Amino and Acetamide Functional Group Effects on the Ionization and Fragmentation of Sugar Chains in Positive-Ion Mass Spectrometry

To elucidate the influence of amino (-NH₂) and acetamide (-NHCOCH₃, -NAc) groups in sugar chains on their ionization and fragmentation, cycloamyloses (cyclodextrins, CyDs) and lacto-oligosaccharide are analyzed by MALDI TOF/TOF and ESI Q-TOF mass spectrometry. CyD derivatives substituted by amino or acetamide groups are ideal analytes to extract the function group effects, which are amino-CyD with one hexosamine (HexNH₂) and acetamide-CyD with one N-acetyl hexosamine (HexNAc). Interestingly, the relative ion intensities and isotope-like patterns in their product ion spectra depend on the functional groups and ion forms of sugar chains. Consequently, the results indicate that a proton (H⁺) localizes on the amino group of the amino sugar, and that the proton (H⁺) induces their fragmentation. Sodium cation (Na⁺) attachment is independent from amino group and exerts no influence on their fragmentation patterns in amino group except for mono- and disaccharide fragment ions because there is the possibility of the reducing end effect. In contrast, a sodium cation localizes much more frequently on the acetamide group in acetamide-CyDs because the chemical species with HexNAc are stable. Thus, their ions with HexNAc are abundant. These results are consistent with the fragmentation of lacto-neo-N-tetraose and maltotetraose, suggesting that a sodium cation generally localizes much more frequently on the acetamide group in sugar chains.