Kinetics of two-electron oxidations by the compound I derivative of chloroperoxidase, a model for cytochrome P450 oxidants

Factors controlling the reactivity of synthetic compound-I Analogs

A high-valent iron(IV)-oxo porphyrin radical cation (FeIV(O)(porph+•) serves as a key, reactive intermediate for a range of heme enzymes, including cytochrome P450 (CYP), horseradish peroxidase (HRP), and catalase (CAT). Synthetic analogs of this intermediate, known as Compound-I (Cpd-I) in the heme enzyme literature, have been generated with different tetrapyrrolic, macrocyclic ligands, including porphyrin derivatives, and the closely related ring-contracted macrocycles, corroles and corrolazines. These synthetic analogs have been useful for assigning and understanding structural and spectroscopic features and examining the reactivity of Cpd-I-like species in controlled and well-defined environments. This review focuses on summarizing recent developments in the synthesis and reactivity of high-valent iron-oxo porphyrinoid complexes in two main classes of reactions, proton-coupled electron transfer (PCET) and oxygen atom transfer (OAT). The relationship between the structure of the complexes and their reactivity is emphasized, including the influence of axial ligation and peripheral macrocyclic substitution, as well as the effects of solvent and secondary coordination spheres on the reactivity of the Cpd-I analogs. In bringing together the latest findings on Cpd-I analogs, this review intends to broaden our current understanding of the factors that control the stability and reactivity of Cpd-I species. This new knowledge should, in turn, point toward new synthetic strategies for constructing catalysts that rely on Cpd-I-like reactive intermediates.

Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions

In this work, chromium(IV)-oxo porphyrins [Cr^IV(Por)(O)] (2) (Por = porphyrin) were produced either by oxidation of [Cr^III(Por)Cl] (1) with iodobenzene diacetate or visible light photolysis of porphyrin‑chromium(III) chlorates. Subsequent oxidation of 2 with silver perchlorate gave chromium(V)-oxo porphyrins [Cr^V(Por)(O)](ClO₄) (3) in three porphyrin ligands, including 5,10,15,20-tetramesitylporphyrin(TMP, a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin(TDFPP, b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP, c). Complexes 2 and 3 reacted with thioanisoles to produce the corresponding sulfoxides, and their kinetics of sulfoxidation reactions with a series of aryl methyl sulfides(thioanisoles) were studied in organic solutions. Chromium(V)-oxo porphyrins are several orders of magnitudes more reactive than chromium(IV)-oxo species, and representative second-order rate constants (k_ox) for the oxidation of thioansole are (0.40 ± 0.01) M^-1 s^-1 (3a), and (2.82 ± 0.20) × 10² M^-1 s^-1 (3b), and (2.20 ± 0.01) × 10³ M^-1 s^-1 (3c). The order of reactivity for 2 and 3 follows TPFPP > TDFPP > TMP, in agreement with the electrophilic nature of metal-oxo complexes. Hammett analyses indicate significant charge transfer in the transition states for oxidation of para-substituted thioanisoles by [Cr^V(Por)(O)]⁺. The ρ⁺ constants are -1.69 for 3a, -2.63 for 3b, and - 2.89 for 3c, respectively, mirror values found previously for related metal-oxo species. A mechanism involving the electrophilic attack of the Cr^V-oxo at sulfides to form a sulfur cation intermediate in the rate-determining step is suggested. Competition studies with chromium(III) porphyrin chloride and PhI(OAc)₂ gave relative rate constants for oxidations of competing thioanisoles that closely match ratios of absolute rate constants from chromium(V)-oxo species, which are true oxidants under catalytic conditions.

Histidine versus Cysteine-Bearing Heme-Dependent Halogen Peroxidases: Parallels and Differences for Cl- Oxidation

The homodimeric myeloperoxidase (MPO) features a histidine as a proximal ligand and a sulfonium linkage covalently attaching the heme porphyrin ring to the protein. MPO is able to catalyze Cl^- oxidation with about the same efficiency as chloroperoxidase at pH 7.0. In this study, we seek to explore the parallels and differences between the histidine and cysteine heme-dependent halogen peroxidases. Transition states, reaction barriers, and relevant thermodynamic properties are calculated on protein models. Together with electronic structure calculations, it gives an overview of the reaction mechanisms and of the factors that determine the selectivity between one- and two-electron paths. Conclusions point to the innate oxidizing nature of MPO with the ester and sulfonium linkages hiking up the reactivity to enable chloride oxidation. The installation of a deprotonated imidazolate as a proximal ligand does not shift the equilibrium from one- to two-electron events without influencing the chemistry of the oxidation reaction.

