Hydrolysis turns the resultant thiosuccinimide linker into a cleavable crosslinker in MALDI-MS/MS and ESI-MS/MS analysis. Type 2 peptides with the hydrolyzed thiosuccinimide linker undergoes a linker fragmentation of a metastable origin in gas phase. This fragmentation is initiated by a proton transfer of the newly generated linker carboxyl group to the nearest pseudopeptide bond. The destabilized amide bond in the linker undergoes an internal rearrangement through a five-membered ring that generates two linker fragment ions named here as P+71 and C+98. P+71 ion contains a β-Ala residue linked to the epsilon amino group of the conjugated lysine residue. In the structure of C+98 ion, the cysteine residue is S-alkylated by a succinic anhydride molecule.