Differentiating structural isomers of sialylated glycans by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry

Using model acidic glycans, we demonstrate the benefits of permethylation for matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) tandem mass spectrometry. With both the linear and branched structures, extensive cross-ring fragmentation product ions were generated, yielding valuable information on sugar linkages. Elimination of the negative charges commonly associated with sialylated structures through permethylation allowed their structural analysis in the positive ion mode. Extensive A- and X-type ions were observed for the linear structures, and slightly weaker signals for the branched sialylated structures. The diagnostic cross-ring fragments, permitting a distinction between α2-3 and α2-6 linkages of the sialic acid residues, were seen in abundance. Importantly, the cross-ring fragmentation with the branched structures provides adequate information to assign sialic acid residues, with a specific linkage, to a particular antenna.