Visible light generation of high-valent metal-oxo intermediates and mechanistic insights into catalytic oxidations

High-valent metal-oxo complexes play central roles as active oxygen atom transfer (OAT) agents in many enzymatic and synthetic oxidation catalysis. This review focuses on our recent advances in application of photochemical approaches to probe the oxidizing metal-oxo species with different metals and macrocyclic ligands. Under visible light irradiation, a variety of important metal-oxo species including iron-oxo porphyrins, manganese-oxo porphyrin/corroles, ruthenium-oxo porphyrins, and chromium-oxo salens have been successfully generated. Kinetical studies in real time have provided mechanistic insights as to the reactivity and reaction pathways of the metal-oxo intermediates in their oxidation reactions. In photo-induced ligand cleavage reactions, metals in n⁺ oxidation state with the oxygen-containing ligands bromate, chlorate, or nitrites were photolyzed. Homolytic cleavage of the O-X bond in the ligand gives (n + 1)⁺ oxidation state metal-oxo species, and heterolytic cleavage gives (n + 2)⁺ oxidation state metal-oxo species. In photo-disproportionation reactions, reactive Mⁿ⁺¹-oxo species can be formed by photolysis of μ-oxo dimeric Mⁿ⁺ complexes with the concomitant formation of M^n-1 products. Importantly, the oxidation of M^n-1 products by molecular oxygen (O₂) to regenerate the μ-oxo dimeric Mⁿ⁺ complexes in photo-disproportionation reactions represents an attractive and green catalytic cycle for the development of photocatalytic aerobic oxidations.

Epoxidation Catalyzed by the Nonheme Iron(II)- and 2-Oxoglutarate-Dependent Oxygenase, AsqJ: Mechanistic Elucidation of Oxygen Atom Transfer by a Ferryl Intermediate

Mechanisms of enzymatic epoxidation via oxygen atom transfer (OAT) to an olefin moiety is mainly derived from the studies on thiolate-heme containing epoxidases, such as cytochrome P450 epoxidases. The molecular basis of epoxidation catalyzed by nonheme-iron enzymes is much less explored. Herein, we present a detailed study on epoxidation catalyzed by the nonheme iron(II)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenase, AsqJ. The native substrate and analogues with different para substituents ranging from electron-donating groups (e.g., methoxy) to electron-withdrawing groups (e.g., trifluoromethyl) were used to probe the mechanism. The results derived from transient-state enzyme kinetics, Mössbauer spectroscopy, reaction product analysis, X-ray crystallography, density functional theory calculations, and molecular dynamic simulations collectively revealed the following mechanistic insights: (1) The rapid O2 addition to the AsqJ Fe(II) center occurs with the iron-bound 2OG adopting an online-binding mode in which the C1 carboxylate group of 2OG is trans to the proximal histidine (His134) of the 2-His-1-carboxylate facial triad, instead of assuming the offline-binding mode with the C1 carboxylate group trans to the distal histidine (His211); (2) The decay rate constant of the ferryl intermediate is not strongly affected by the nature of the para substituents of the substrate during the OAT step, a reactivity behavior that is drastically different from nonheme Fe(IV)-oxo synthetic model complexes; (3) The OAT step most likely proceeds through a stepwise process with the initial formation of a C(benzylic)-O bond to generate an Fe-alkoxide species, which is observed in the AsqJ crystal structure. The subsequent C3-O bond formation completes the epoxide installation.

Significantly shorter Fe-S bond in cytochrome P450-I is consistent with greater reactivity relative to chloroperoxidase

Cytochrome P450 (P450) and chloroperoxidase (CPO) are thiolate ligated heme proteins that catalyze the activation of carbon hydrogen bonds. The principal intermediate in these reactions is a ferryl radical species called compound I. P450 compound I (P450-I) is significantly more reactive than CPO-I, which only cleaves activated C-H bonds. To provide insight into the differing reactivities of these intermediates, we examined CPO-I and P450-I with variable temperature Mössbauer and X-ray absorption spectroscopies. These measurements indicate that the Fe-S bond is significantly shorter in P450-I than in CPO-I. This difference in Fe-S bond lengths can be understood in terms of variations in hydrogen bonding patterns within the “cys-pocket” (a portion of the proximal helix that encircles the thiolate ligand). Weaker hydrogen bonding in P450-I results in a shorter Fe-S bond, which enables greater electron donation from the axial-thiolate ligand. This observation may in part explain P450's greater propensity for C-H bond activation.

