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Unusual fragmentation of Pro-Ser/Thr-containing peptides detected in collision-induced dissociation spectra.

Authors
  • Medzihradszky, Katalin F
  • Trinidad, Jonathan C
Type
Published Article
Journal
Journal of the American Society for Mass Spectrometry
Publisher
Springer-Verlag
Publication Date
Apr 01, 2012
Volume
23
Issue
4
Pages
602–607
Identifiers
DOI: 10.1007/s13361-011-0216-7
PMID: 21952759
Source
Medline
License
Unknown

Abstract

During collision-induced dissociation (CID)-, phosphoserine- and phosphothreonine-containing peptides frequently undergo neutral loss of phosphoric acid. Subsequent amide bond cleavage N-terminal to the site of phosphorylation results in a y ion with a mass 18 Da lower than the corresponding unmodified y fragment. We report here that when the phosphoserine or phosphothreonine is directly preceded by a proline, an unusual fragment with a mass 10 Da higher than the corresponding unmodified y ion is frequently observed. Accurate mass measurements are consistent with elimination of the phosphoric acid followed by fragmentation between the α carbon and the carbonyl group of the proline residue. We propose a cyclic oxazoline structure for this fragment. Our observation may be explained by the charge-directed S(N)2 neighboring group participation reaction proposed for the phosphoric acid elimination by Palumbo et al. [Palumbo, A. M., Tepe, J. J., Reid, G. E. Mechanistic Insights into the Multistage Gas-Phase Fragmentation Behavior of Phosphoserine- and Phosphothreonine-Containing Peptides. J. Protein Res. 7(2), 771-779 (2008)]. Considering such specific fragment ions for confirmation purposes after regular database searches may boost the confidence of peptide identifications as well as phosphorylation site assignments.

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