title: Unusual fragmentation of Pro-Ser/Thr-containing peptides detected in collision-induced dissociation spectra.
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.
authors.affiliation: Mass Spectrometry Facility, School of Pharmacy, University of California San Francisco, 600 16th Street Genentech Hall, Suite N472A, Box 2240, San Francisco, CA 94158-2517, USA. folkl@cgl.ucsf.edu
authors.forename: Katalin F
authors.initials: KF
authors.lastname: Medzihradszky
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Trinidad
mesh_terms: D000595:Amino Acid Sequence; D030562:Databases, Protein; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000737:chemistry*; D010080:Oxazoles / Q000737:chemistry; D010756:Phosphoric Acids / Q000737:chemistry; D010768:Phosphoserine / Q000737:chemistry*; D010769:Phosphothreonine / Q000737:chemistry*; D011392:Proline / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry
chemical_list: D009842:Oligopeptides; D010080:Oxazoles; D010756:Phosphoric Acids; D010769:Phosphothreonine; D010768:Phosphoserine; D011392:Proline; C030242:phosphoric acid