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

Abstract

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*)(CH ₃CN)](CF ₃SO ₃) 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 H ₂O ₂ into O ₂ and H ₂O 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 k ₂( 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 H ₂O ₂ 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 H ₂O ₂ 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.