Models for peroxidase compound I: generation and spectroscopic characterization of new oxoferryl porphyrin .pi. cation radical species

Comparative oxidative ability of mononuclear and dinuclear high-valent iron-oxo species towards the activation of methane: does the axial/bridge atom modulate the reactivity?

Over the years, mononuclear Fe^IVO species have been extensively studied, but the presence of dinuclear Fe^IVO species in soluble methane monooxygenase (sMMO) has inspired the development of biomimic models that could activate inert substrates such as methane. There are some successful attempts; particularly the [(Por)(m-CBA) Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- species has been reported to activate methane and yield decent catalytic turnover numbers and therefore regarded as the closest to the sMMO enzyme functional model, as no mononuclear Fe^IVO analogues could achieve this feat. In this work, we have studied a series of mono and dinuclear models using DFT and ab initio DLPNO-CCSD(T) calculations to probe the importance of nuclearity in enhancing the reactivity. We have probed the catalytic activities of four complexes: [(HO)Fe^IV(O)(Por)]^- (1), [(HO)Fe^IV(O)(Por˙⁺)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-O)Fe^IV(O) (Por˙⁺)]^- (3) and N-bridged dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)Fe^IV(O)(Por˙⁺)]ⁿ {X = N 4a (n = -2), NH 4b (n = -1) and NH₂4c (n = 0)} to understand the role of nuclearity in the reactivity. DFT calculations performed on species 1-4 suggest an interesting variation among them, with species 1-3 possessing an intermediate spin (S = 1) as a ground state and species 4 possessing a high-spin (S = 2) as a ground state. Furthermore, the two Fe^IV centres in species 3 and 4 are antiferromagnetically coupled, yielding a singlet state with a distinct difference in their electronic structure. On the other hand, species 2 exhibits a ferromagnetic coupling between the Fe^IV and the Por˙⁺ moiety. Our calculations suggest that the higher barriers for the C-H bond activation of methane and the rebound step for species 1 and 3 are very high in energy, rendering them unreactive towards methane, while species 2 and 4 have lower barriers, suggesting their reactivity towards methane. Studies on the system reveal that model 4a has multiple FeN bonds facilitating greater reactivity, whereas the other two models have longer Fe-N bonds and less radical character with steeper barriers. Strong electronic cooperativity is found to be facilitated by the bridging nitride atom, and this cooperativity is suppressed by substituents such as oxygen, rendering them inactive. Thus, our study unravels that apart from enhancing the nuclearity, bridging atoms that facilitate strong cooperation between the metals are required to activate very inert substrates such as methane, and our results are broadly in agreement with earlier experimental findings.

Rate-Limiting Step of Epoxidation Reaction of the Oxoiron(IV) Porphyrin π-Cation Radical Complex: Electron Transfer Coupled Bond Formation Mechanism

Epoxidation reactions catalyzed by high-valent metal-oxo species are key reactions in various biological and chemical processes. Because the redox potentials of alkenes are higher than those of most high-valent metal-oxo species, the electron transfer (ET) from the alkene to the high-valent metal-oxo species in the epoxidation reaction is endergonic and must be coupled with another exergonic process. To reveal the mechanism of the ET, we performed a Marcus plot analysis for the epoxidation reaction of the oxoiron(IV) porphyrin π-cation radical complex (compound I) with alkene. The Marcus plots can be simulated with a linear line with the gradient of 0.50 when the redox potential of compound I varies and 0.07 when the redox potential of alkene varies. These results indicate that the ET process is involved in the rate-limiting step and coupled with the following O-C bond formation process: ET coupled bond formation mechanism. The DFT calculations support this conclusion and disclose the details of the mechanism. As the alkene comes close to the oxo ligand, the energy of the highest occupied molecular orbital (HOMO) of the alkene increases and the energy for the ET becomes small enough to allow the ET. Finally, the ET occurs from the HOMO of the alkene to the porphyrin π-radical orbital. The shift of one electron from the HOMO of the alkene by the ET simultaneously results in the O-C half bond formation between the oxo ligand and the alkene. The ET process itself is still endergonic and reversible, but the bond formation coupled with the ET changes the overall process to exergonic and irreversible. We also discuss the similarity with the aromatic hydroxylation reaction and the relevance to the epoxidation reactions of other metal-oxo complexes and peracid.

