The fragmentation in gas phase of the newly generated pseudopeptide bond present in the thiazine and the hydrolyzed thiosuccinimide linkers (Figure ) generates an ion (named here as P+71) with identical molecular mass, and 71.0347 Da mass higher that the unmodified Lys containing peptide . This linker fragment ion has been proposed for the differentiation by mass spectrometry of isobaric type 2 peptides cross-linked by a thiosuccinimide and thiazine linkers .