Enzymes are highly efficient catalysts in Nature that often react with high stereo-, chemo-, and regioselectivity. Usually, enzymes achieve this selectivity through substrate binding and positioning in the active site. The second-coordination sphere effects (also called noncovalent interactions) position the substrate and oxidant in close vicinity, and their effects include electrostatic interactions, hydrogen-bonding interactions, salt-bridges, and also long-range charge-effects from bound cations and anions. Each of these environmental perturbations can affect the kinetics and selectivity of reactions differently. Over the past couple of years, a variety of biomimetic model complexes have been developed and designed that have a similar first-coordination sphere to mononuclear iron-containing enzymes. However, sometimes the reactivity patterns of the biomimetic models in solution are different from the analogous enzymatic systems, and often the selectivity of the reaction is lost. To understand the functional differences between enzymes and biomimetic models, large catalytic clusters have been developed that incorporate second-coordination sphere effects that influence spectroscopic features as well as reactivity patterns. In this Review, we summarize and highlight recent advances in biomimetic chemistry on the creation and design of iron catalysts and the insights that have been obtained when elaborate ligand features are added, which influence the substrate approach to the catalytic center. We start with a highlight of the axial and distal ligand effects of metal centers and how these can be perturbed by hydrogen bonding as well as steric restraints. The syntheses of the active oxidants through the addition of a proton-donating or -accepting groups to the structure have also been discussed in detail in this article. As shown in this work, second-coordination sphere effects can be useful not only to trap and characterize short-lived intermediates but also to enable high selectivity and specificity of a chemical reaction in analogy to enzymatic systems. These biomimetic models appear highly useful for biotechnological and engineering applications with reasonable turnover numbers and consequently have great potential for the future of stereo-and chemoselective synthetic catalytic reactions.

Aldehyde deformylation is one of the useful reactions in biology and organic syntheses and this review provides mechanistic insights into the same.

Ethylene is an important signaling molecule in plants that triggers the growth of leaves, flowers, and fruits. One of the enzymes involved in the biosynthesis of ethylene is the ethylene-forming enzyme (EFE), which is an usual nonheme iron enzyme that biodegrades α-ketoglutarate into three CO 2 molecules and ethylene. As the detailed mechanism of EFE in the biosynthesis of ethylene remains controversial and particularly the function of the co-substrate L-arginine, we decided to pursue a density functional theory study on the possible pathways of the enzyme leading to ethylene biosynthesis and test many possible pathways and mechanisms. A large active site cluster model of 322 atoms was created, which contains all the features of the firstand second-coordination sphere of the active site and substrate (αketoglutarate) binding pockets. The calculations identify a persuccinate intermediate that triggers a bifurcation pathway in the enzyme and either react with a molecule of CO 2 to form a carbonate or forms a high-valent iron(IV)-oxo species through heterolytic dioxygen bond cleavage. Our studies show that both the bifurcation pathways converge to the same intermediate again and can lead to ethylene products, although the two pathways have different kinetics. Interestingly, our studies also show that the iron(IV)-oxo itself can form a carbonate and ethylene but through much higher barriers. As a matter of fact, these barriers are higher in energy than the typical aliphatic hydroxylation barriers and may not be competitive with arginine hydroxylation. Inclusion of the L-arginine co-substrate into the model leads to minor changes in the structure and fold, but its charge and dipole moment does not seem to affect the first stage of the catalytic cycle. Moreover, the key activation barriers seem less affected by the inclusion of L-arginine into the model. We, therefore, believe that the role of L-arginine is to lock α-ketoglutarate and its products into a tight binding pocket to enable its degradation and to prevent early release of CO 2 . Our studies show that due to the distinct differences in α-ketoglutarate positioning between different arginine activating nonheme iron dioxygenases in the co-substrate binding pocket and its tighter binding in EFE, we predict that the release of CO 2 is prevented in the first stage of the oxygen activation mechanism. This enables attack of CO 2 on a persuccinate complex to form carbonate products, leading to ethylene formation. This work gives suggestions on the engineering of EFE into a hydroxylase or improving the ethylene biosynthesis.

Mononuclear high-valent iron(IV)-oxo intermediates are excellent oxidants towards oxygenation reactions by heme and nonheme metalloenzymes and their model systems. One of the most important functions of these intermediates in nature...

High-valent iron(IV)-oxo intermediates are versatile oxidants in the biotransformation of various substrates by metalloenzymes and catalyze essential reactions for human health as well as in the biodegradation of toxic organic pollutants in the environment. Herein, we report a biomimetic system that efficiently reacts with halophenols through defluorination reactions and characterize various short-lived intermediates along the reaction mechanism. We study the reactivity pattern of a nonheme iron(IV)-oxo species with a series of trihalophenols (X=F, Cl, Br). A combined experimental and computational study reveals that the oxidative dehalogenation of 2,4,6-trifluorophenol is initiated with an Hatom abstraction from the phenolic group by the iron(IV)-oxo species resulting in the formation of a phenolate radical and an iron(III)-hydroxo species. This iron(III)-hydroxo species forms an adduct with the oxidized substrate with λ max at 558 nm which subsequently decays to give quinones as products.