Isopeptide-cyclized product formation of the model peptide substrate shown in a , quantified by RP-HPLC (relative peak integrals; n = 3 independent experiments; mean +/- s.d.; error bars are too small to see at some points). Representative RP-HPLC chromatograms and MALDI–TOF MS spectra are shown in Supplementary Fig. 7 .
