The MALDI-MS/MS spectra of (P1-P2)T fragmented by collision induced dissociation (Figure ) allowed a full-sequence coverage of both crosslinked peptide sequences (Fig. ). The fragmentation of the pseudopeptide bond newly generated by the transcyclization reaction (Figure ) generates two ions at m/z 886.37 and m/z 927.30 that were tentatively assigned as (P1+71) and (P2+80), respectively (highlighted in red, see Figures and). The intensities of these fragment ions (Figure ) were comparable to other backbone fragment ions (bn or yn) of P1 and P2 peptides. The expanded region (m/z 804-1034) of these two MALDI-MS/MS spectra (Figure and 2b) are shown in Fig. .