Iron(IV)hydroxide pK(a) and the role of thiolate ligation in C-H bond activation by cytochrome P450

A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn^V(O)(ttppc) (4) and Mn^III(H₂O)(ttppc) (1·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di- tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k₂ = 2.73(12) × 10⁴ M^-1 s^-1, which is ∼3 orders of magnitude larger than 4 ( k₂ = 17.4(1) M^-1 s^-1). Reaction of 3 with a series of para-substituted 2,6-di- tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k₂ = 510(10)-36(1.4) M^-1 s^-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Artificial Metalloproteins Containing Co4O4 Cubane Active Sites

Artificial metalloproteins (ArMs) containing Co4O4 cubane active sites were constructed via biotin-streptavidin technology. Stabilized by hydrogen bonds (H-bonds), terminal and cofacial CoIII-OH2 moieties are observed crystallographically in a series of immobilized cubane sites. Solution electrochemistry provided correlations of oxidation potential and pH. For variants containing Ser and Phe adjacent to the metallocofactor, 1e-/1H+ chemistry predominates until pH 8, above which the oxidation becomes pH-independent. Installation of Tyr proximal to the Co4O4 active site provided a single H-bond to one of a set of cofacial CoIII-OH2 groups. With this variant, multi-e-/multi-H+ chemistry is observed, along with a change in mechanism at pH 9.5 that is consistent with Tyr deprotonation. With structural similarities to both the oxygen-evolving complex of photosystem II (H-bonded Tyr) and to thin film water oxidation catalysts (Co4O4 core), these findings bridge synthetic and biological systems for water oxidation, highlighting the importance of secondary sphere interactions in mediating multi-e-/multi-H+ reactivity.

The Nature and Reactivity of Ferryl Heme in Compounds I and II

Aerobic organisms have evolved to activate oxygen from the atmosphere, which allows them to catalyze the oxidation of different kinds of substrates. This activation of oxygen is achieved by a metal center (usually iron or copper) buried within a metalloprotein. In the case of iron-containing heme enzymes, the activation of oxygen is achieved by formation of transient iron-oxo (ferryl) intermediates; these intermediates are called Compound I and Compound II. The Compound I and II intermediates were first discovered in the 1930s in horseradish peroxidase, and it is now known that these same species are used across the family of heme enzymes, which include all of the peroxidases, the heme catalases, the P450s, cytochrome c oxidase, and NO synthase. Many years have passed since the first observations, but establishing the chemical nature of these transient ferryl species remains a fundamental question that is relevant to the reactivity, and therefore the usefulness, of these species in biology. This Account summarizes experiments that were conceived and conducted at Leicester and presents our ideas on the chemical nature, stability, and reactivity of these ferryl heme species. We begin by briefly summarizing the early milestones in the field, from the 1940s and 1950s. We present comparisons between the nature and reactivity of the ferryl species in horseradish peroxidase, cytochrome c peroxidase, and ascorbate peroxidase; and we consider different modes of electron delivery to ferryl heme, from different substrates in different peroxidases. We address the question of whether the ferryl heme is best formulated as an (unprotonated) Fe^IV═O or as a (protonated) Fe^IV-OH species. A range of spectroscopic approaches (EXAFS, resonance Raman, Mossbauer, and EPR) have been used over many decades to examine this question, and in the last ten years, X-ray crystallography has also been employed. We describe how information from all of these studies has blended together to create an overall picture, and how the recent application of neutron crystallography has directly identified protonation states and has helped to clarify the precise nature of the ferryl heme in cytochrome c peroxidase and ascorbate peroxidase. We draw comparisons between the Compound I and Compound II species that we have observed in peroxidases with those found in other heme systems, notably the P450s, highlighting possible commonality across these heme ferryl systems. The identification of proton locations from neutron structures of these ferryl species opens the door for understanding the proton translocations that need to occur during O-O bond cleavage.

How Do Ring Size and π-Donating Thiolate Ligands Affect Redox-Active, α-Imino-N-heterocycle Ligand Activation?

