Figure ), and the energy barrier for the back reaction from IM1→Cpd I is higher than the one for the forward reaction (IM1→IM2). This observation qualitatively agrees with the experimental study by Grant et a where they did not find any trace of Cpd I once Cpd I decays to IM1 ( k-2 ~2 s -1 ) and accumulation of intermediate IM1. Another study by the same group shows that Cpd I, decays very quickly (in 1 s), to a species that is indistinguishable from the ferric-low spin substratefree form of the enzyme, and the time course for this decay required two summed exponentials for adequate fitting, suggesting a complex decomposition process that may involve multiple steps.