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What Drives the Rate-determining Step for Oxygen Atom Transfer by Heme Compound I?

Understanding the catalytic properties of reactive species is an important concept in chemical catalysis, and in particular, computational modelling has been shown to provide useful insights into reaction mechanisms that lead to products and by-products. In recent years, several approaches have been reported that tried to generalize the reactivity trends of substrate oxidation reactions, for instance, to rationalize the oxidative patterns of the active species of heme peroxidases and the cytochrome P450s. Particularly useful are valence bond curve crossing diagrams that explain the mechanisms by dissecting the rate-determining barrier height for the reaction into components related to bond formation and bond breaking (or orbital formation and orbital breaking) processes and electron transfer steps. Very recently, we developed a novel two-parabola valence bond model for reactivity trends that enables one to predict enthalpies of activation, and consequently rate constants, from empirical values. Details of the methodology are described and examples are given on how to apply the two-parabola valence bond model in catalysis. Several case studies are given on heme Compound I reactivity that predict experimental reaction rates ab initio, but also predict regio- and chemoselectivities. It is worth noting that trends in hydrogen atom abstraction reactions by a cytochrome P450 model as calculated using density functional theory can be perfectly reproduced with the new model with a slope of unity and less than 1 kcal mol−1 systematic error.

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03 Oct 2018
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