An organometallic catalase mimic with exceptional activity, H2O2 stability, and catalase/peroxidase selectivity

Manganese-porphyrin and -salen redox therapeutics catalyze redox reactions involving O₂˙^-, H₂O₂, and other reactive oxygen species, thereby modulating cellular redox states. Many of these complexes perform catalase reactions via high-valent Mn-oxo or -hydroxo intermediates that oxidize H₂O₂ to O₂, but these intermediates can also oxidize other molecules (e.g., thiols), which is peroxidase reactivity. Whether catalase or peroxidase reactivity predominates depends on the metal-ligand set and the local environment, complicating predictions of what therapeutic effects (e.g., promoting vs. suppressing apoptosis) a complex might produce in a given disease. We recently reported an organoruthenium complex (Ru1) that catalyzes ABTS˙^- reduction to ABTS^2- with H₂O₂ as the terminal reductant. Given that H₂O₂ is thermodynamically a stronger oxidant than ABTS˙^-, we reasoned that the intermediate that reduced ABTS˙^- would also be able to reduce H₂O₂ to H₂O. Herein we demonstrate Ru1-catalyzed H₂O₂ disproportionation into O₂ and H₂O, exhibiting an 8,580-fold faster catalase TOF vs. peroxidase TOF, which is 89.2-fold greater than the highest value reported for a Mn-porphyin or -salen complex. Furthermore, Ru1 was 120-fold more stable to H₂O₂ than the best MnSOD mimic (TON = 4000 vs. 33.4) Mechanistic studies provide evidence that the mechanism for Ru1-catalyzed H₂O₂ disproportionation is conserved with the mechanism for ABTS˙^- reduction. Therapeutic effects of redox catalysts can be predicted with greater accuracy for catalysts that exhibit exclusively catalase activity, thereby facilitating the development of future redox therapeutic strategies for diseases.

57Fe Mössbauer parameters from domain based local pair-natural orbital coupled-cluster theory

We report on applications of the domain based local pair-natural orbital (PNO) coupled-cluster method within the singles and doubles approximation (DLPNO-CCSD) to the calculation of ⁵⁷Fe isomer shifts and quadrupole splittings in a small training set of iron complexes consisting of large molecular ligands and iron atoms in varying charge, spin, and oxidation states. The electron densities and electric field gradients needed for these calculations were obtained within the recently implemented analytic derivative scheme. A method for the direct treatment of scalar relativistic effects in the calculation of effective electron densities is described by using the first-order Douglas-Kroll-Hess Hamiltonian and a Gaussian charge distribution model for the nucleus. The performance of DLPNO-CCSD is compared with four modern-day density functionals, namely, RPBE, TPSS, B3LYP, and B2PLYP, as well as with the second-order Møller-Plesset perturbation theory. An excellent correlation between the calculated electron densities and the experimental isomer shifts is attained with the DLPNO-CCSD method. The correlation constant a obtained from the slope of the linear correlation plot is found to be ≈-0.31 a.u.³ mm s^-1, which agrees very well with the experimental calibration constant α = -0.31 ± 0.04 a.u.³ mm s^-1. This value of a is obtained consistently using both nonrelativistic and scalar relativistic DLPNO-CCSD electron densities. While the B3LYP and B2PLYP functionals achieve equally good correlation between theory and experiment, the correlation constant a is found to deviate from the experimental value. Similar trends are observed also for quadrupole splittings. The value of the nuclear quadrupole moment for ⁵⁷Fe is estimated to be 0.15 b at the DLPNO-CCSD level. This is consistent with previous results and is here supported by a higher level of theory. The DLPNO-CCSD results are found to be insensitive to the intrinsic approximations in the method, in particular the PNO occupation number truncation error, while the results obtained with density functional theory (DFT) are found to depend on the choice of the functional. In a statistical sense, i.e., on the basis of the linear regression analysis, however, the accuracies of the DFT and DLPNO-CCSD results can be considered comparable.

Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count

Metallocorroles wherein the metal ion is Mn^III and formally Fe^IV are studied here using field- and frequency-domain electron paramagnetic resonance techniques. The Mn^III corrole, Mn(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole trianion), exhibits the following S = 2 zero-field splitting (zfs) parameters: D = -2.67(1) cm^-1, |E| = 0.023(5) cm^-1. This result and those for other Mn^III tetrapyrroles indicate that when D ≈ - 2.5 ± 0.5 cm^-1 for 4- or 5-coordinate and D ≈ - 3.5 ± 0.5 cm^-1 for 6-coordinate complexes, the ground state description is [Mn^III(Cor^3-)]⁰ or [Mn^III(P^2-)]⁺ (Cor = corrole, P = porphyrin). The situation for formally Fe^IV corroles is more complicated, and it has been shown that for Fe(Cor)X, when X = Ph (phenyl), the ground state is a spin triplet best described by [Fe^IV(Cor^3-)]⁺, but when X = halide, the ground state corresponds to [Fe^III(Cor^•2-)]⁺, wherein an intermediate spin (S = ³/₂) Fe^III is antiferromagnetically coupled to a corrole radical dianion (S = ¹/₂) to also give an S = 1 ground state. These two valence isomers can be distinguished by their zfs parameters, as determined here for Fe(tpc)X, X = Ph, Cl (tpc = 5,10,15-triphenylcorrole trianion). The complex with axial phenyl gives D = 21.1(2) cm^-1, while that with axial chloride gives D = 14.6(1) cm^-1. The D value for Fe(tpc)Ph is in rough agreement with the range of values reported for other Fe^IV complexes. In contrast, the D value for Fe(tpc)Cl is inconsistent with an Fe^IV description and represents a different type of iron center. Computational studies corroborate the zfs for the two types of iron corrole complexes. Thus, the zfs of metallocorroles can be diagnostic as to the electronic structure of a formally high oxidation state metallocorrole, and by extension to metalloporphyrins, although such studies have yet to be performed.

