Identification and Relative-quantification of Glycans by Matrix-assisted Laser Desorption/Ionization In-Source Decay with Hydrogen Abstraction

The use of specific matrices allows enhancing the scope of in-source decay (ISD) applications
in matrix-assisted laser desorption ionization (MALDI) thanks to the specificity of
analyte-matrix chemistry. The use of an oxidizing matrix, 5-nitrosalicylic acid (5-NSA) for
MALDI-ISD of glycans is shown to promote fragmentation pathways involving radical
precursors. Both glycosidic and cross-ring cleavages are promoted by hydrogen abstraction
from hydroxyl group of glycans by 5-NSA molecules. Cross-ring cleavage ions are potentially
useful in linkage analysis, one of the most critical steps of glycan characterization. Moreover,
we show here that isobaric glycans could be distinguished by structure specific ISD ions, and
that the molar ratio of glycan isomers in the mixture can be estimated from their fragment
ions abundance. The use of 5-NSA also opens the possibility to perform pseudo-MS3 analysis
of glycans. Therefore, MALDI-ISD with 5-NSA is a useful method for identification of
glycans and semi-quantitative analysis of mixture of glycan isomers.