Figure 1.
Mass spectral time courses for the HalM2/His6P-HalA2 and ProcM/ProcA2.8 reactions. Both reactions were conducted with 1 μM LanM and 40 μM LanA. Time-dependent changes in the electrospray ionization mass spectra are shown for peptide samples recovered from the HalM2- and ProcM-catalyzed reactions (panels A–B and C–D, respectively) at the indicated time points. The peptide samples were alkylated with N-ethylmaleimide. All peptides from both reactions were detected in several different multiply charged ionic states (panels A and C). A closer view of the 8+ ion families is shown in panels B and D. The starting material, reaction intermediates, and final products for the two reactions are numbered. The assignment of these MS signals to specific peptide ions is detailed in Figures S3 and S4 and in Tables S1 and S2 of the Supporting Information. The t = 0 min spectra show that partially alkylated precursor peptide derivatives could not be detected following alkylation of the samples with NEM under our conditions. Instead, only the fully NEM-alkylated forms of His6P-HalA2 and ProcA2.8 (7 and 26, respectively) were observed.