Post-source decay fragmentation analyses of linkage isomers of Lewis-type oligosaccharides in curved-field reflectron matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: combined in-source decay/post-source decay experiments and relative ion abundance analysis

Linkage isomers of LewisX trisaccharide (LeX) and Lewisa trisaccharide (Lea) were distinguished by the post‐source decay (PSD) fragment spectra obtained by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS) without permethylation. Both Y‐ and Z‐type fragmentations were observed at the C‐3 position of N‐acetylhexosamine. β‐Elimination at C‐3 of the reducing‐end N‐acetylglucosamine in LeX formed a double bond, which conjugated to an N‐acetyl group, making the chemical species stable. In contrast, the double bond formed in the reducing end glucose of 3‐fucosyllactose was unstable owing to the lack of a conjugated system. Therefore, β‐elimination of N‐acetylglucosamine occurred predominantly rather than that of hexose in MALDI‐PSD fragmentation. The measurements of the PSD fragment mass spectra using pseudo precursor ions originating from in‐source decay were useful for the analyses of the fragmentation mechanisms and for the assignments of the chemical species of the fragment ions. The combined in‐source decay/post‐source decay experiments revealed the formation of a double bond between C‐2 and C‐3 in N‐acetylglucosamine of LeX. Abundance analysis of the PSD ions indicated that the 1–3 glycosyl linkage cleaves more easily than does the 1–4 linkage in MALDI‐PSD fragmentation. Ion abundance analyses were useful in estimating the degree of Y‐ and Z‐type fragmentation at the C‐3 position of hexose and N‐acetylhexosamine. The analysis of the relative ion abundances was a powerful tool for the assignments of the chemical species of the PSD ions. Copyright © 2000 John Wiley & Sons, Ltd.