Figure 4.

Time courses for peptide species involved in the HalM2/His₆P-HalA2 reaction. The charge-state-weighted fractional abundances of peptide species were determined as described in the methods, converted to peptide concentration, and plotted vs reaction time. The major species included the starting material and final product (7 and 4, respectively, panel A), species with no thioether ring and 1–3 dehydrations (8–10, panel B), species with 1 thioether ring and 1–5 additional dehydrations (14–18, panel C), and species with two thioether rings and 2–5 additional dehydrations (19–22, panel D). The progress curves overlying the data were generated by numerical simulation with the kinetic model shown in Scheme 3 and the rate constants given in Table 1. The reaction does not go to completion, leading to slight discrepancies between the model and the data at longer time points. The FitSpace Explorer calculation for this global fit, showing the extent to which the variable parameters in the kinetic model are constrained by the data, is shown in Figure S13 of the Supporting Information. Two additional low-abundance species, each containing three thioether rings (compounds 23 and 24) were detected in the mass spectra for the reaction but were not included in this model. A preliminary kinetic model that accounts for these two species is presented in the Supporting Information.