Considerable effort has been devoted to the development of first-row transition-metal catalysts containing redox-active imino-pyridine ligands that are capable of storing multiple reducing equivalents. This property allows abundant and inexpensive first-row transition metals, which favor sequential one-electron redox processes, to function as competent catalysts in the concerted two-electron reduction of substrates. Herein we report the syntheses and characterization of a series of iron complexes that contain both π-donating thiolate and π-accepting (α-imino)-N-heterocycle redox-active ligands, with progressively larger N-heterocycle rings (imidazole, pyridine, and quinoline). A cooperative interaction between these complementary redox-active ligands is shown to dictate the properties of these complexes. Unusually intense charge-transfer (CT) bands, and intraligand metrical parameters, reminiscent of a reduced (α-imino)-N-heterocycle ligand (L•-), initially suggested that the electron-donating thiolate had reduced the N-heterocycle. Sulfur K-edge X-ray absorption spectroscopic (XAS) data, however, provides evidence for direct communication, via backbonding, between the thiolate sulfur and the formally orthogonal (α-imino)-N-heterocycle ligand π*-orbitals. DFT calculations provide evidence for extensive delocalization of bonds over the sulfur, iron, and (α-imino)-N-heterocycle, and TD-DFT shows that the intense optical CT bands involve transitions between a mixed Fe/S donor, and (α-imino)-N-heterocycle π*-acceptor orbital. The energies and intensities of the optical and S K-edge pre-edge XAS transitions are shown to correlate with N-heterocycle ring size, as do the redox potentials. When the thiolate is replaced with a thioether, or when the low-spin S = 0 Fe(II) is replaced with a high-spin S = 3/2 Co(II), the N-heterocycle ligand metrical parameters and electronic structure do not change relative to the neutral L0 ligand. With respect to the development of future catalysts containing redox-active ligands, the energy cost of storing reducing equivalents is shown to be lowest when a quinoline, as opposed to imidazole or pyridine, is incorporated into the ligand backbone of the corresponding Fe complex.

Density Functional Theory Study on the Demethylation Reaction between Methylamine, Dimethylamine, Trimethylamine, and Tamoxifen Catalyzed by a Fe(IV)-Oxo Porphyrin Complex

In this work, we studied computationally the N-demethylation reaction of methylamine, dimethylamine, and trimethylamine as archetypal examples of primary, secondary, and tertiary amines catalyzed by high-field low-spin Fe-containing enzymes such as cytochromes P450. Using DFT calculations, we found that the expected C-H hydroxylation process was achieved for trimethylamine. When dimethylamine and methylamine were studied, two different reaction mechanisms (C-H hydroxylation and a double hydrogen atom transfer) were computed to be energetically accessible and both are equally preferred. Both processes led to the formation of formaldehyde and the N-demethylated substrate. Finally, as an illustrative example, the relative contribution of the three primary oxidation routes of tamoxifen was rationalized through energetic barriers obtained from density functional calculations and docking experiments involving CYP3A4 and CYP2D6 isoforms. We found that the N-demethylation process was the intrinsically favored one, whereas other oxidation reactions required most likely preorganization imposed by the residues close to the active sites.

Structure and function of the cytochrome P450 peroxygenase enzymes

The cytochromes P450 (P450s or CYPs) constitute a large heme enzyme superfamily, members of which catalyze the oxidative transformation of a wide range of organic substrates, and whose functions are crucial to xenobiotic metabolism and steroid transformation in humans and other organisms. The P450 peroxygenases are a subgroup of the P450s that have evolved in microbes to catalyze the oxidative metabolism of fatty acids, using hydrogen peroxide as an oxidant rather than NAD(P)H-driven redox partner systems typical of the vast majority of other characterized P450 enzymes. Early members of the peroxygenase (CYP152) family were shown to catalyze hydroxylation at the α and β carbons of medium-to-long-chain fatty acids. However, more recent studies on other CYP152 family P450s revealed the ability to oxidatively decarboxylate fatty acids, generating terminal alkenes with potential applications as drop-in biofuels. Other research has revealed their capacity to decarboxylate and to desaturate hydroxylated fatty acids to form novel products. Structural data have revealed a common active site motif for the binding of the substrate carboxylate group in the peroxygenases, and mechanistic and transient kinetic analyses have demonstrated the formation of reactive iron-oxo species (compounds I and II) that are ultimately responsible for hydroxylation and decarboxylation of fatty acids, respectively. This short review will focus on the biochemical properties of the P450 peroxygenases and on their biotechnological applications with respect to production of volatile alkenes as biofuels, as well as other fine chemicals.