Detection and kinetic characterization of a highly reactive heme-thiolate peroxygenase compound I

The extracellular heme-thiolate peroxygenase from Agrocybe aegerita (AaeAPO) has been shown to hydroxylate alkanes and numerous other substrates using hydrogen peroxide as the terminal oxidant. We describe the kinetics of formation and decomposition of AaeAPO compound I upon its reaction with mCPBA. The UV-vis spectral features of AaeAPO-I (361, 694 nm) are similar to those of chloroperoxidase-I and the recently described cytochrome P450-I. The second-order rate constant for AaeAPO-I formation was 1.0 (±0.4) × 10(7) M(-1) s(-1) at pH 5.0, 4 °C. The relatively slow decomposition rate, 1.4 (±0.03) s(-1), allowed the measurement of its reactivity toward a panel of substrates. The observed rate constants, k2', spanned 5 orders of magnitude and correlated linearly with bond dissociation enthalpies (BDEs) of strong C-H bond substrates with a log k2' vs BDE slope of ∼0.4. However, the hydroxylation rate was insensitive to a C-H BDE below 90 kcal/mol, similar to the behavior of the tert-butoxyl radical. The shape and slope of the Brønsted-Evans-Polanyi plot indicate a symmetrical transition state for the stronger C-H bonds and suggest entropy control of the rate in an early transition state for weaker C-H bonds. The AaeAPO-II Fe(IV)O-H BDE was estimated to be ∼103 kcal/mol. All results support the formation of a highly reactive AaeAPO oxoiron(IV) porphyrin radical cation intermediate that is the active oxygen species in these hydroxylation reactions.

Formation of hydroxymethyl DNA adducts in rats orally exposed to stable isotope labeled methanol

Methanol is a large volume industrial chemical and widely used solvent and fuel additive. Methanol's well known toxicity and use in a wide spectrum of applications has raised long-standing environmental issues over its safety, including its carcinogenicity. Methanol has not been listed as a carcinogen by any regulatory agency; however, there are debates about its carcinogenic potential. Formaldehyde, a metabolite of methanol, has been proposed to be responsible for the carcinogenesis of methanol. Formaldehyde is a known carcinogen and actively targets DNA and protein, causing diverse DNA and protein damage. However, formaldehyde-induced DNA adducts arising from the metabolism of methanol have not been reported previously, largely due to the absence of suitable DNA biomarkers and the inability to differentiate what was due to methanol compared with the substantial background of endogenous formaldehyde. Recently, we developed a unique approach combining highly sensitive liquid chromatography-mass spectrometry methods and exposure to stable isotope labeled chemicals to simultaneously quantify formaldehyde-specific endogenous and exogenous DNA adducts. In this study, rats were exposed daily to 500 or 2000 mg/kg [¹³CD₄]-methanol by gavage for 5 days. Our data demonstrate that labeled formaldehyde arising from [¹³CD₄]-methanol induced hydroxymethyl DNA adducts in multiple tissues in a dose-dependent manner. The results also demonstrated that the number of exogenous DNA adducts was lower than the number of endogenous hydroxymethyl DNA adducts in all tissues of rats administered 500 mg/kg per day for 5 days, a lethal dose to humans, even after incorporating an average factor of 4 for reduced metabolism due to isotope effects of deuterium-labeled methanol into account.

Oxidation of chloride and subsequent chlorination of organic compounds by oxoiron(IV) porphyrin π-cation radicals

Ironing it out: oxoiron(IV) porphyrin π-cation radical complexes serve as models for the oxidation of Cl(-) into an active chlorinating reagent that chlorinates various organic compounds. Evidence suggests that Cl(-) is oxidized to Cl(2) via Cl·. The mechanism involving either direct electron transfer or iron(III) hypochlorite formation, and then homolysis of the Cl-O bond is discussed.

