other residues have been found in the CID spectra of [M + H]+ and [M + 2H]2+ ions, but not [M + Na]+ ions, generated from several O-linked carbohydrates and milk sugars derivatized at their reducing termini with aromatic amines such as 2-aminobenzamide. Such rearrangements, which are similar to those reported by other investigators from several underivatized carbohydrates and glycosides, cause an apparent loss of sugar residues from within a carbohydrate chain and can produce ambiguous results during spectral interpretation. A mechanism, involving initial protonation of the amine nitrogen atom of the derivative, is proposed to account for the formation of the observed ions.