Assessment of the intramolecular magnetic interactions in the highly saddled iron(iii) porphyrin π-radical cations: the change from planar to saddle conformations

The intramolecular magnetic interactions in one-electron oxidized iron(iii) porphyrin π-radical cations, [Fe(OETPP˙)Cl][SbCl6] (1), [Fe(OMTPP˙)Cl][SbCl6] (2) and [Fe(TPP˙)Cl][SbCl6] (3), have been compared by means of X-ray crystallography, SQUID magnetometry, cyclic voltammetry, UV-Vis spectroelectrochemical analysis, NMR spectroscopy analysis and unrestricted DFT calculations. Unlike a generally recognized antiferromagnetic coupling dxy↑dxz↑dyz↑dz2↑dx2-y2↑P˙+(a2u)↓ (S = 2) state via a weak bonding interaction as in (3), we have disclosed that a strong bonding interaction among iron dx2-y2 and porphyrin a2u orbitals forms in (1) into a highly delocalized Ψπ = [P˙+(a2u) + FeIII(dx2-y2, dz2)] orbital that is able to accommodate two spin-paired electrons to form the Ψπ2dxy1dxz1dyz1, dz21 (S = 2) ground state. Concurrently, the spin polarization effect is exerted on the paired spins in the Ψπ orbital by magnetic induction from the remaining unpaired electrons in the iron d orbitals. The interpretation mentioned above is further verified by the diamagnetic nature of the saddled copper(ii) porphyrin π-cation radical, CuII(OETPP˙)(ClO4) (S = 0), where the strong bonding interaction leads to the Ψπ2dxy2dxz2dyz2dz22 (S = 0) ground state but no spin polarization exists. Thus, the magnetic nature of the iron(iii) porphyrin π-radical cation is tuneable by saddling the ring planarity.

Experimental and theoretical studies of the porphyrin ligand effect on the electronic structure and reactivity of oxoiron(iv) porphyrin π-cation-radical complexes

Oxoiron(IV) porphyrin π-cation-radical complexes (Cpd I) have been studied as models for reactive intermediates called compound I in cytochromes P450, peroxidases, and catalases. It has been well known that the electronic structure and reactivity of Cpd I are modulated by the substituted position and the electron-withdrawing ability of the substituent. However, there still remain two major questions: (1) how many electronegative halogen atoms should be introduced in the meso-phenyl group to switch the porphyrin π-cation-radical state of Cpd I? (2) How does the electron-withdrawing effect of the substituent modulate the reactivity of Cpd I? To answer these two questions, we here performed experimental and theoretical studies on the electron-withdrawing effect of the meso-substituent. We gradually increased the electron-withdrawing effect by increasing the number of fluorine atoms in the meso-phenyl group. Spectroscopic analyses of these Cpd I models reveal that the porphyrin radical state shifts from having a_2u radical character to having a_1u radical character with an increase in the number of the fluorine atoms in the phenyl group, and the ground state of Cpd I switches from the a_2u state to the a_1u state when four fluorine atoms are introduced in the meso-phenyl group. The switch of the radical state is predicted well by LC-BLYP, but not by the commonly used B3LYP. The theoretical calculations indicate that the electron-withdrawing substituent makes Cpd I more reactive by stabilizing the ferric porphyrin state (product state) more than the Cpd I state (reactant state), generating a larger free energy change in the oxygenation reaction (ΔG) of Cpd I.

Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes

Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [Fe^V(N)(TPP)] (1, TPP^2– = tetraphenylporphyrinato), and [Fe^V(N)(cyclam-ac)]⁺ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and ⁵⁷Fe Mössbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [Fe^V(N)(MePy₂tacn)](PF₆)₂ (3, MePy₂tacn = methyl-N′,N″-bis(2-picolyl)-1,4,7-triazacyclononane), [Fe^V(N){PhB(t-BuIm)₃}]⁺ (4, PhB(^tBuIm)₃^– = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [Fe^V(O)(TAML)]⁻ (5, TAML^4– = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and -oxo complexes 1–3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g_|| < g_⊥ ≤ 2 were measured for them, and their g_|| and g_⊥ values were found to obey a simple relation of g_⊥² + (2 – g_∥)² = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g_|| < 2 < g_⊥. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and -oxo complexes feature electronic structures akin to those found for complexes 1–3 and 5. Therefore, the characteristic EPR signals determined for 1–3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.

Manganese(ii) complexes of tetradentate 4N ligands with diazepane backbones for catalytic olefin epoxidation: effect of nucleophilicity of peroxo complexes on reactivity

Mononuclear Mn(ii) complexes of linear 4N ligands have been studied as catalysts for deformylation of aldehydes and epoxidation (H₂O₂) of unfunctionalised olefins to understand the role of ligand stereoelectronic factors.