The structure of a one-electron oxidized Mn(iii)-bis(phenolate)dipyrrin radical complex and oxidation catalysis control via ligand-centered redox activity

The tetradentate ligand dppH3, which features a half-porphyrin and two electron-rich phenol moieties, was prepared and chelated to manganese. The mononuclear Mn(iii)-dipyrrophenolate complex 1 was structurally characterized. The metal ion lies in a square pyramidal environment, the apical position being occupied by a methanol molecule. Complex 1 displays two reversible oxidation waves at 0.00 V and 0.47 V vs. Fc⁺/Fc, which are assigned to ligand-centered processes. The one-electron oxidized species 1+ SbF6- was crystallized, showing an octahedral Mn(iii) center with two water molecules coordinated at both apical positions. The bond distance analysis and DFT calculations disclose that the radical is delocalized over the whole aromatic framework. Complex 1+ SbF6- exhibits an S_tot = 3/2 spin state due to the antiferromagnetic coupling between Mn(iii) and the ligand radical. The zero field splitting parameters are D = 1.6 cm^-1, E/D = 0.18(1), g_⊥ = 1.99 and g_∥ = 1.98. The dication 12+ is an integer spin system, which is assigned to a doubly oxidized ligand coordinated to a Mn(iii) metal center. Both 1 and 1+ SbF6- catalyze styrene oxidation in the presence of PhIO, but the nature of the main reaction product is different. Styrene oxide is the main reaction product when using 1, but phenylacetaldehyde is formed predominantly when using 1+ SbF6-. We examined the ability of complex 1+ SbF6- to catalyze the isomerization of styrene oxide and found that it is an efficient catalyst for the anti-Markovnikov opening of styrene oxide. The formation of phenylacetaldehyde from styrene therefore proceeds in a tandem E-I (epoxidation-isomerization) mechanism in the case of 1+ SbF6-. This is the first evidence of control of the reactivity for styrene oxidation by changing the oxidation state of a catalyst based on a redox-active ligand.

Spectroscopic studies of the cytochrome P450 reaction mechanisms

The cytochrome P450 monooxygenases (P450s) are thiolate heme proteins that can, often under physiological conditions, catalyze many distinct oxidative transformations on a wide variety of molecules, including relatively simple alkanes or fatty acids, as well as more complex compounds such as steroids and exogenous pollutants. They perform such impressive chemistry utilizing a sophisticated catalytic cycle that involves a series of consecutive chemical transformations of heme prosthetic group. Each of these steps provides a unique spectral signature that reflects changes in oxidation or spin states, deformation of the porphyrin ring or alteration of dioxygen moieties. For a long time, the focus of cytochrome P450 research was to understand the underlying reaction mechanism of each enzymatic step, with the biggest challenge being identification and characterization of the powerful oxidizing intermediates. Spectroscopic methods, such as electronic absorption (UV-Vis), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electron nuclear double resonance (ENDOR), Mössbauer, X-ray absorption (XAS), and resonance Raman (rR), have been useful tools in providing multifaceted and detailed mechanistic insights into the biophysics and biochemistry of these fascinating enzymes. The combination of spectroscopic techniques with novel approaches, such as cryoreduction and Nanodisc technology, allowed for generation, trapping and characterizing long sought transient intermediates, a task that has been difficult to achieve using other methods. Results obtained from the UV-Vis, rR and EPR spectroscopies are the main focus of this review, while the remaining spectroscopic techniques are briefly summarized. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

Probing Ligand Exchange in the P450 Enzyme CYP121 from Mycobacterium tuberculosis: Dynamic Equilibrium of the Distal Heme Ligand as a Function of pH and Temperature

CYP121 is a cytochrome P450 enzyme from Mycobacterium tuberculosis that catalyzes the formation of a C-C bond between the aromatic groups of its cyclodityrosine substrate (cYY). The crystal structure of CYP121 in complex with cYY reveals that the solvent-derived ligand remains bound to the ferric ion in the enzyme-substrate complex. Whereas in the generally accepted P450 mechanism, binding of the primary substrate in the active-site triggers the release of the solvent-derived ligand, priming the metal center for reduction and subsequent O2 binding. Here we employed sodium cyanide to probe the metal-ligand exchange of the enzyme and the enzyme-substrate complex. The cyano adducts were characterized by UV-vis, EPR, and ENDOR spectroscopies and X-ray crystallography. A 100-fold increase in the affinity of cyanide binding to the enzyme-substrate complex over the ligand-free enzyme was observed. The crystal structure of the [CYP121(cYY)CN] ternary complex showed a rearrangement of the substrate in the active-site, when compared to the structure of the binary [CYP121(cYY)] complex. Transient kinetic studies showed that cYY binding resulted in a lower second-order rate constant (kon (CN)) but a much more stable cyanide adduct with 3 orders of magnitude slower koff (CN) rate. A dynamic equilibrium between multiple high- and low-spin species for both the enzyme and enzyme-substrate complex was also observed, which is sensitive to changes in both pH and temperature. Our data reveal the chemical and physical properties of the solvent-derived ligand of the enzyme, which will help to understand the initial steps of the catalytic mechanism.

