An iron(III) methoxide complex reacts with para-substituted triarylmethyl radicals to give iron(II) and methoxyether products. Second-order rate constants for the radical derivatives were obtained. Hammett and Marcus plots suggest the radical transfer reactions proceed via a concerted process. Calculations support the concerted nature of these reactions involving a single transition state with no initial charge-transfer. These findings have implications for the radical "rebound" step invoked in nonheme iron oxygenases, halogenases, and related synthetic catalysts.

Vanadium haloperoxidases are one of the few enzymes in nature that utilize a vanadium center and catalyze the halogenation of substrates through the biosynthesis of hypohalite. Vanadium chloroperoxidases (VCPOs) bind and activate hydrogen peroxide and in a reaction with chloride convert it into hypochlorite as a precursor for a substrate chlorination reaction. Despite the fact that these enzymes have been studied extensively, surprisingly little is known on their catalytic cycle and particularly on the function of the vanadium atom in the reaction mechanism. In order to gain insight into the intricate details of the catalytic cycle of VCPOs, we performed an extensive computational study using large cluster model complexes, where we tested many possible pathways and active-site protonation states. Our work establishes that the biosynthesis of hypochlorite proceeds in two steps: H 2 O 2 activation on the vanadium center to form an end-on V(V)-hydroperoxo complex, followed by OH + transfer from hydroperoxo to chloride on the vanadium center to form hypochlorite. We show that the initial reaction starts with a proton transfer from H 2 O 2 to the equatorial OH group of the V V (O) 2 (OH) 2 − active site, followed by hydroperoxo binding and water release to form the highly stable vanadium-hydroperoxo-dioxo-hydroxo complex. A further proton transfer from an active-site His or Lys residue can lead to the vanadium-peroxo-hydroxo-oxo complex, which we assign as a dead-end complex unable to react further to hypochlorite products. The mechanisms were considered under various protonation state, and it is shown to be the most effective with His 404 singly protonated. The work shows that vanadium is a spectator ion that does not change its oxidation state during the reaction mechanism but holds and positions the H 2 O 2 substrate and guides its proton-relay steps through its oxo and hydroxo ligands. The effect of the protonation state of firstand second-coordination sphere residues and ligands was tested and shows that the reaction is highly sensitive to local changes in the protonation state. Finally, the computations show that the oxygen atom of HOCl exclusively derives from H 2 O 2 .

Heme superoxides are one of the most versatile metallo-intermediates in biology, and they mediate a vast variety of oxidation and oxygenation reactions involving O¬2(g). Overall proton-coupled electron transfer (PCET) processes...

Vanadium haloperoxidases (VHPOs) are unique enzymes in biology that catalyze a challenging halogen transfer reaction and convert a strong aromatic C–H bond into C–X (X = Cl, Br, I) with the use of a vanadium cofactor and H2O2. The VHPO catalytic cycle starts with the conversion of hydrogen peroxide and halide (X = Cl, Br, I) into hypohalide on the vanadate cofactor, and the hypohalide subsequently reacts with a substrate. However, it is unclear whether the hypohalide is released from the enzyme or otherwise trapped within the enzyme structure for the halogenation of organic substrates. A substrate-binding pocket has never been identified for the VHPO enzyme, which questions the role of the protein in the overall reaction mechanism. Probing its role in the halogenation of small molecules will enable further engineering of the enzyme and expand its substrate scope and selectivity further for use in biotechnological applications as an environmentally benign alternative to current organic chemistry synthesis. Using a combined experimental and computational approach, we elucidate the role of the vanadium haloperoxidase protein in substrate halogenation. Activity studies show that binding of the substrate to the enzyme is essential for the reaction of the hypohalide with substrate. Stopped-flow measurements demonstrate that the rate-determining step is not dependent on substrate binding but partially on hypohalide formation. Using a combination of molecular mechanics (MM) and molecular dynamics (MD) simulations, the substrate binding area in the protein is identified and even though the selected substrates (methylphenylindole and 2-phenylindole) have limited hydrogen-bonding abilities, they are found to bind relatively strongly and remain stable in a binding tunnel. A subsequent analysis of the MD snapshots characterizes two small tunnels leading from the vanadate active site to the surface that could fit small molecules such as hypohalide, halide, and hydrogen peroxide. Density functional theory studies using electric field effects show that a polarized environment in a specific direction can substantially lower barriers for halogen transfer. A further analysis of the protein structure indeed shows a large dipole orientation in the substrate-binding pocket that could enable halogen transfer through an applied local electric field. These findings highlight the importance of the enzyme in catalyzing substrate halogenation by providing an optimal environment to lower the energy barrier for this challenging aromatic halide insertion reaction.