Fragmentation of Lewis-type trisaccharides in the gas phase: Experimental and theoretical studies

The mechanisms of fragmentation of the chlorinated Lewis-type trisaccharides Lewis a and Lewis X were studied by using experimental and theoretical methods. In the fragment ion spectra obtained by ESI-CID-MS, C- and Z-type fragmentations corresponding to 4- and 3-linked glycosyl bond cleavages of the N-acetylglucosamine moiety were observed. The relationships of the relative ion intensities of C- and Z-type fragmentations were Z > C in Lewis a and C > Z in Lewis X, suggesting that the relationships of the activation energies were C > Z in Lewis a and Z > C in Lewis X. Anomeric and acetoamide groups were assumed to be included in the mechanisms of C- and Z-type fragmentation; we therefore analyzed 3-fucosyllactose and methylated Lewis-type trisaccharides, which are the derivatives of Lewis-type trisaccharides. These analyses revealed that C-type fragmentation was little influenced by anomeric group and that Z-type fragmentation was influenced by both the anomeric and the acetoamide groups. Fragmentation mechanisms were proposed on the basis of the experimental results and were calculated at the HF/3-21G(d), HF/6-31G(d) levels. From the calculations, C-type fragmentation of the 4-linkage was considered to take place by electron transfer from the acetoamide group, and Z-type fragmentation at the 3-linkage was considered to take place in two steps: (i) elimination of the 3-linked saccharide moiety, and (ii) deprotonation of an anomeric proton by a chloride anion. The activation energies for C- and Z-type fragmentations, as determined by HF calculations, were consistent with the experimentally assumed trends.