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide. These species are composed of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates. These complexes have been studied to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this Review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. This Review is organized by structural type of metal-oxygen intermediate and broken into subsections based on the metal (manganese and iron) and porphyrinoid ligand (porphyrin, corrole, and corrolazine).

Direct Observation of Oxygen Rebound with an Iron-Hydroxide Complex

The rebound mechanism for alkane hydroxylation was invoked over 40 years ago to help explain reactivity patterns in cytochrome P450, and subsequently has been used to provide insight into a range of biological and synthetic systems. Efforts to model the rebound reaction in a synthetic system have been unsuccessful, in part because of the challenge in preparing a suitable metal-hydroxide complex at the correct oxidation level. Herein we report the synthesis of such a complex. The reaction of this species with a series of substituted radicals allows for the direct interrogation of the rebound process, providing insight into this uniformly invoked, but previously unobserved process.

meso-Tetraphenylporphyrin Iron Chloride Catalyzed Selective Oxidative Cross-Coupling of Phenols

A novel catalytic system for oxidative cross-coupling of readily oxidized phenols with poor nucleophilic phenolic partners based on an iron meso-tetraphenylporphyrin chloride (Fe[TPP]Cl) complex in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) was developed. The unique chemoselectivity of this reaction is attributed to the coupling between a liberated phenoxyl radical with an iron-ligated phenolic coupling partner. The conditions are scalable for preparing a long list of unsymmetrical biphenols assembled from a less reactive phenolic unit substituted with alkyl or halide groups.

Hole Hopping through Tryptophan in Cytochrome P450

Electron-transfer kinetics have been measured in four conjugates of cytochrome P450 with surface-bound Ru-photosensitizers. The conjugates are constructed with enzymes from Bacillus megaterium (CYP102A1) and Sulfolobus acidocaldarius (CYP119). A W96 residue lies in the path between Ru and the heme in CYP102A1, whereas H76 is present at the analogous location in CYP119. Two additional conjugates have been prepared with (CYP102A1)W96H and (CYP119)H76W mutant enzymes. Heme oxidation by photochemically generated Ru³⁺ leads to P450 compound II formation when a tryptophan residue is in the path between Ru and the heme; no heme oxidation is observed when histidine occupies this position. The data indicate that heme oxidation proceeds via two-step tunneling through a tryptophan radical intermediate. In contrast, heme reduction by photochemically generated Ru⁺ proceeds in a single electron tunneling step with closely similar rate constants for all four conjugates.

The Enigmatic P450 Decarboxylase OleT Is Capable of, but Evolved To Frustrate, Oxygen Rebound Chemistry

OleT is a cytochrome P450 enzyme that catalyzes the removal of carbon dioxide from variable chain length fatty acids to form 1-alkenes. In this work, we examine the binding and metabolic profile of OleT with shorter chain length (n ≤ 12) fatty acids that can form liquid transportation fuels. Transient kinetics and product analyses confirm that OleT capably activates hydrogen peroxide with shorter substrates to form the high-valent intermediate Compound I and largely performs C-C bond scission. However, the enzyme also produces fatty alcohol side products using the high-valent iron oxo chemistry commonly associated with insertion of oxygen into hydrocarbons. When presented with a short chain fatty acid that can initiate the formation of Compound I, OleT oxidizes the diagnostic probe molecules norcarane and methylcyclopropane in a manner that is reminiscent of reactions of many CYP hydroxylases with radical clock substrates. These data are consistent with a decarboxylation mechanism in which Compound I abstracts a substrate hydrogen atom in the initial step. Positioning of the incipient substrate radical is a crucial element in controlling the efficiency of activated OH rebound.

Ni(II) Complexes of the Redox-Active Bis(2-aminophenyl)dipyrrin: Structural, Spectroscopic, and Theoretical Characterization of Three Members of an Electron Transfer Series

