Histidine, tyrosine, and furfuryl alcohol share a common initial reaction step through cycloaddition of 1 O 2 to the imidazole, phenyl, and furan moieties of the probe compounds. It is thus reasonable to assume that the similar O-AKIE values for O 2 consumption and consistent trends with k 3 [PC]/k 2 originate from an intrinsic KIE 3 of similar magnitude within the uncertainty of 0.003 AKIE-units identified here (Figure ). O-AKIEs from experiments with methionine, by contrast, are offset from the other three probe compounds. One possible explanation for this offset is that the initial reaction step of methionine with to the oxygenation and eletron transfer reaction between 1 O 2 and organic moieties.