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Low-energy CID spectrum of the EPI-nitrene-modified Pro98−Arg122 tryptic peptide, precursor ion at m/z 728.4 (4+). Met116 is modified. The C-terminal y ions were detected unmodified up to y6 (fragments in blue), while all y fragments from y7 (in red) show the 221 Da mass shift. Labeling involves the sequence ions and only a few other fragments.