Further Studies on the Oxidation State of Iron in μ-Oxo Dimeric Phthalocyanine Complexes

The reaction with sodium cyanide of the μ-oxo-bridged complex of tetra-4-tert-butyl-substituted iron phthalocyanine (form ‘690’) and that of the product of its treatment with organic bases such as Py, Im, etc. (form ‘627’) result in the formation of the same ferrous bis-cyanide complex Na 2[ Pc t Fe II ( CN )2] which can be readily oxidized to the analogous ferric complex Na [ Pc t Fe III ( CN )2]. Form ‘690’ has been oxidized to the corresponding ferric μ-oxo complex (form ‘630’). Data for all μ-oxo-bridged complexes (chemical behavior; electronic, NMR, Mössbauer, X-ray photoelectron and ESR spectra; magnetic susceptibility) are discussed, and based on them, the following structures are proposed: ( HPc t Fe II )2 O (form ‘690’), H 2[( LPc t Fe II )2 O ] (form ‘627’) and ( Pc t Fe III )2 O (form ‘630’).

Ground and Excited State Electronic Properties of Halogenated Tetraarylporphyrins: Tuning the Building Blocks for Porphyrin-based Photonic Devices

The rational design of molecular photonic devices relies on the ability to select components with predictable electronic structure, excited state lifetimes and redox chemistry. Electronic communication in multiporphyrin arrays depends critically on the relative energies and electron density distributions of the frontier molecular orbitals, especially the energetically close highest occupied molecular orbitals (a2u and a1u). To explore how these ground and excited state properties can be modulated, we have synthesized and characterized 40 free base ( Fb ), magnesium and zinc tetraarylporphyrins. The porphyrins bear meso-substituents with the following substitution patterns: (1) four identical substituents (phenyl, o-chlorophenyl, p-chlorophenyl, o,o'-difluorophenyl, pentafluorophenyl, mesityl); (2) one, two, three or four o,o'-dichlorophenyl substituents; (3) one p-ethynylphenyl group and three mesityl or pentafluorophenyl groups; (4) one p-ethynyl-o,o″-dichlorophenyl or p-ethynyl-o,o″-dimethylphenyl and three phenyl groups. For each neutral complex the ground state electronic properties were investigated using electrochemical methods and optical absorption spectroscopy. Similarly the absorption, emission, and relaxation properties of the lowest singlet excited state were probed by time-resolved absorption and fluorescence methods. Each oxidized complex was investigated by static absorption and liquid and frozen solution EPR spectroscopy. The collective results of these investigations have provided insights into the direct (orbital overlap) and indirect (inductive/conjugative) mechanisms by which halogenated phenyl rings influence the static and dynamic electronic properties of neutral and oxidized porphyrinic chromophores. Three key findings are as follows. (1) The effective electron-withdrawing strength of halogenated phenyl rings required to reverse the ordering of the a2u and a1u HOMOs in Mg versus Zn tetraarylporphyrins has been elucidated. (2) Appropriate halogenation can significantly increase the excited state lifetime of a Zn porphyrin relative to the unsubstituted complex. (3) Halogenation can be used to modulate redox potentials in a manner that complements the enhancement of other electronic properties. The insights gained from study of this library of porphyrins provide a foundation for tuning the electronic properties of monomeric porphyrins as building blocks for multichromophoric assemblies in optoelectronics and other applications.

High-valent iron porphyrins

The term ‘high-valent’ refers to iron complexes of porphyrins and related macrocycles in which the oxidation state of the iron center exceeds III. High-valent iron porphyrins and chlorins are important biological transients whose intermediacy has been demonstrated in numerous peroxidase and catalase enzymes. Two species, compounds I and II, are spectroscopically detectable upon stoichiometric addition of monooxygen donors to resting ferric enzymes. Compounds I and II are formally two and one oxidizing equivalents respectively above the ferric state. In compound II the oxidizing equivalent has been shown by spectroscopic studies to be located on iron as an oxoiron(IV) unit. The spectroscopic and magnetic properties of compound I support the structural assignment of an S = 1 oxoiron(IV) unit magnetically coupled to a heme π-cation radical (S = 1/2). Studies on model hemes have contributed much to the understanding of protein chemistry. Much work has been accomplished with meso-tetaarylporphyrins and, more recently, with physiologically congruent meso-unsubstituted pyrrole β-substituted complexes. Compounds I of both proteins and synthetic models have been characterized by a wide array of spectroscopic methods, including UV-vis, NMR, resonance Raman, EPR, variable-temperature/variable-field magnetic Mössbauer, magnetic circular dichroism and extended X-ray absorption fine structure spectroscopy. Results of these studies are summarized. Recent developments, which promise to yield a detailed picture of electronic structure, are variable-temperature magnetic circular dichroism, studies in the pre-K-edge region and L-edge X-ray absorption spectroscopy. Time-resolved X-ray diffraction techniques have been applied to obtain the first structural data on the protein forms of compound I.