Cytochrome P450 compound I: capture, characterization, and C-H bond activation kinetics

Cytochrome P450 enzymes are responsible for the phase I metabolism of approximately 75% of known pharmaceuticals. P450s perform this and other important biological functions through the controlled activation of C-H bonds. Here, we report the spectroscopic and kinetic characterization of the long-sought principal intermediate involved in this process, P450 compound I (P450-I), which we prepared in approximately 75% yield by reacting ferric CYP119 with m-chloroperbenzoic acid. The Mössbauer spectrum of CYP119-I is similar to that of chloroperoxidase compound I, although its electron paramagnetic resonance spectrum reflects an increase in |J|/D, the ratio of the exchange coupling to the zero-field splitting. CYP119-I hydroxylates the unactivated C-H bonds of lauric acid [D(C-H) ~ 100 kilocalories per mole], with an apparent second-order rate constant of k(app) = 1.1 × 10(7) per molar per second at 4°C. Direct measurements put a lower limit of k ≥ 210 per second on the rate constant for bound substrate oxidation, whereas analyses involving kinetic isotope effects predict a value in excess of 1400 per second.

Probing the oxyferrous and catalytically active ferryl states of Amphitrite ornata dehaloperoxidase by cryoreduction and EPR/ENDOR spectroscopy. Detection of compound I

Dehaloperoxidase (DHP) from Amphitrite ornata is a heme protein that can function both as a hemoglobin and as a peroxidase. This report describes the use of 77 K cryoreduction EPR/ENDOR techniques to study both functions of DHP. Cryoreduced oxyferrous [Fe(II)-O(2)] DHP exhibits two EPR signals characteristic of a peroxoferric [Fe(III)-O(2)(2-)] heme species, reflecting the presence of conformational substates in the oxyferrous precursor. (1)H ENDOR spectroscopy of the cryogenerated substates shows that H-bonding interactions between His N(ε)H and heme-bound O(2) in these conformers are similar to those in the β-chain of oxyferrous hemoglobin A (HbA) and oxyferrous myoglobin, respectively. Decay of cryogenerated peroxoferric heme DHP intermediates upon annealing at temperatures above 180 K is accompanied by the appearance of a new paramagnetic species with an axial EPR signal with g(⊥) = 3.75 and g(∥) = 1.96, characteristic of an S = 3/2 spin state. This species is assigned to Compound I (Cpd I), in which a porphyrin π-cation radical is ferromagnetically coupled with an S = 1 ferryl [Fe(IV)═O] ion. This species was also trapped by rapid freeze-quench of the ambient-temperature reaction mixture of ferric [Fe(III)] DHP and H(2)O(2). However, in the latter case Cpd I is reduced very rapidly by a nearby tyrosine to form Cpd ES [(Fe(IV)═O)(porphyrin)/Tyr(•)]. Addition of the substrate analogue 2,4,6-trifluorophenol (F(3)PhOH) suppresses formation of the Cpd I intermediate during annealing of cryoreduced oxyferrous DHP at 190 K but has no effect on the spectroscopic properties of the remaining cryoreduced oxyferrous DHP intermediates and kinetics of their decay. These observations indicate that substrate (i) binds to oxyferrous DHP outside of the distal pocket and (ii) can reduce Cpd I to Cpd II [Fe(IV)═O]. These assumptions are also supported by the observation that F(3)PhOH has only a small effect on the EPR properties of radiolytically cryooxidized and cryoreduced ferrous [Fe(II)] DHP. EPR spectra of cryoreduced ferrous DHP disclose the multiconformational nature of the ferrous DHP precursor. The observation and characterization of Cpds I, II, and ES in the absence and in the presence of F(3)PhOH provides definitive evidence of a mechanism involving consecutive one-electron steps and clarifies the role of all intermediates formed during turnover.