The sterically hindered bis(2-aminophenyl)dipyrrin ligand H₃^NL was prepared. X-ray diffraction discloses a bifurcated hydrogen bonding network involving the dipyrrin and one aniline ring. The reaction of H₃^NL with one equivalent of nickel(II) in the air produces a paramagnetic neutral complex, which absorbs intensively in the Vis-NIR region. Its electron paramagnetic resonance spectrum displays resonances at g₁ = 2.033, g₂ = 2.008, and g₃ = 1.962 that are reminiscent of an (S = 1/2) system having a predominant organic radical character. Both the structural investigation (X-ray diffraction) and density functional theory calculations on [Ni^II(^NL^•)] points to an unprecedented mixed "pyrrolyl-anilinyl" radical character. The neutral complex [Ni^II(^NL^•)] exhibits both a reversible oxidation wave at -0.28 V vs Fc⁺/Fc and a reversible reduction wave at -0.91 V. The anion was found to be highly air-sensitive, but could be prepared by reduction with cobaltocene and structurally characterized. It comprises a Ni(II) ion coordinated to a closed-shell trianionic ligand and hence can be formulated as [Ni^II(^NL)]^-. The cation was generated by reacting [Ni^II(^NL^•)] with one equivalent of silver hexafluoroantimonate. By X-ray diffraction we established that it contains an oxidized, closed-shell ligand coordinated to a nickel(II) ion. We found that a reliable hallmark for both the oxidation state of the ligand and the extent of delocalization within the series is the bond connecting the dipyrrin and the aniline, which ranges between 1.391 Å (cation) and 1.449 Å (anion). The cation and anion exhibit a rich Vis-NIR spectrum, despite their nonradical nature. The low energy bands correspond to ligand-based electronic excitations. Hence, the HOMO-LUMO gap is small, and the redox processes in the electron transfer series are exclusively ligand-centered.

Correlation between the potency of flavonoids for cytochrome c reduction and inhibition of cardiolipin-induced peroxidase activity

There are large differences between flavonoids to protect against apoptosis, a process in which cytochrome c (Cyt c) plays a key role. In this work, we show that 7 of 13 flavonoids studied have a capacity to reduce Cyt c similar or higher than ascorbate, the flavonols quercetin, kaempferol and myricetin, flavanol epigallocatechin-gallate, anthocyanidins cyanidin and malvidin, and the flavone luteolin. In contrast, the kaempferol 3(O)- and 3,4'(O)-methylated forms, the flavanone naringenin, and also apigenin and chrysin, had a negligible reducing capacity. Equilibrium dialysis and quenching of 1,6-diphenyl-1,3,5-hexatriene fluorescence experiments showed that flavonoids did not interfere with Cyt c binding to cardiolipin (CL)/phosphatidylcholine (PC) vesicles. However, the CL-induced loss of Cyt c Soret band intensity was largely attenuated by flavonoids, pointing out a stabilizing action against Cyt c unfolding in the complex. Moreover, flavonoids that behave as Cyt c reductants also inhibited the pro-apoptotic CL-induced peroxidase activity of Cyt c, indicating that modulation of Cyt c signaling are probable mechanisms behind the protective biological activities of flavonoids. © 2016 BioFactors, 43(3):451-468, 2017.

A new look at the role of thiolate ligation in cytochrome P450

Protonated ferryl (or iron(IV)hydroxide) intermediates have been characterized in several thiolate-ligated heme proteins that are known to catalyze C-H bond activation. The basicity of the ferryl intermediates in these species has been proposed to play a critical role in facilitating this chemistry, allowing hydrogen abstraction at reduction potentials below those that would otherwise lead to oxidative degradation of the enzyme. In this contribution, we discuss the events that led to the assignment and characterization of the unusual iron(IV)hydroxide species, highlighting experiments that provided a quantitative measure of the ferryl basicity, the iron(IV)hydroxide pKa. We then turn to the importance of the iron(IV)hydroxide state, presenting a new way of looking at the role of thiolate ligation in these systems.

Fast Hydrogen Atom Abstraction by a Hydroxo Iron(III) Porphyrazine

A reactive hydroxoferric porphyrazine complex, [(PyPz)Fe^III(OH) (OH₂)]⁴⁺ (1, PyPz = tetramethyl-2,3-pyridino porphyrazine), has been prepared via one-electron oxidation of the corresponding ferrous species [(PyPz)Fe^II(OH₂)₂]⁴⁺ (2). Electrochemical analysis revealed a pH-dependent and remarkably high Fe^III-OH/Fe^II-OH₂ reduction potential of 680 mV vs Ag/AgCl at pH 5.2. Nernstian behavior from pH 2 to pH 8 indicates a one-proton, one-electron interconversion throughout that range. The O-H bond dissociation energy of the Fe^II-OH₂ complex was estimated to be 84 kcal mol^-1. Accordingly, 1 reacts rapidly with a panel of substrates via C-H hydrogen atom transfer (HAT), reducing 1 to [(PyPz)Fe^II(OH₂)₂]⁴⁺ (2). The second-order rate constant for the reaction of [(PyPz)Fe^III(OH) (OH₂)]⁴⁺ with xanthene was 2.22 × 10³ M^-1 s^-1, 5-6 orders of magnitude faster than other reported Fe^III-OH complexes and faster than many ferryl complexes.

