Electrospray mass spectrometry and fragmentation of N-linked carbohydrates derivatized at the reducing terminus

Derivatives were prepared from N-linked glycans by reductive amination from 2-aminobenzamide, 2-aminopyridine, 3-aminoquinoline, 2-aminoacridone, 4-amino-N-(2-diethylaminoethyl)benzamide, and the methyl, ethyl, and butyl esters of 4-aminobenzoic acid. Their electrospray and collision-induced dissociation (CID) fragmentation spectra were examined with a Q-TOF mass spectrometer. The strongest signals were obtained from the [M + Na](+) ions for all derivatives except sugars derivatized with 4-amino-N-(2-diethylaminoethyl)benzamide which gave very strong doubly charged [M + H + Na](2+) ions. The strongest [M + Na](+) ion signals were obtained from the butyl ester of 4-aminobenzoic acid and the weakest from 2-aminopyridine. The most informative spectra were recorded from the [M + Li](+) or [M + Na](+) ions. These spectra were dominated by ions produced by sequence-revealing glycosidic cleavages and "internal" fragments. Linkage-revealing cross-ring cleavage ions were reasonably abundant, particularly from high-mannose glycans. Although the nature of the derivative was found to have little effect upon the fragmentation pattern, 3-aminoquinoline derivatives gave marginally more abundant cross-ring fragments than the other derivatives. [M + H](+) ions formed only glycosidic fragments with few, if any, cross-ring cleavage ions. Doubly charged molecular ions gave less informative spectra; singly charged fragments were weak, and molecular ions containing hydrogen ([M + 2H](2+) and [M + H + Na](2+)) fragmented as the [M + H](+) singly charged ions with no significant cross-ring cleavages. (J Am Soc Mass Spectrom 2000, 11, 900-915) (C) 2000 American Society for Mass Spectrometry.