To explain origin of the metastable fragmentation that generates intense P1+71 and P2+98 fragment ions (Figure ) in (P1-P2)H (Figures and) we hypothesized that the proton located at the linker carboxyl group become mobile and migrates spontaneously to the nearest carbonyl group in the newly formed pseudopeptide bond by hydrolysis (highlighted in red, see Figure ). This process destabilizes favorably this pseudopeptide bonds in a similar way that protonation does the same for Asp-Pro linker peptide bonds in linear peptides to generate backbone fragment ions (Figure ) .