Triphenyltin degradation and proteomic response by an engineered Escherichia coli expressing cytochrome P450 enzyme

Although triphenyltin (TPT) degradation pathway has been determined, information about the enzyme and protein networks involved was severely limited. To this end, a cytochrome P450 hydroxylase (CYP450) gene from Bacillus thuringiensis was cloned and expressed in Escherichia coli BL21 (DE3), namely E. coli pET32a-CYP450, whose dosage at 1gL^-1 could degrade 54.6% TPT at 1mgL^-1 within 6 d through attacking the carbon-tin bonds of TPT by CYP450. Sequence analysis verified that the CYP450 gene had a 1214bp open reading frame, encoding a protein with 404 amino acids. Proteomic analysis determined that 60 proteins were significantly differentially regulated expression in E. coli pET32a-CYP450 after TPT degradation. The up-regulated proteins enriched in a network related to transport, cell division, biosynthesis of amino acids and secondary metabolites, and microbial metabolism in diverse environments. The current findings demonstrated for the first time that P450 received electrons transferring from NADH could effectively cleave carbon-metal bonds.

Evidence for a Di-μ-oxo Diamond Core in the Mn(IV)/Fe(IV) Activation Intermediate of Ribonucleotide Reductase from Chlamydia trachomatis

High-valent iron and manganese complexes effect some of the most challenging biochemical reactions known, including hydrocarbon and water oxidations associated with the global carbon cycle and oxygenic photosynthesis, respectively. Their extreme reactivity presents an impediment to structural characterization, but their biological importance and potential chemical utility have, nevertheless, motivated extensive efforts toward that end. Several such intermediates accumulate during activation of class I ribonucleotide reductase (RNR) β subunits, which self-assemble dimetal cofactors with stable one-electron oxidants that serve to initiate the enzyme's free-radical mechanism. In the class I-c β subunit from Chlamydia trachomatis, a heterodinuclear Mn(II)/Fe(II) complex reacts with dioxygen to form a Mn(IV)/Fe(IV) intermediate, which undergoes reduction of the iron site to produce the active Mn(IV)/Fe(III) cofactor. Herein, we assess the structure of the Mn(IV)/Fe(IV) activation intermediate using Fe- and Mn-edge extended X-ray absorption fine structure (EXAFS) analysis and multifrequency pulse electron paramagnetic resonance (EPR) spectroscopy. The EXAFS results reveal a metal-metal vector of 2.74-2.75 Å and an intense light-atom (C/N/O) scattering interaction 1.8 Å from the Fe. Pulse EPR data reveal an exchangeable deuterium hyperfine coupling of strength |T| = 0.7 MHz, but no stronger couplings. The results suggest that the intermediate possesses a di-μ-oxo diamond core structure with a terminal hydroxide ligand to the Mn(IV).

A metalloporphyrin-based porous organic polymer as an efficient catalyst for the catalytic oxidation of olefins and arylalkanes

A metalloporphyrin-based porous organic polymer contains both micropores and mesopores, which are favourable for mass transfer in heterogeneous catalysis.

Autocatalytic dioxygen activation to produce an iron(v)-oxo complex without any reductants

An iron(v)-oxo complex with a tetraamido macrocyclic ligand, [(TAML)Fe^V(O)]⁻, was produced by reacting [(TAML)Fe^III]⁻ with dioxygen without any electron source in acetone at 298 K.

Spectroscopic Investigations of Catalase Compound II: Characterization of an Iron(IV) Hydroxide Intermediate in a Non-thiolate-Ligated Heme Enzyme

We report on the protonation state of Helicobacter pylori catalase compound II. UV/visible, Mössbauer, and X-ray absorption spectroscopies have been used to examine the intermediate from pH 5 to 14. We have determined that HPC-II exists in an iron(IV) hydroxide state up to pH 11. Above this pH, the iron(IV) hydroxide complex transitions to a new species (pKa = 13.1) with Mössbauer parameters that are indicative of an iron(IV)-oxo intermediate. Recently, we discussed a role for an elevated compound II pKa in diminishing the compound I reduction potential. This has the effect of shifting the thermodynamic landscape toward the two-electron chemistry that is critical for catalase function. In catalase, a diminished potential would increase the selectivity for peroxide disproportionation over off-pathway one-electron chemistry, reducing the buildup of the inactive compound II state and reducing the need for energetically expensive electron donor molecules.

