Thiazine linker does not have any source of mobile protons in their structure that promote its own destabilization therefore the metastable fragmentation for (P1-P2)T is not favored. The formation P1+71 is probably also mediated by another mobile proton available in (P1-P2)T. In this sense, this pseudopeptide bond and other peptide bonds in (P1-P2)T seem to have a similar affinity for protonation taking into account their similar intensities observed in the MALDI-MS spectra (Figures and).