Structural characterization by IRMPD spectroscopy and DFT calculations of deprotonated phosphorylated amino acids in the gas phase

Gas-phase infrared spectra of deprotonated phosphorylated amino acids ([pAA-H]^-) - phosphoserine ([pSer-H]^-), phospho-threonine ([pThr-H]^-), and phosphotyrosine ([pTyr-H]^-) - and of the dihydrogen phosphate anion H₂POS₄^- have been recorded in the mid-IR region (650-2000 cm^-1) under tandem mass spectrometry conditions. The experimental setup involved a Paul ion trap equipped with an electrospray ionization source coupled with a tunable free electron laser (FEL). Spectral assignment of the observed IRMPD bands and identification of the vibrational signatures of the phosphorylation have been performed by comparison with DFT calculations. The H₂PO ₄^- anion has been used as a simple model of a free deprotonated phosphate group, helping the identification of the IR signatures of phosphorylation. Our results show that deprotonation occurs on the phosphate group for the three amino acids. A comparison between the deprotonated and protonated phosphorylated amino acids is reported for the most important vibrational features.