The reaction was a single-step process with no discernible accumulation of intermediate species, as indicated by the several isosbestic points in the spectra (Fig. 5, upper panel) and by the single-exponential decay of the absorbance signal at two different wavelengths (Fig. 5, upper panel inset), which followed the rate of FeIINO complex disappearance (436 nm) and the rate of ferric enzyme format