Fragmentation mechanism of product ions from protonated proline-containing tripeptides in electrospray ionization mass spectrometry

The dissociation chemistry of primary fragment ions from the protonated proline-containing tripeptides glycylprolylglycine, prolylglycylglycine, and prolylprolylglycine was investigated by electrospray ionization multi-stage mass spectrometry. Calculations showed the a₂ ions generated from b₂ ions were cyclic, which is energetically more favorable than the linear form. The prolyl residue in the structure affected the energy hypersurface of the dissociation reaction from the b₂ ion to the a₂ ion. In the fragmentation of a₂ ions, the iminium-imine complex corresponding to loss of CO from the a₂ ion was suggested to be an ion-neutral complex (INC). The a₁ ion was generated from direct separation of this INC, and the internal iminium ion, which was absent in PGG, was generated from another INC that was formed from the first INC via proton-bridged complex-mediated intramolecular proton transfer. Although these intermediates are unstable, their existence is supported by experiments and density functional theory calculations.