Spectroscopic Evidence for an Oxazolone Structure in Anionic b-Type Peptide Fragments

• Published in: Journal of the American Society for Mass Spectrometry 2012-02, Vol.23 (2), p.290-300
• Main Authors: Grzetic, Josipa, Oomens, Jos
• Format: Article
• Language: English
• Subjects: Aminoacids, peptides. Hormones. Neuropeptides; Analytical Chemistry; Analytical, structural and metabolic biochemistry; Anions - chemistry; Bioinformatics; Biological and medical sciences; Biotechnology; Chemistry; Chemistry and Materials Science; Fragmentation; Fragments; Fundamental and applied biological sciences. Psychology; Infrared spectra; Infrared spectroscopy; Isomers; Mass spectrometry; Mass Spectrometry - methods; Organic Chemistry; Oxazolone - chemistry; Peptide Fragments - chemistry; Peptides; Phase stability; Proteins; Proteomics; Research Article; Spectroscopy, Fourier Transform Infrared; Spectrum analysis; Structural analysis
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1007/s13361-011-0297-3

Infrared spectra of anionic b -type fragments generated by collision induced dissociation (CID) from deprotonated peptides are reported. Spectra of the b 2 fragments of deprotonated AlaAlaAla and AlaTyrAla have been recorded over the 800–1800 cm –1 spectral range by multiple-photon dissociation (MPD) spectroscopy using an FTICR mass spectrometer in combination with the free electron laser FELIX. Structural characterization of the b -type fragments is accomplished by comparison with density functional theory calculated spectra at the B3LYP/6-31++G(d,p) level for different isomeric structures. Although diketopiperazine structures represent the energetically lowest isomers, the IR spectra suggest an oxazolone structure for the b 2 fragments of both peptides. Deprotonation is shown to occur on the oxazolone α-carbon, which leads to a conjugated structure in which the negative charge is practically delocalized over the entire oxazolone ring, providing enhanced gas-phase stability.