Spectroscopic Evidence for Lactam Formation in Terminal Ornithine b2+ and b3+ Fragment Ions

Autor: Jonathan Martens, Vincent Steinmetz, Árpád Somogyi, Jonathan R. Scheerer, Giel Berden, Xiye Wang, Zachary M. Smith, John C. Poutsma, Jos Oomens, Vicki H. Wysocki
Rok vydání: 2019
Předmět:
Zdroj: Journal of the American Society for Mass Spectrometry, 30, 1565-1577
J Am Soc Mass Spectrom
Journal of the American Society for Mass Spectrometry, 30, 9, pp. 1565-1577
ISSN: 1044-0305
DOI: 10.1007/s13361-019-02244-0
Popis: Infrared multiple photon dissociation action spectroscopy was performed on the AlaOrn b(2)(+) and AlaAlaOrn b(3)(+) fragment ions from ornithine-containing tetrapeptides. Infrared spectra were obtained in the fingerprint region (1000 – 2000 cm(−1)) using the infrared free electron lasers at the Centre Laser Infrarouge d’Orsay (CLIO) facility in Orsay, France and the Free Electron Lasers for Infrared eXperiments (FELIX) facility in Nijmegen, the Netherlands. A novel terminal ornithine lactam AO(+) b2(+) structure was synthesized for experimental comparison and spectroscopy confirms that the b2(+) fragment ion from AOAA forms a lactam structure. Comparison of experimental spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory shows that AO(+) b(2)(+) forms a terminal lactam protonated either on the lactam carbonyl oxygen or the N-terminal nitrogen atom. Several low-lying conformers of these isomers are likely populated following IRMPD dissociation. Similarly, a comparison of the experimental IRMPD spectrum with calculated spectra shows that AAO(+) b(3)(+)-ions also adopt a lactam structure, again with multiple different protonation sites, during fragmentation. This study provides spectroscopic confirmation for the lactam cyclization proposed for the “ornithine effect” and represents an alternative b(n)(+) structure to the oxazolone and diketopiperazine/macrocycle structures most often formed.
Databáze: OpenAIRE
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