Study of the fragmentation patterns of the phosphate-arginine noncovalent bond

Our previous work has highlighted the role of certain amino acid residues, mainly two or more adjacent arginine on one peptide and two or more adjacent glutamate, or aspartate, or a phosphorylated residue on the other in the formation of noncovalent complexes (NCX) between peptides. In the present study, we employ ESI-MS to investigate the gas-phase stability and dissociation pathways of the NCX of a basic peptide VLR-RRRKRVN, an epitope from the third intracellular loop of the dopamine D₂ receptor, with the phosphopetide SVST-DpTpSAE, an epitope from the cannabinoid CB1 carboxyl terminus. ESI-MS/MS analysis of the NCX between VLRRRRKRVN and SVSTDpTpSAE suggests two dissociation pathways for the NCX. The major pathway is the disruption of the electrostatic interactions between the Arg residues and the phosphate groups, while an alternative pathway is also recorded, in which the complex is dissociated along the covalent bond between the oxygen from either Thr or Ser and HPO₃. To verify the alternative pathway, we have used an ion trap instrument to conduct MS³ analysis on the product ions of both dissociation pathways.