Oxoferryl-porphyrin radical catalytic intermediate in cytochrome bd oxidases protects cells from formation of reactive oxygen species

The quinol-linked cytochrome bd oxidases are terminal oxidases in respiration. These oxidases harbor a low spin heme b(558) that donates electrons to a binuclear heme b(595)/heme d center. The reaction with O(2) and subsequent catalytic steps of the Escherichia coli cytochrome bd-I oxidase were investigated by means of ultra-fast freeze-quench trapping followed by EPR and UV-visible spectroscopy. After the initial binding of O(2), the O-O bond is heterolytically cleaved to yield a kinetically competent heme d oxoferryl porphyrin π-cation radical intermediate (compound I) magnetically interacting with heme b(595). Compound I accumulates to 0.75-0.85 per enzyme in agreement with its much higher rate of formation (~20,000 s(-1)) compared with its rate of decay (~1,900 s(-1)). Compound I is next converted to a short lived heme d oxoferryl intermediate (compound II) in a phase kinetically matched to the oxidation of heme b(558) before completion of the reaction. The results indicate that cytochrome bd oxidases like the heme-copper oxidases break the O-O bond in a single four-electron transfer without a peroxide intermediate. However, in cytochrome bd oxidases, the fourth electron is donated by the porphyrin moiety rather than by a nearby amino acid. The production of reactive oxygen species by the cytochrome bd oxidase was below the detection level of 1 per 1000 turnovers. We propose that the two classes of terminal oxidases have mechanistically converged to enzymes in which the O-O bond is broken in a single four-electron transfer reaction to safeguard the cell from the formation of reactive oxygen species.

Acid-catalyzed disproportionation of oxoiron(IV) porphyrins to give oxoiron(IV) porphyrin radical cations

Disproportionation of oxoiron(IV) porphyrin (Compound II) to oxoiron(IV) porphyrin radical cation (Compound I) was studied in three P450 model systems with different electronic structures. Direct conversion of Compound II to Compound I has been observed for 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin (TDCPP) in acid-catalyzed reactions in a mixed solvent of acetonitrile and water (1:1, v/v) containing excess m-CPBA oxidant, with a second-order rate constant of (1.3 ± 0.2) × 10(2) M(-1) s(-1). The acid-catalyzed disproportionation heavily depends on the electron demand of the substituted aryl groups on the porphyrin macrocycle. The disproportionation equilibrium constants show drastic change for the three porphyrin systems.

A highly reactive p450 model compound I

The detection and kinetic characterization of a cytochrome P450 model compound I, [OFe(IV)-4-TMPyP](+) (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of Fe(III)-4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin pi-radical cation. This intermediate was subsequently transformed into the well-characterized OFe(IV)-4-TMPyP. Global analysis afforded a second-order rate constant k(1) = (1.59 +/- 0.06) x 10(7) M(-1) s(-1) for the formation of 1 followed by a first-order decay with k(2) = 8.8 +/- 0.1 s(-1). (1)H and (13)C NMR determined 9-xanthydrol to be the major product (approximately 90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% (18)OH(2) indicated 21% (18)O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d(2) revealed a modest kinetic isotope effect, k(H)/k(D) = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k(3) = (3.6 +/- 0.3) x 10(6) M(-1) s(-1). Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C-H hydroxylation that correlated well with the scissile C-H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C-H bond cleavage onto the Brønsted-Evans-Polanyi relationship for similar substrates determined the H-OFe(IV)-4-TMPyP bond dissociation energy to be approximately 100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.

Iron-Catalyzed Oxidation of Thioethers by Iodosylarenes: Stereoselectivity and Reaction Mechanism

Catalytic properties of a series of iron(III)-salen (salen=N,N'-bis(salicylidene)ethylenediamine dianion) and related complexes in asymmetric sulfoxidation reactions, with iodosylarenes as terminal oxidants, have been explored. These catalysts have been found to efficiently catalyze oxidation of alkyl aryl sulfides to sulfoxides with high chemoselectivity (up to 100 %) and moderate-to-high enantioselectivity (up to 84 % with isopropylthiobenzene and iodosylmesitylene), the TON (TON=turnover number) approaching 500. The influence of the ligand (electronic and steric effects of the substituents), oxidant, and substrate structures on the oxidation stereoselectivity has been investigated systematically. The structure of the reactive intermediates (complexes of the type [Fe(III)(ArIO)(salen)] and the reaction mechanism have been revealed by both mechanistic studies with different iodosylarenes and direct in situ (1)H NMR observation of the formation of the reactive species and its reaction with the substrate.

