where [ ) from experiments with furfuryl alcohol (FFA, red squares), histidine (His, blue triangles), methionine (Met, green diamonds), and tyrosine (Tyr, yellow circles). The black solid and green dashed lines were calculated with eq 10, a KIE 1 of 1, and KIE 3 values of 1.028 and 1.031, respectively. Note that the x-axis has a logarithmic scale for improved resolution of the data points. masked (i.e., O-AKIE ≈ 1) at high k 3 [PC]/k 2 , that is in kinetic regimes of so-called forward commitment, where the forward reaction of 1 O 2 out-competes its decay to O 2 . Conversely, O-AKIE values approach a maximum value of 1.028 and 1.031 (solid and dashed lines in Figure ) and thus the intrinsic kinetic isotope effect (KIE 3 ) when the 1 O 2 formation/decay equilibrium is faster than 1 O 2 oxygenation and electron transfer.