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C-H activation

Since our initial report in 1976, the oxygen rebound mechanism has become the consensus mechanistic feature for an expanding variety of enzymatic C-H functionalization reactions and small molecule biomimetic catalysts. For both the biotransformations and models, an initial hydrogen atom abstraction from the substrate (R-H) by high-valent iron-oxo species (Feⁿ=O) generates a substrate radical and a reduced iron hydroxide, [Fe^n-1-OH ·R]. This caged radical pair then evolves on a complicated energy landscape through a number of reaction pathways, such as oxygen rebound to form R-OH, rebound to a non-oxygen atom affording R-X, electron transfer of the incipient radical to yield a carbocation, R⁺, desaturation to form olefins, and radical cage escape. These various flavors of the rebound process, often in competition with each other, give rise to the wide range of C-H functionalization reactions performed by iron-containing oxygenases. In this review, we first recount the history of radical rebound mechanisms, their general features, and key intermediates involved. We will discuss in detail the factors that affect the behavior of the initial caged radical pair and the lifetimes of the incipient substrate radicals. Several representative examples of enzymatic C-H transformations are selected to illustrate how the behaviors of the radical pair [Fe^n-1-OH ·R] determine the eventual reaction outcome. Finally, we discuss the powerful potential of "radical rebound" processes as a general paradigm for developing novel C-H functionalization reactions with synthetic, biomimetic catalysts. We envision that new chemistry will continue to arise by bridging enzymatic "radical rebound" with synthetic organic chemistry.

Direct visualization of a Fe(IV)-OH intermediate in a heme enzyme

Catalytic heme enzymes carry out a wide range of oxidations in biology. They have in common a mechanism that requires formation of highly oxidized ferryl intermediates. It is these ferryl intermediates that provide the catalytic engine to drive the biological activity. Unravelling the nature of the ferryl species is of fundamental and widespread importance. The essential question is whether the ferryl is best described as a Fe(IV)=O or a Fe(IV)–OH species, but previous spectroscopic and X-ray crystallographic studies have not been able to unambiguously differentiate between the two species. Here we use a different approach. We report a neutron crystal structure of the ferryl intermediate in Compound II of a heme peroxidase; the structure allows the protonation states of the ferryl heme to be directly observed. This, together with pre-steady state kinetic analyses, electron paramagnetic resonance spectroscopy and single crystal X-ray fluorescence, identifies a Fe(IV)–OH species as the reactive intermediate. The structure establishes a precedent for the formation of Fe(IV)–OH in a peroxidase.

The nature of the ferryl intermediate generated in reactions catalysed by heme-containing enzymes is uncertain, due to the ambiguity of X-ray crystallography data. Here, the authors apply neutron diffraction, kinetics and other spectroscopy to directly observe a protonated ferryl intermediate in a heme peroxidase.

Reactivity of an FeIV-Oxo Complex with Protons and Oxidants

High-valent Fe-OH species are often invoked as key intermediates but have only been observed in Compound II of cytochrome P450s. To further address the properties of non-heme FeIV-OH complexes, we demonstrate the reversible protonation of a synthetic FeIV-oxo species containing a tris-urea tripodal ligand. The same protonated FeIV-oxo species can be prepared via oxidation, suggesting that a putative FeV-oxo species was initially generated. Computational, Mössbauer, XAS, and NRVS studies indicate that protonation of the FeIV-oxo complex most likely occurs on the tripodal ligand, which undergoes a structural change that results in the formation of a new intramolecular H-bond with the oxido ligand that aids in stabilizing the protonated adduct. We suggest that similar protonated high-valent Fe-oxo species may occur in the active sites of proteins. This finding further argues for caution when assigning unverified high-valent Fe-OH species to mechanisms.

Catalytic strategy for carbon-carbon bond scission by the cytochrome P450 OleT

OleT is a cytochrome P450 that catalyzes the hydrogen peroxide-dependent metabolism of Cn chain-length fatty acids to synthesize Cn-1 1-alkenes. The decarboxylation reaction provides a route for the production of drop-in hydrocarbon fuels from a renewable and abundant natural resource. This transformation is highly unusual for a P450, which typically uses an Fe(4+)-oxo intermediate known as compound I for the insertion of oxygen into organic substrates. OleT, previously shown to form compound I, catalyzes a different reaction. A large substrate kinetic isotope effect (≥8) for OleT compound I decay confirms that, like monooxygenation, alkene formation is initiated by substrate C-H bond abstraction. Rather than finalizing the reaction through rapid oxygen rebound, alkene synthesis proceeds through the formation of a reaction cycle intermediate with kinetics, optical properties, and reactivity indicative of an Fe(4+)-OH species, compound II. The direct observation of this intermediate, normally fleeting in hydroxylases, provides a rationale for the carbon-carbon scission reaction catalyzed by OleT.