Low temperature photo-oxidation of chloroperoxidase Compound II

Oxidation of the heme-thiolate enzyme chloroperoxidase (CPO) from Caldariomyces fumago with peroxynitrite (PN) gave the Compound II intermediate, which was photo-oxidized with 365 nm light to give a reactive oxidizing species. Cryo-solvents at pH ≈ 6 were employed, and reactions were conducted at temperatures as low as -50° C. The activity of CPO as evaluated by the chlorodimedone assay was unaltered by treatment with PN or by production of the oxidizing transient and subsequent reaction with styrene. EPR spectra at 77K gave the amount of ferric protein at each stage in the reaction sequence. The PN oxidation step gave a 6:1 mixture of Compound II and ferric CPO, the photolysis step gave an approximate 1:1 mixture of active oxidant and ferric CPO, and the final mixture after reaction with excess styrene contained ferric CPO in 80% yield. In single turnover reactions at -50°C, styrene was oxidized to styrene oxide in high yield. Kinetic studies of styrene oxidation at -50°C displayed saturation kinetics with an equilibrium constant for formation of the complex of K(bind)=3.8 x 10(4)M(-1) and an oxidation rate constant of k(ox)=0.30s(-1). UV-Visible spectra of mixtures formed in the photo-oxidation sequence at ca. -50° C did not contain the signature Q-band absorbance at 690 nm ascribed to CPO Compound I prepared by chemical oxidation of the enzyme, indicating that different species were formed in the chemical oxidation and the photo-oxidation sequence.

Explaining the atypical reaction profiles of heme enzymes with a novel mechanistic hypothesis and kinetic treatment

Many heme enzymes show remarkable versatility and atypical kinetics. The fungal extracellular enzyme chloroperoxidase (CPO) characterizes a variety of one and two electron redox reactions in the presence of hydroperoxides. A structural counterpart, found in mammalian microsomal cytochrome P450 (CYP), uses molecular oxygen plus NADPH for the oxidative metabolism (predominantly hydroxylation) of substrate in conjunction with a redox partner enzyme, cytochrome P450 reductase. In this study, we employ the two above-mentioned heme-thiolate proteins to probe the reaction kinetics and mechanism of heme enzymes. Hitherto, a substrate inhibition model based upon non-productive binding of substrate (two-site model) was used to account for the inhibition of reaction at higher substrate concentrations for the CYP reaction systems. Herein, the observation of substrate inhibition is shown for both peroxide and final substrate in CPO catalyzed peroxidations. Further, analogy is drawn in the “steady state kinetics” of CPO and CYP reaction systems. New experimental observations and analyses indicate that a scheme of competing reactions (involving primary product with enzyme or other reaction components/intermediates) is relevant in such complex reaction mixtures. The presence of non-selective reactive intermediate(s) affords alternate reaction routes at various substrate/product concentrations, thereby leading to a lowered detectable concentration of “the product of interest” in the reaction milieu. Occam's razor favors the new hypothesis. With the new hypothesis as foundation, a new biphasic treatment to analyze the kinetics is put forth. We also introduce a key concept of “substrate concentration at maximum observed rate”. The new treatment affords a more acceptable fit for observable experimental kinetic data of heme redox enzymes.

Quantitative production of compound I from a cytochrome P450 enzyme at low temperatures. Kinetics, activation parameters, and kinetic isotope effects for oxidation of benzyl alcohol

Cytochrome P450 enzymes are commonly thought to oxidize substrates via an iron(IV)-oxo porphyrin radical cation transient termed Compound I, but kinetic studies of P450 Compounds I are essentially nonexistent. We report production of Compound I from cytochrome P450 119 (CYP119) in high conversion from the corresponding Compound II species at low temperatures in buffer mixtures containing 50% glycerol by photolysis with 365 nm light from a pulsed lamp. Compound I was studied as a reagent in oxidations of benzyl alcohol and its benzylic mono- and dideuterio isotopomers. Pseudo-first-order rate constants obtained at -50 degrees C with concentrations of substrates between 1.0 and 6.0 mM displayed saturation kinetics that gave binding constants for the substrate in the Compound I species (K(bind)) and first-order rate constants for the oxidation reactions (k(ox)). Representative results are K(bind) = 214 M(-1) and k(ox) = 0.48 s(-1) for oxidation of benzyl alcohol. For the dideuterated substrate C(6)H(5)CD(2)OH, kinetics were studied between -50 and -25 degrees C, and a van't Hoff plot for complexation and an Arrhenius plot for the oxidation reaction were constructed. The H/D kinetic isotope effects (KIEs) at -50 degrees C were resolved into a large primary KIE (P = 11.9) and a small, inverse secondary KIE (S = 0.96). Comparison of values extrapolated to 22 degrees C of both the rate constant for oxidation of C(6)H(5)CD(2)OH and the KIE for the nondeuterated and dideuterated substrates to values obtained previously in laser flash photolysis experiments suggested that tunneling could be a significant component of the total rate constant at -50 degrees C.

