Fragmentation of cationized phosphotyrosine containing peptides by atmospheric pressure MALDI/Ion trap mass spectrometry

An investigation of phosphate loss from sodium-cationized phosphotyrosine containing peptide ions was conducted using liquid infrared (2.94 μm) atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) coupled to an ion trap mass spectrometer (ITMS). Previous experiments in our laboratory explored the fragmentation patterns of protonated phosphotyrosine containing peptides, which experience a loss of 98 Da under CID conditions in the ITMS. This loss of 98 Da is unexpected for phosphotyrosine, given the structure of its side chain. Phosphate loss from phosphotyrosine residues seems to be dependent on the presence of arginine or lysine residues in the peptide sequence. In the absence of a basic residue, the protonated phosphotyrosine peptides do not undergo losses of HPO₃ (Δ 80 Da) nor HPO₃ + H₂O (Δ 98 Da) in their CID spectra. However, sodium cationized phosphotyrosine containing peptides that do not contain arginine or lysine residues within their sequences do undergo losses of HPO₃ (Δ 80 Da) and HPO₃ + H₂O (Δ 98 Da) in their CID spectra.