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

Abstract

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. © 2012 The Author(s).