Although the mobile proton provided by the linker carboxyl group could also be transferred to the other pseudo peptide bond in the linker (between the amino terminal group of P1) or any peptide bond (P1-P2)H, its fragmentation seems to be not equally favored. Probably, the linker carboxyl group involved in the internal rearrangement and other amide bonds are distant in the structure and could not yield a five membered ring intermediate required for this metastable fragmentation (Figure ).