Figure 4: Electrospray ionization tandem spectrometry (MS/MS) spectrum and analysis of the 13-mer peptide obtained by tryptic digestion of enzyme labelled with compound 1.

The peptide sequence is noted at the top the MS/MS spectrum. The site of modification is denoted by the lowercase d in the peptide sequence shown at top. The doubly charged precursor ion is annotated as pre[2+]. The amino terminus-containing ions (b-ion series) are shown in blue and the carboxyl terminus-containing ions (y-ion series) are shown in red. These observed ions are noted beside the corresponding peaks in the MS/MS spectrum. Only major identified peaks are labelled for purposes of clarity.