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Characteristic NH 3 and CO losses from sodiated peptides C ‐terminated by glutamine residues
Author(s) -
Guan Xinshu,
Wang Bing,
Wang Huixin,
Liu Jinrong,
Li Ying,
Guo Xinhua
Publication year - 2017
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.7831
Subject(s) - chemistry , protonation , electrospray ionization , ion , dissociation (chemistry) , mass spectrometry , peptide , glutamine , amino acid , molecule , mass spectrum , electrospray , density functional theory , tandem mass spectrometry , crystallography , stereochemistry , computational chemistry , chromatography , organic chemistry , biochemistry
Rationale Under certain conditions some amino acid (AA) residues undergo special reactions in the gas phase, generating characteristic neutral losses and product ions. Taking these special fragments into account and understanding the effect of AA residues on peptide cleavages will consummate database search algorithms and manual data interpretation in peptide sequencing by mass spectrometry (MS). In this study, the details of the characteristic NH 3 and CO losses of glutamine (Gln) residues located at the C ‐terminus of peptides are presented. Methods A number of selected peptides were fragmented under collision‐induced dissociation (CID) in electrospray ionization quadrupole time‐of‐flight mass spectrometry (ESI‐QTOF‐MS). Density functional theory (DFT) quantum mechanical calculations at the B3LYP/6‐31+G(d,p) level were carried out to optimize the geometry of peptide ions and provide energy barriers of ions in each step during fragmentations. Results Two characteristic peaks appear near the precursor ions of sodiated Gln C ‐terminated peptides, suggesting the loss of neutral NH 3 and CO via a two‐step process. The proposed mechanism of their formation is as follows: after losing NH 3 , a non‐classical b n * ion is formed with a glutaric anhydride structure that further dissociates to lose CO. The sodiated peptides show more intensive peaks corresponding to the loss of neutral molecules than the protonated ones. This type of neutral loss can also occur at the Gln residue that is rearranged to the C ‐terminus of sodiated peptides. Conclusions The experiments and calculations suggest that the two‐step characteristic NH 3 and CO loss of sodiated peptides is energetically favored, and can be applied to identify C ‐terminated Gln residues. This study provides a mechanistic insight into the role of sodium ion during peptide fragmentation. Copyright © 2017 John Wiley & Sons, Ltd.