Progress curves for mGFPsol maturation of unlabeled protein (top panel) and isotopically enriched protein (bottom panel). Red (-●-), rise in red fluorescence due to resorufin production from hydrogen peroxide; Green (-■-), rise in green fluorescence due to formation of the mature GFP chromophore. The symbols represent experimental data (for clarity, only 10% of the collected data points are shown). The solid lines describe theoretical progress curves calculated from the rate constants extracted by global curve fitting of experimental data. Black, precursor protein; Blue, pre-oxidation intermediate (α-enolate); Purple, post-oxidation intermediate (cyclic imine); Red, resorufin (hydrogen peroxide indicator); Green, mature protein. Inset: Schematic of pre-cyclization structures with Cβ-di-hydro (unlabeled) and Cβ-di-deutero (labeled) Tyr66.