Mechanistic Studies on Peroxide Activation by a Water-Soluble Iron(III)–Porphyrin: Implications for OO Bond Activation in Aqueous and Nonaqueous Solvents

The reactions of a water-soluble iron(III)-porphyrin, [meso-tetrakis(sulfonatomesityl)porphyrinato]iron(III), [Fe(III)(tmps)] (1), with m-chloroperoxybenzoic acid (mCPBA), iodosylbenzene (PhIO), and H(2)O(2) at different pH values in aqueous methanol solutions at -35 degrees C have been studied by using stopped-flow UV/Vis spectroscopy. The nature of the porphyrin product resulting from the reactions with all three oxidants changed from the oxo-iron(IV)-porphyrin pi-cation radical [Fe(IV)(tmps(*+))(O)] (1(++)) at pH<5.5 to the oxo-iron(IV)-porphyrin [Fe(IV)(tmps)(O)] (1(+)) at pH>7.5, whereas a mixture of both species was formed in the intermediate pH range of 5.5-7.5. The observed reactivity pattern correlates with the E degrees' versus pH profile reported for 1, which reflects pH-dependent changes in the relative positions of E degrees'(Fe(IV)/Fe(III) ) and E degrees'(P(*+)/P) for metal- and porphyrin-centered oxidation, respectively. On this basis, the pH-dependent redox equilibria involving 1(++) and 1(+) are suggested to determine the nature of the final products that result from the oxidation of 1 at a given pH. The conclusions reached are extended to water-insoluble iron(III)-porphyrins on the basis of literature data concerning the electrochemical and catalytic properties of [Fe(III)(P)(X)] species in nonaqueous solvents. Implications for mechanistic studies on [Fe(P)]-catalyzed oxidation reactions are briefly addressed.

Toward Identification of the Compound I Reactive Intermediate in Cytochrome P450 Chemistry: A QM/MM Study of Its EPR and Mössbauer Parameters

Quantum mechanical/molecular mechanical (QM/MM) methods have been used in conjunction with density functional theory (DFT) and correlated ab initio methods to predict the electron paramagnetic resonance (EPR) and Mossbauer (MB) properties of Compound I in P450(cam). For calibration purposes, a small Fe(IV)-oxo complex [Fe(O)(NH(3))(4)(H(2)O)](2+) was studied. The (3)A(2) and (5)A(1) states (in C(4)(v)() symmetry) are found to be within 0.1-0.2 eV. The large zero-field splitting (ZFS) of the (FeO)(2+) unit in the (3)A(2) state arises from spin-orbit coupling with the low-lying quintet and singlet states. The intrinsic g-anisotropy is very small. The spectroscopic properties of the model complex [Fe(O)(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are well reproduced by theory. In the model complexes [Fe(O)(TMP)(X)](+) (TMP = tetramesitylporphyrin, X = nothing or H(2)O) the computations again account for the observed spectroscopic properties and predict that the coupling of the (5)A(1) state of the (FeO)(2+) unit to the porphyrin radical leads to a low-lying sextet/quartet manifold approximately 12 kcal/mol above the quartet ground state. The calculations on cytochrome P450(cam), with and without the simulation of the protein environment by point charges, predict a small antiferromagnetic coupling (J approximately -13 to -16 cm(-)(1); H(HDvV) = - 2JS(A)S(B)) and a large ZFS > 15 cm(-)(1) (with E/D approximately 1/3) which will compete with the exchange coupling. This leads to three Kramers doublets of mixed multiplicity which are all populated at room temperature and may therefore contribute to the observed reactivity. The MB and ligand hyperfine couplings ((14)N, (1)H) are fairly sensitive to the protein environment which controls the spin density distribution between the porphyrin ring and the axial cysteinate ligand.

Spectroscopic characterization of the iron-oxo intermediate in cytochrome P450

From analogy to chloroperoxidase from Caldariomyces fumago, it is believed that the electronic structure of the intermediate iron-oxo species in the catalytic cycle of cytochrome P450 corresponds to an iron(IV) porphyrin-pi-cation radical (compound I). However, our recent studies on P450cam revealed that after 8 ms a tyrosine radical and iron(IV) were formed in the reaction of ferric P450 with external oxidants in the shunt pathway. The present study on the heme domain of P450BM3 (P450BMP) shows a similar result. In addition to a tyrosine radical, a contribution from a tryptophan radical was found in the electron paramagnetic resonance (EPR) spectra of P450BMP. Here we present comparative multi-frequency EPR (9.6, 94 and 285 GHz) and Mössbauer spectroscopic studies on freeze-quenched intermediates produced using peroxy acetic acid as oxidant for both P450 cytochromes. After 8 ms in both systems, amino acid radicals occurred instead of the proposed iron(IV) porphyrin-pi-cation radical, which may be transiently formed on a much faster time scale. These findings are discussed with respect to other heme thiolate proteins. Our studies demonstrate that intramolecular electron transfer from aromatic amino acids is a common feature in these enzymes. The electron transfer quenches the presumably transiently formed porphyrin-pi-cation radical, which makes it extremely difficult to trap compound I.

The elusive oxidant species of cytochrome P450 enzymes: characterization by combined quantum mechanical/molecular mechanical (QM/MM) calculations

The primary oxidant of cytochrome P450 enzymes, Compound I, is hard to detect experimentally; in the case of cytochrome P450(cam), this intermediate does not accumulate in solution during the catalytic cycle even at temperatures as low as 200 K (ref 4). Theory can play an important role in characterizing such elusive species. We present here combined quantum mechanical/molecular mechanical (QM/MM) calculations of Compound I of cytochrome P450(cam) in the full enzyme environment as well as density functional studies of the isolated QM region. The calculations assign the ground state of the species, quantify the effect of polarization and hydrogen bonding on its properties, and show that the protein environment and its specific hydrogen bonding to the cysteinate ligand are crucial for sustaining the Fe-S bond and for preventing the full oxidation of the sulfur.

