Insights into the fragmentation pathways of gas-phase protonated sulfoserine

The fragmentation chemistry of protonated sulfoserine was probed using a combination of collision-induced dissociation (CID) mass spectrometry, infrared multiple photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The IRMPD spectra of the dominant fragment ions at m/z 106 and 88 (i.e., loss of SO3 and H2SO4) were obtained and used to determine the corresponding structures. By comparison to a synthetic standard and calculations, it was determined that the m/z 106 ion is structurally identical to protonated serine. The m/z 88 fragment ion was assigned an aziridine structure based on a comparison to theory, analogous to the structure previously proposed by others for phosphoric acid loss from phosphoserine. This work provides the first spectroscopic insights into the dissociation pathways of a sulfated amino acid, laying the groundwork for future studies on related amino acids and peptides with this important, labile post-translational modification.