Kinetics and activation parameters for oxidations of styrene by Compounds I from the cytochrome P450(BM-3) (CYP102A1) heme domain and from CYP119

Cytochrome P450 (CYP or P450) enzymes are ubiquitous in nature where they catalyze a vast array of oxidation reactions. The active oxidants in P450s have long been assumed to be iron(IV)-oxo porphyrin radical cations termed Compounds I, but P450 Compounds I have proven to be difficult to prepare. The recent development of an entry to these transients by photo-oxidation of the corresponding iron(IV)-oxo neutral porphyrin species (Compounds II) permits spectroscopic and kinetic studies. We report here application of the photo-oxidation method for production of Compound I from the heme domain of CYP102A1 (cytochrome P450(BM-3)), and product and kinetic studies of reactions of styrene with this Compound I transient and also Compound I from CYP119. The studies were performed at low temperatures in 1:1 (v:v) mixtures of glycerol and phosphate buffer. Single-turnover reactions at 0 degrees C gave styrene oxide in good yields. In kinetic studies conducted between -10 and -50 degrees C, both Compounds I displayed saturation kinetics permitting determinations of binding constants and first-order oxidation rate constants. Temperature-dependent functions for the binding constants and rate constants were determined for both Compounds I. In the temperature range studied, the Compound I transient from the CYP102A1 heme domain bound styrene more strongly than Compound I from CYP119, but the rate constants for oxidations of styrene by the latter were somewhat larger than those for the former. The temperature-dependent functions for the first-order oxidation reactions are as follows: log k = 13.2-15.2/2.303RT and log k = 13.3-14.6/2.303RT (kilocalories per mole) for Compounds I from the CYP102A1 heme domain and CYP119, respectively.

The mechanism of oxidative halophenol dehalogenation by Amphitrite ornata dehaloperoxidase is initiated by H2O2 binding and involves two consecutive one-electron steps: role of ferryl intermediates

The enzymatic globin, dehaloperoxidase (DHP), from the terebellid polychaete Amphitrite ornata is designed to catalyze the oxidative dehalogenation of halophenol substrates. In this study, the ability of DHP to catalyze this reaction by a mechanism involving two consecutive one-electron steps via the normal order of addition of the oxidant cosubstrate (H(2)O(2)) before organic substrate [2,4,6-trichlorophenol (TCP)] is demonstrated. Specifically, 1 equiv of H(2)O(2) will fully convert 1 equiv of TCP to 2,6-dichloro-1,4-benzoquinone, implicating the role of multiple ferryl [Fe(IV)O] species. A significant amount of heterolytic cleavage of the O-O bond of cumene hydroperoxide, consistent with transient formation of a Compound I [Fe(IV)O/porphyrin pi-cation radical] species, is observed upon its reaction with ferric DHP. In addition, a more stable high-valent Fe(IV)O-containing DHP intermediate [Compound II (Cpd II) or Compound ES] is characterized by UV-visible absorption and magnetic circular dichroism spectroscopy. Spectral similarities are seen between this intermediate and horse heart myoglobin Cpd II. It is also shown in single-turnover experiments that the DHP Fe(IV)O intermediate is an active oxidant in halophenol oxidative dehalogenation. Furthermore, reaction of DHP with 4-chlorophenol leads to a dimeric product. The results presented herein are consistent with a normal peroxidase order of addition of the oxidant cosubstrate (H(2)O(2)) followed by organic substrate (TCP) and indicate that the enzymatic mechanism of DHP-catalyzed oxidative halophenol dehalogenation involves two consecutive one-electron steps with a dissociable radical intermediate.

Highly reactive porphyrin-iron-oxo derivatives produced by photolyses of metastable porphyrin-iron(IV) diperchlorates