Reactions of nickel(II) 2,21-dimethyl-2-aza-21-carbaporphyrin with phenyl Grignard reagents, phenyllithium, and n-butyllithium

Addition of a phenyl Grignard reagent to a toluene solution of the nickel(II) chloride complex of a dimethylated inverted porphyrin, (2-NCH3-21-CH3CTPP)NiIICl (1), at 203 K results in the formation of a rare paramagnetic (sigma-phenyl)nickel(II) species, (2-NCH3-21-CH3CTPP)NiIIPh (2). The coordination of the sigma-phenyl in 2 is determined by a unique pattern of three sigma-phenyl resonances (ortho 375.0 ppm; meta 108.94 ppm; para 35.68 ppm (at 283 K)) in the 1H NMR and 2H NMR spectra. The (sigma-phenyl)nickel(II) compound 2 is in the high-spin ground electronic state (dxy)2(dxz)2(dyz)2(dz2)1(dx2-y2)1, as confirmed by similarity of the NMR spectra of the equatorial ligand in 1 and 2. Titration of 1 with phenyllithium produces (2-NCH3-21-CH3CTPP)NiIIPh (2). One-electron reduction with excess PhLi yields [(2-NCH3-21-CH3CTPP)NiIIPh]- (3), which can be also generated by independent routes, e.g., by reduction of (2-NCH3-21-CH3CTPP)NiIIPh using lithium triethylborohydride or tetrabutylammonium borohydride. The spectroscopic data indicate that (2-NCH3-21-CH3CTPP)NiIIPh (2) undergoes one-electron reduction without a substantial disruption of the molecular geometry. The presence of two paramagnetic centers in 3, i.e., the high-spin nickel(II) and the carbaporphyrin anion radical, produces remarkable variations in a spectral patterns, such as the upfield and downfield positions of pyrrole resonances (103.78, 96.66, -25.35, -50.97, -92.15, -114.83 ppm (at 253 K)) and sign alternations of the meso-phenyl resonances (ortho -77.81, -79.34 ppm; meta 48.77, 48.04 ppm; para -85.65, -86.46 ppm (at 253 K)). A single species, 4, is detected in the 1H NMR titration of 1 with n-butyllithium. The formation of one- or two-electron-reduced species, [(2-NCH3-21-CH3CTPP)NiBu]- or [(2-NCH3-21-CH3CTPP)NiBu]2-, respectively, is considered to account for the spectroscopic properties of 4 (pyrrole 17.33, 15.45, -5.79, -7.74, -14.62, -58.14 ppm; 21-CH3 3 ppm (at 203 K)). The temperature dependence of the hyperfine shifts of 4 demonstrates pronounced anti-Curie behavior, interpreted in terms of a temperature-dependent spin equilibrium between diamagnetic and paramagnetic states with diamagnetic properties approached as the temperature is lowered. Warming of 2-4 results in complete decomposition via homolytic/heterolytic cleavage of an axial nickel-apical carbon bond. In the case of 2 or 3, the process yields a mixture of two compounds, 5 and 6, which are detected by EPR spectroscopy, demonstrating the anisotropy of the g tensor (5, g1 = 2.237, g2 = 2.092, g3 = 2.090; 6, g1 = 2.115, g2 = 2.030, g3 = 1.940 (in frozen toluene solution at 77 K)).

Intermediates in the reaction of substrate-free cytochrome P450cam with peroxy acetic acid

Freeze-quenched intermediates of substrate-free cytochrome 57Fe-P450(cam) in reaction with peroxy acetic acid as oxidizing agent have been characterized by EPR and Mossbauer spectroscopy. After 8 ms of reaction time the reaction mixture consists of approximately 90% of ferric low-spin iron with g-factors and hyperfine parameters of the starting material; the remaining approximately 10% are identified as a free radical (S' = 1/2) by its EPR and as an iron(IV) (S= 1) species by its Mossbauer signature. After 5 min of reaction time the intermediates have disappeared and the Mossbauer and EPR-spectra exhibit 100% of the starting material. We note that the spin-Hamiltonian analysis of the spectra of the 8 ms reactant clearly reveals that the two paramagnetic species, e.g. the ferryl (iron(IV)) species and the radical, are not exchanged coupled. This led to the conclusion that under the conditions used, peroxy acetic acid oxidized a tyrosine residue (probably Tyr-96) into a tyrosine radical (Tyr*-96), and the iron(III) center of substrate-free P450(cam) to iron(IV).

