Next, an spontaneous internal rearrangement of a cyclic five-membered ring intermediate leads to the dissociation of this destabilized linker pseudopeptide bond (Figure ) to yield the two intense signals assigned to (P1+71) and (P2+98) in (P1-P2)H (Figures and). Both linearized peptides P1 and P2 contain modified amino acids that reveal information on the conjugation site. The P+71 fragment ion contains a βalanine amino acid linked by a pseudopeptide bond to the epsilon amino group of the crosslinked lysine residue (Figure ). In the case of P2+98 fragment ion, the formerly crosslinked cysteine residue, is alkylated by a succinic anhydride structure (Figure ). This rearrangement proposed for the hydrolyzed thiosuccinimide linker shares similarities with the described by Soberblom et al. for an Asp-Pro cleavable crosslinker .