Characterization of Collision-Induced Dissociation of Deprotonated Peptides of 4–16 Amino Acids Using High-Resolution Mass Spectrometry

Mass spectrometry (MS) analysis of peptides has traditionally been conducted in the positive ion mode on protonated peptides, although negative ion-mode analysis of deprotonated peptides can provide complementary and sometimes critical structural information. This is partly due to insufficient understanding of the fragmentation behaviors of deprotonated peptides. Here, using ion-trap collision-induced dissociation (CID) and higher-energy collisional dissociation (HCD, a beam-type CID), we characterized the fragmentation patterns of 36 deprotonated peptides of 4–16 amino acids on a high-resolution, high-mass accuracy instrument. Our study finds that among the backbone cleavage products, y-, c- and z-type ions (using a nomenclature similar to that of positive ions) are the most dominant species in both CID and HCD spectra of deprotonated peptides, accompanied by abundant neutral loss (NL) peaks. Similar to the charge-dependency of collisional energy of protonated peptides, we find that singly charged deprotonated peptides require significantly higher collisional energy than their doubly or multiply charged counterparts to reach 50% fragmentation. HCD is generally better than CID for peptide sequencing in the negative ion mode, since HCD generates more backbone cleavage products whereas CID produces predominately NL peaks of precursors. For disulfide-bonded peptides and C-terminally amidated peptides, unusual fragmentation is observed in the negative ion mode. The fragmentation behaviors of deprotonated peptides reported in this study will promote further investigation of the fundamental mechanisms and facilitate algorithm development for peptide sequencing in the negative mode.