Methane-Oxidizing Enzymes: An Upstream Problem in Biological Gas-to-Liquids Conversion

Biological conversion of natural gas to liquids (Bio-GTL) represents an immense economic opportunity. In nature, aerobic methanotrophic bacteria and anaerobic archaea are able to selectively oxidize methane using methane monooxygenase (MMO) and methyl coenzyme M reductase (MCR) enzymes. Although significant progress has been made toward genetically manipulating these organisms for biotechnological applications, the enzymes themselves are slow, complex, and not recombinantly tractable in traditional industrial hosts. With turnover numbers of 0.16-13 s(-1), these enzymes pose a considerable upstream problem in the biological production of fuels or chemicals from methane. Methane oxidation enzymes will need to be engineered to be faster to enable high volumetric productivities; however, efforts to do so and to engineer simpler enzymes have been minimally successful. Moreover, known methane-oxidizing enzymes have different expression levels, carbon and energy efficiencies, require auxiliary systems for biosynthesis and function, and vary considerably in terms of complexity and reductant requirements. The pros and cons of using each methane-oxidizing enzyme for Bio-GTL are considered in detail. The future for these enzymes is bright, but a renewed focus on studying them will be critical to the successful development of biological processes that utilize methane as a feedstock.

Rapid kinetic methods to dissect steroidogenic cytochrome P450 reaction mechanisms

All cytochrome P450 enzyme reactions involve a catalytic cycle with several discreet physical or chemical steps. This cycle ends with the formation of the reactive heme iron-oxygen complex, which oxygenates substrate. While the steps might be very similar for each P450 enzyme, the rates of each step varies tremendously for each enzyme and sometimes even for different reactions catalyzed by the same enzyme. For example, the rate-limiting step for most bacterial P450 enzymes, with turnover numbers over 1000s(-1), is the second electron transfer. In contrast, steroidogenic P450s from eukaryotes catalyze much slower reactions, with turnover numbers of ∼5-250min(-1); therefore, assumptions about kinetic properties for the mammalian P450 enzymes based on the bacterial enzymes are tenuous. In order to dissect the rates for individual steps, special techniques that isolate individual steps and/or single turnovers are required. This article will review the theoretical principles and practical considerations for several of these techniques, with illustrative published examples. The reader should gain an appreciation for the appropriate methods used to interrogate particular steps in the P450 reaction cycle.

Mechanism of 17α,20-Lyase and New Hydroxylation Reactions of Human Cytochrome P450 17A1: 18O LABELING AND OXYGEN SURROGATE EVIDENCE FOR A ROLE OF A PERFERRYL OXYGEN

Cytochrome P450 (P450) reactions can involve C-C bond cleavage, and several of these are critical in steroid and sterol biosynthesis. The mechanisms of P450s 11A1, 17A1, 19A1, and 51A1 have been controversial, in the context of the role of ferric peroxide (FeO2 (-)) versus perferryl (FeO(3+), compound I) chemistry. We reinvestigated the 17α-hydroxyprogesterone and 17α-hydroxypregnenolone 17α,20-lyase reactions of human P450 17A1 and found incorporation of one (18)O atom (from (18)O2) into acetic acid, consonant with proposals for a ferric peroxide mechanism (Akhtar, M., Lee-Robichaud, P., Akhtar, M. E., and Wright, J. N. (1997) J. Steroid Biochem. Mol. Biol. 61, 127-132; Akhtar, M., Wright, J. N., and Lee-Robichaud, P. (2011) J. Steroid Biochem. Mol. Biol. 125, 2-12). However, the reactions were supported by iodosylbenzene (a precursor of the FeO(3+) species) but not by H2O2 We propose three mechanisms that can involve the FeO(3+) entity and that explain the (18)O label in the acetic acid, two involving the intermediacy of an acetyl radical and one a steroid 17,20-dioxetane. P450 17A1 was found to perform 16-hydroxylation reactions on its 17α-hydroxylated products to yield 16,17α-dihydroxypregnenolone and progesterone, suggesting the presence of an active perferryloxo active species of P450 17A1 when its lyase substrate is bound. The 6β-hydroxylation of 16α,17α-dihydroxyprogesterone and the oxidation of both 16α,17α-dihydroxyprogesterone and 16α,17α-dihydroxypregnenolone to 16-hydroxy lyase products were also observed. We provide evidence for the contribution of a compound I mechanism, although contribution of a ferric peroxide pathway in the 17α,20-lyase reaction cannot be excluded.