Magnetic interactions in the high-spin iron(III) oxooctaethylchlorinato derivative [Fe(oxoOEC)(Cl)] and its pi-cation radical [Fe(oxoOEC.)(Cl)]SbCl6

The preparation and characterization of the beta-oxochlorin derivative [3,3,7,8,12,13,17,18-octaethyl-(3H)-porphin-2-onato(2-)]iron(III) chloride, [Fe(oxoOEC)(Cl)], and its pi-cation radical derivative [Fe(oxoOEC.)(Cl)]SbCl6 is described. Both compounds have been characterized by single-crystal X-ray structure determinations, IR, UV/vis/near-IR, and Mössbauer spectroscopies, and temperature-dependent magnetic susceptibility measurements. The macrocycles of [Fe(oxoOEC)(Cl)] and [Fe(oxoOEC.)(Cl)]SbCl6 are both saddled, and [Fe(oxoOEC.)(Cl)]-SbCl6 is slightly ruffled as well. [Fe(oxoOEC)(Cl)] shows a laterally shifted dimeric unit in the solid state, with a mean plane separation of 3.39 A and a lateral shift of 7.39 A. Crystal data for [Fe(oxoOEC)(Cl)]: triclinic, space group P1, Z = 2, a = 9.174(2) A, b = 13.522(3) A, c = 14.838(3) A, alpha = 95.79(3) degrees, beta = 101.46(2) degrees, gamma = 104.84(3) degrees. Upon oxidation, the inter-ring geometric parameters increase; the mean plane separation and the lateral shift of the dimeric unit of [Fe(oxoOEC.)(Cl)]SbCl6 are 4.82 and 8.79 A, respectively. Crystal data for [Fe(oxoOEC.)(Cl)]SbCl6: monoclinic, space group Cc, Z = 4, a = 19.8419(13) A, b = 10.027(2) A, c = 22.417(4) A, beta = 96.13(2) degrees. A broad near-IR absorption band appears at 1415 nm for the pi-cation radical, [Fe(oxoOEC.)(Cl)]SbCl6. Zero-field Mössbauer measurements at 4.2 K for both [Fe(oxoOEC)(Cl)] and [Fe(oxoOEC.)(Cl)]SbCl6 confirmed that the oxidation state of the iron atom did not change upon chemical oxidation. Solid-state magnetic susceptibility measurements for [Fe(oxoOEC.)(Cl)]SbCl6 resulted in a large temperature dependence of the magnetic moment that can best be fit with a model that includes a zero-field splitting parameter of D = 6 cm-1, antiferromagnetic intermolecular iron-iron coupling (2JFe-Fe = -0.14 cm-1), antiferromagnetic intramolecular iron-radical coupling (2JFe-r = -76 cm-1), and antiferromagnetic radical-radical coupling (2Jr-r = -13 cm-1).

Generation of oxoiron (IV) tetramesitylporphyrin pi-cation radical complexes by m-CPBA oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at - 78 degrees C. Evidence for the formation of six-coordinate oxoiron (IV) tetramesitylporphyrin pi-cation radical complexes FeIV = O(tmp*)X (X = Cl-, Br-), by Mössbauer and X-ray absorption spectroscopy

The generation of six-coordinate oxoiron (IV) tetramesitylporphyrin pi-caption radical complexes by m-CPBA (meta-chloroperbenzoic acid) oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at - 78 degrees C was investigated. UV-Vis and EPR spectroscopies indicate that the axial ligand present in the ferric starting derivatives is retained in the high-valent iron complexes. Indirect evidence for the formation of six-coordinate oxoiron (IV) tetramesitylporphyrin complexes FeIV = O(tmp*)X (X=Cl-, Br-) by m-CPBA oxidation of FeX(tmp) (X=Cl-, Br-) in butyronitrile at - 78 degrees C was also obtained by Mössbauer spectroscopy. Direct confirmation of the presence of a halide ion as second axial ligand of iron in these high-valent iron species was obtained by X-ray absorption spectroscopy. The EXAFS spectra of the samples obtained by m-CPBA oxidation of FeX(tmp) (X=Cl-, Br-) were refined using two different coordination models including both four porphyrinato-nitrogens and the axial oxo group. The two models include (model I) or exclude (model II) the axial halogen. The statistical tests indicate the presence of a halide ion as second axial ligand of iron in both derivatives. The refinements led to the following bond distances: FeIV=O(tmp*)Cl(3):Fe-O=1.66(1),Fe-Cl=2.39(2) and Fe-Np=1.99(1) A;FeIV=O(tmp*)Br(4):Fe-O=1.65(1),Fe-Br=2.93(2), Fe-Np=2.02(1) A. The lengthening of the Fe-X(X=Cl-, Br-) distances relative to those occurring in the ferric precursor porphyrins is, most probably, related to the strong trans influence of the oxoiron(IV) fragment present in 3 or 4.

Resonance Raman spectroscopy of soybean peroxidase

Resonance Raman spectra (600-1700 cm-1) for the heme enzyme soybean peroxidase (Rz = 2.5) were obtained using Soret band excitation at 406.7 nm. The vibrational frequencies and depolarization data indicate a strong similarity between the active sites of soybean and horseradish peroxidase. This similarity suggests that the active site in the resting form of soybean peroxidase contains a ferric iron, is a high-spin 5-coordinate heme binding His as a fifth axial ligand.