Quantum chemical mass spectrometry: Ab initio study of b 2 -ion formation mechanisms for the singly protonated Gln-His-Ser tripeptide

• Published in: Rapid communications in mass spectrometry 2020-06, Vol.34 (12), p.e8778
• Main Authors: Cautereels, Julie, Giribaldi, Julien, Enjalbal, Christine, Blockhuys, Frank
• Format: Article
• Language: English
• Subjects: Amides; Amides - chemistry; Chemical Sciences; Diketopiperazines; Diketopiperazines - chemistry; Glycine; Glycine - chemistry; Histidine; Histidine - chemistry; Ions; Ions - chemistry; MAss Spectrometry; Mass Spectrometry - methods; Oligopeptides; Oligopeptides - analysis; Oligopeptides - chemistry; Oxazolone; Oxazolone - chemistry; Protons; Thermodynamics
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.8778

Both amide bond protonation triggering peptide fragmentations and the controversial b -ion structures have been subjects of intense research. The involvement of histidine (H), with its imidazole side chain that induces specific dissociation patterns involving inter-side-chain (ISC) interactions, in b -ion formation was investigated, focusing on the QHS model tripeptide. To identify the effect of histidine on fragmentations issued from ISC interactions, QHS was selected for a comprehensive analysis of the pathways leading to the three possible b -ion structures, using quantum chemical calculations performed at the DFT/B3LYP/6-311+G* level of theory. Electrospray ionization ion trap mass spectrometry allowed the recording of MS and MS tandem mass spectra, whereas the Quantum Chemical Mass Spectrometry for Materials Science (QCMS ) method was used to predict fragmentation patterns. Whereas it is very difficult to differentiate among protonated oxazolone, diketopiperazine, or lactam b -ions using MS and MS mass spectra, the calculations indicated that the QH b -ion (detected at m/z 266) is probably a mixture of the lactam and oxazolone structures formed after amide nitrogen protonation, making the formation of diketopiperazine less likely as it requires an additional step for its formation. In contrast to glycine-histidine-containing b -ions, known to be issued from the backbone-imidazole cyclization, we found that interactions between the side chains were not obvious to perceive, neither from a thermodynamics nor from a fragmentation perspective, emphasizing the importance of the whole sequence on the dissociation behavior usually demonstrated from simple glycine-containing tripeptides.