Stability and Catalase-Like Activity of a Mononuclear Non-Heme Oxoiron(IV) Complex in Aqueous Solution

Heme-type catalase is a class of oxidoreductase enzymes responsible for the biological defense against oxidative damage of cellular components caused by hydrogen peroxide, where metal-oxo species are proposed as reactive intermediates. To get more insight into the mechanism of this curious reaction a non-heme structural and functional model was carried out by the use of a mononuclear complex [Fe^II(N4Py*)(CH₃CN)](CF₃SO₃)₂ (N4Py* = N,N-bis(2-pyridylmethyl)- 1,2-di(2-pyridyl)ethylamine) as a catalyst, where the possible reactive intermediates, high-valent Fe^IV=O and Fe^III-OOH are known and spectroscopically well characterized. The kinetics of the dismutation of H₂O₂ into O₂ and H₂O was investigated in buffered water, where the reactivity of the catalyst was markedly influenced by the pH, and it revealed Michaelis-Menten behavior with K_M = 1.39 M, k_cat = 33 s^-1 and k₂(k_cat/K_M) = 23.9 M^-1s^-1 at pH 9.5. A mononuclear [(N4Py)Fe^IV=O]²⁺ as a possible intermediate was also prepared, and the pH dependence of its stability and reactivity in aqueous solution against H₂O₂ was also investigated. Based on detailed kinetic, and mechanistic studies (pH dependence, solvent isotope effect (SIE) of 6.2 and the saturation kinetics for the initial rates versus the H₂O₂ concentration with K_M = 18 mM) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the iron-bound substrate and the Fe(IV)-oxo species.