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*)(CH(3)CN)](CF(3)SO(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(2)O(2) into O(2) and H(2)O 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(−1)s(−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(2)O(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(2)O(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.