SUMMARYMononuclear non-haem iron (NHFe) enzymes catalyse a wide variety of oxidative reactions including halogenation, hydroxylation, ring closure, desaturation, and aromatic ring cleavage. These are highly important for mammalian somatic processes such as phenylalanine metabolism, production of neurotransmitters, hypoxic response, and the biosynthesis of natural products.1–3 The key reactive intermediate in the catalytic cycles of these enzymes is an S = 2 FeIV=O species, which has been trapped for a number of NHFe enzymes4–8 including the halogenase SyrB2, the subject of this study. Computational studies to understand the reactivity of the enzymatic NHFe FeIV=O intermediate9–13 are limited in applicability due to the paucity of experimental knowledge regarding its geometric and electronic structures, which determine its reactivity. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes of Fe on the nature of the FeIV=O active site.14–16 Here we present the first NRVS structural characterisation of the reactive FeIV=O intermediate of a NHFe enzyme. This FeIV=O intermediate reacts via an initial H-atom abstraction step, with its subsquent halogenation (native) or hydroxylation (non-native) rebound reactivity being dependent on the substrate.17 A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate is able to direct the orientation of the FeIV=O intermediate, presenting specific frontier molecular orbitals (FMOs) which can activate the selective halogenation versus hydroxylation reactivity.

Oxygen-containing mononuclear iron species—iron(III)–peroxo, iron(III)–hydroperoxo and iron(IV)–oxo—are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes1–7. It has been difficult to generate synthetic analogues of these three active iron–oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)–peroxo complex, [Fe(III)(TMC)(OO)]+. We also report a series of chemical reactions in which this iron(III)–peroxo complex is cleanly converted to the iron(III)–hydroperoxo complex, [Fe(III)(TMC)(OOH)]2+, via a short-lived intermediate on protonation. This iron(III)–hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)]2+, via homolytic O–O bond cleavage of the iron(III)–hydroperoxo species. All three of these iron species—the three most biologically relevant iron–oxygen intermediates—have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)–hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)–oxo complex in C–H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)–hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.

Conspectus Mononuclear non-heme Fe (NHFe) enzymes play key roles in antibiotic biosynthesis, hypoxic response, DNA repair, anticancer therapy and many other biological processes. On a molecular level these enzymes catalyze a diverse range of oxidation reactions, including hydroxylation, halogenation, ring closure, desaturation and electrophilic aromatic substitution (EAS). Most of these enzymes use an FeII site to activate dioxygen. These ferrous active sites had been inaccessible to traditional spectroscopic methods. A methodology has been developed that provides detailed geometric and electronic structure insight for these NHFeII active sites. This has defined a general mechanistic strategy utilized by a wide range of these enzymes to control O2 activation by FeII coordination unsaturation only in the presence of cosubstrates to limit autooxidation and self-hydroxylation. Depending on the type of enzyme, O2 activation either involves a 2e− reduced FeIII–OOH intermediate or a 4e− reduced FeIV=O intermediate. The nature of these intermediates has been defined in terms of geometric structure using nuclear resonance vibrational spectroscopy (NRVS) and electronic structure using magnetic circular dichroism (MCD) to define the frontier molecular orbitals (FMOs) that control reactivity. For FeIII–OOH intermediates the anticancer drug Activated Bleomycin is shown to be the non-heme Fe analog of compound 0 in heme (e.g. P450) chemistry but undergoes different reactivity where the low-spin (LS) FeIII–OOH can directly abstract an H atom from DNA. It is also shown that the transition states of LS and high-spin (HS) FeIII–OOH are fundamentally different in that the former goes through a hydroxyl radical while the latter is activated for EAS without O-O cleavage, which is important in one class of NHFe enzymes that utilizes a HS FeIII–OOH intermediate in dioxygenation. For FeIV=O intermediates the LS form is shown to have a π-type FMO activated for attack perpendicular to the Fe–O bond while the HS form (present in the NHFe enzymes) has both π and σ FMOs that are activated for attack both perpendicular to and along the Fe–O bond, respectively. For the NHFe enzymes these π vs σ FMOs direct reactivity for EAS vs H-atom abstraction, and for the latter halogenation vs hydroxylation. This study emphasizes that experimental spectroscopy is critical in evaluating the results of electronic structure calculations and thus key to bridging structure and reactivity with mechanism.

The geometric and electronic structures and reactivity of an S = 5/2 (HS) mononuclear non-heme (TMC)FeIII–OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of S = 1/2 (LS) FeIII–OOH complexes to understand parallels and differences in mechanisms of O–O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)FeIII–OOH]2+ complex is found to involve axial ligand coordination and a crossing to the LS surface for O–O bond homolysis. Both HS and LS FeIII–OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS FeIII–OOH, the transition state is late in O–O and early in C–H coordinates. However, for the HS FeIII–OOH, the transition state is early in O–O and further along in the C–H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS FeIII–OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS FeIII–OOH, the H-atom abstraction reactivity of HS FeIII–OOH is found to be highly dependent on both the ionization potential and C–H bond strength of the substrate. LS FeIII–OOH is found to be more effective in H-atom abstraction for strong C–H bonds, while the higher reduction potential of HS FeIII–OOH allows it to be active in electrophilic reactions without the requirement of O–O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS FeIII–OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds.

Binuclear non-heme iron enzymes activate O2 for diverse chemistries that include oxygenation of organic substrates and hydrogen atom abstraction. This process often involves the formation of peroxo-bridged biferric intermediates, only some of which can perform electrophilic reactions. To elucidate the geometric and electronic structural requirements to activate peroxo reactivity, the active peroxo intermediate in 4-aminobenzoate N-oxygenase (AurF) has been characterized spectroscopically and computationally. A magnetic circular dichroism study of reduced AurF shows that its electronic and geometric structures are poised to react rapidly with O2. Nuclear resonance vibrational spectroscopic definition of the peroxo intermediate formed in this reaction shows that the active intermediate has a protonated peroxo bridge. Density functional theory computations on the structure established here show that the protonation activates peroxide for electrophilic/single-electron-transfer reactivity. This activation of peroxide by protonation is likely also relevant to the reactive peroxo intermediates in other binuclear non-heme iron enzymes.