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.Calcula-tions showed the a 2 ions generated from b 2 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 b 2 ion to the a 2 ion.In the fragmen-tation of a 2 ions,the iminium-imine complex corresponding to loss of CO from the a 2 ion was suggested to be an ion-neutral com-plex (INC).The a 1 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.