Structural Characterization by IRMPD Spectroscopy and DFT Calculations of Deprotonated Phosphorylated Amino Acids in the Gas Phase

Spectral assignment of the IRMPD bands and identification of the vibrational signatures of deprotonated phosphorylated amino acids are attained. The H 2 PO 4 − anion is used as a simple model of a free deprotonated phosphate group to identify the IR signatures of phosphorylation. The fragmentation efficiency is lower for [pSer‐H] − (left scale in figure) than for [pThr‐H] − and [pTyr‐H] − (right scale).Gas‐phase infrared spectra of deprotonated phosphorylated amino acids ([pAA‐H] − )—phosphoserine ([pSer‐H] − ), phosphothreonine ([pThr‐H] − ), and phosphotyrosine ([pTyr‐H] − )—and of the dihydrogen phosphate anion H 2 PO 4 − 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 H 2 PO 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.