Spectroscopic and Kinetic Evidence for the Crucial Role of Compound 0 in the P450cam -Catalyzed Hydroxylation of Camphor by Hydrogen Peroxide

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

H2O2 Oxidation by FeIII-OOH Intermediates and Its Effect on Catalytic Efficiency

The oxidation of the C–H and C=C bonds of hydrocarbons with H₂O₂ catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive Fe^IV=O species such as [(N4Py)Fe^IV=O]²⁺ formed by homolytic cleavage of the O–O bond of an Fe^III–OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine). We show here that at low H₂O₂ concentrations the Fe^IV=O species formed is detectable in methanol. Furthermore, we show that the decomposition of H₂O₂ to water and O₂ is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H₂O₂ are present. Although independently prepared [(N4Py)Fe^IV=O]²⁺ oxidizes stoichiometric H₂O₂ rapidly, the rate of formation of Fe^IV=O from the Fe^III–OOH intermediate is too low to account for the rate of H₂O₂ decomposition observed under catalytic conditions. Indeed, with excess H₂O₂, disproportionation to O₂ and H₂O is due to reaction with the Fe^III–OOH intermediate and thereby prevents formation of the Fe^IV=O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H₂O₂, which ensure that the rate of the H₂O₂ oxidation by the Fe^III–OOH intermediate is less than the rate of the O–O bond homolysis and the subsequent reaction of the Fe^IV=O species with a substrate.

Jumpstarting the cytochrome P450 catalytic cycle with a hydrated electron

Cytochrome P450cam (CYP101Fe³⁺) regioselectively hydroxylates camphor. Possible hydroxylating intermediates in the catalytic cycle of this well-characterized enzyme have been proposed on the basis of experiments carried out at very low temperatures and shunt reactions, but their presence has not yet been validated at temperatures above 0 °C during a normal catalytic cycle. Here, we demonstrate that it is possible to mimic the natural catalytic cycle of CYP101Fe³⁺ by using pulse radiolysis to rapidly supply the second electron of the catalytic cycle to camphor-bound CYP101[FeO₂]²⁺ Judging by the appearance of an absorbance maximum at 440 nm, we conclude that CYP101[FeOOH]²⁺ (compound 0) accumulates within 5 μs and decays rapidly to CYP101Fe³⁺, with a k_{440 nm} of 9.6 × 10⁴ s^-1 All processes are complete within 40 μs at 4 °C. Importantly, no transient absorbance bands could be assigned to CYP101[FeO²⁺por^•+] (compound 1) or CYP101[FeO²⁺] (compound 2). However, indirect evidence for the involvement of compound 1 was obtained from the kinetics of formation and decay of a tyrosyl radical. 5-Hydroxycamphor was formed quantitatively, and the catalytic activity of the enzyme was not impaired by exposure to radiation during the pulse radiolysis experiment. The rapid decay of compound 0 enabled calculation of the limits for the Gibbs activation energies for the conversions of compound 0 → compound 1 → compound 2 → CYP101Fe³⁺, yielding a ΔG^‡ of 45, 39, and 39 kJ/mol, respectively. At 37 °C, the steps from compound 0 to the iron(III) state would take only 4 μs. Our kinetics studies at 4 °C complement the canonical mechanism by adding the dimension of time.

Enhanced light-driven catalytic performance of cytochrome P450 confined in macroporous silica

A light-driven approach combined with a macroporous reactor for the enzymatic biocatalytic reaction has been developed by confining the enzyme/photosensitizer nanohybrids in a macroporous material, which exhibits high bio-conversion efficiency due to the fast diffusion and collision between the enzyme/photosensitizer nanohybrid and the substrate in the reactor.