Identification of Phosphorylcholine-Substituted Peptides by Their Characteristic Mass Spectrometric Fragmentation

Phosphorylcholine (PC)-substituted biomolecules are wide-spread, highly relevant antigens of parasites, since this small hapten has been found to be a potent immunomodulatory component which allows the establishment of long lasting infections of the host. Structural data, especially of protein bound PC-substituents, are still rare due to the observation that mass spectrometric analyses are mostly hampered by this zwitterionic substituent resulting in low sensitivities and unusual but characteristic fragmentation patterns. Here, we investigated the fragmentation behavior of synthetic PC-substituted peptides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization ion-trap mass spectrometry. We could show that the predominant neutral loss of a trimethylamine unit (Hoffmann elimination) leads to cyclic phosphate derivatives which prevent further fragmentation of the peptide backbone by stabilizing the positive charge at this particular side chain. Knowledge of this PC-specific fragmentation might help to identify PC-substituted biomolecules and facilitate their structural analysis.