Postsource decay fragmentation of N-linked carbohydrates from ovalbumin and related glycoproteins

N-linked glycans were released from chicken ovalbumin by hydrazinolysis and examined by matrix-assisted laser desorption/ionization mass spectrometry. Postsource decay analysis showed that most fragment ions arose as the result of internal glycosidic cleavages involving loss of nonreducing terminal residues from ions that had lost one or both GlcNAc residues from the chitobiose core [GlcNAcbeta(1 --> 4)GlcNAc]. Cross-ring fragments were abundant from the reducing-terminal GlcNAc but other cross-ring fragments were weak. The ion found to be most useful for determining the composition of the antennae attached to the 3- or 6-linked core mannose residues was an internal cleavage ion formed by loss of both the chitobiose core and the antenna linked to the 3-position of the core branching mannose. This ion was observed to lose water in the absence of a "bisecting" GlcNAc residue (beta1 --> 4 linked to the core mannose) and to lose a GlcNAc molecule (221 mass units) when a bisecting GlcNAc residue was present.