Photolyses of metastable porphyrin-iron(IV) diperchlorates in laser flash photolysis reactions gave highly reactive transients. The systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP). The new species, which decayed within milliseconds in acetonitrile solutions, were shown to react with organic substrates by oxo-transfer reactions involving insertions into carbon-carbon double bonds of alkenes and styrenes or benzylic carbon-hydrogen bonds of arenes. The order of reactivity was OEP > TPP > TMP. Second-order rate constants for reactions with several substrates at 22 degrees C were determined; representative values of rate constants for the TPP derivative were k = 8.6 x 10(5) M(-1) s(-1) for styrene, k = 2.5 x 10(6) M(-1) s(-1) for cyclohexene, and k = 7.7 x 10(4) M(-1) s(-1) for ethylbenzene. These porphyrin-iron-oxo transients reacted 4-5 orders of magnitude faster than the corresponding iron(IV)-oxo porphyrin radical cations with rate constants similar to those of porphyrin-manganese(V)-oxo derivatives. Rate constants for oxidations of benzylic C-H positions of arenes correlated with the C-H bond dissociation energies, and Hammett correlations for reactions with substituted styrenes had rho(+) values ranging from -0.5 to -0.7, reflecting electrophilic character of the oxidants and their high reactivity. On the basis of their unique UV-visible spectra, high reactivities, and oxo-transfer properties, the new transients are tentatively identified as porphyrin-iron(V)-oxo perchlorates, electronic isomers (or valence tautomers) of well-known iron(IV)-oxo porphyrin radical cations.

Formation of stable and metastable porphyrin- and corrole-iron(IV) complexes and isomerizations to iron(III) macrocycle radical cations

Oxidations of three porphyrin-iron(III) complexes (1) with ferric perchlorate, Fe(ClO(4))(3), in acetonitrile solutions at -40 degrees C gave metastable porphyrin-iron(IV) diperchlorate complexes (2) that isomerized to known iron(III) diperchlorate porphyrin radical cations (3) when the solutions were warmed to room temperature. The 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) systems were studied by UV-visible spectroscopy. Low temperature NMR spectroscopy and effective magnetic moment measurements were possible with the TPP and TMP iron(IV) complexes. Reactions of two corrole systems, 5,10,15-tris(pentafluorophenyl)corrole (TPFC) and 5,15-bis(pentafluorophenyl)-10-p-methoxyphenylcorrole (BPFMC), also were studied. The corrole-iron(IV) chlorides reacted with silver salts to give corrole-iron(IV) complexes. The corrole-iron(IV) nitrate complexes were stable at room temperature. (TPFC)-iron(IV) toslyate, (TPFC)-iron(IV) chlorate, and (BPFMC)-iron(IV) chlorate were metastable and rearranged to their electronic isomers iron(III) corrole radical cations at room temperature. (TPFC)-iron(III) perchlorate corrole radical cation was the only product observed from reaction of the corrole-iron(IV) chloride with silver perchlorate. For the metastable iron(IV) species, the rates of isomerizations to the iron(III) macrocycle radical cation electronic isomers in dilute acetonitrile solutions were relatively insensitive to electron demands of the macrocyclic ligand but reflected the binding strength of the ligand to iron. Kinetic studies at varying temperatures and concentrations indicated that the mechanisms of the isomerization reactions are complex, involving mixed order reactivity.

Spectra and kinetic studies of the compound I derivative of cytochrome P450 119

The Compound I derivative of cytochrome P450 119 (CYP119) was produced by laser flash photolysis of the corresponding Compound II derivative, which was first prepared by reaction of the resting enzyme with peroxynitrite. The UV-vis spectrum of the Compound I species contained an asymmetric Soret band that could be resolved into overlapping transitions centered at approximately 367 and approximately 416 nm and a Q band with lambda(max) approximately 650 nm. Reactions of the Compound I derivative with organic substrates gave epoxidized (alkene oxidation) and hydroxylated (C-H oxidation) products, as demonstrated by product studies and oxygen-18 labeling studies. The kinetics of oxidations by CYP119 Compound I were measured directly; the reactions included hydroxylations of benzyl alcohol, ethylbenzene, Tris buffer, lauric acid, and methyl laurate and epoxidations of styrene and 10-undecenoic acid. Apparent second-order rate constants, equal to the product of the equilibrium binding constant (K(bind)) and the first-order oxidation rate constant (k(ox)), were obtained for all of the substrates. The oxidations of lauric acid and methyl laurate displayed saturation kinetic behavior, which permitted the determination of both K(bind) and k(ox) for these substrates. The unactivated C-H positions of lauric acid reacted with a rate constant of k(ox) = 0.8 s(-1) at room temperature. The CYP119 Compound I derivative is more reactive than model Compound I species [iron(IV)-oxo porphyrin radical cations] and similar in reactivity to the Compound I derivative of the heme-thiolate enzyme chloroperoxidase. Kinetic isotope effects (kH/kD) for oxidations of benzyl alcohol and ethylbenzene were small, reflecting the increased reactivity of the Compound I derivative in comparison to models. Nonetheless, CYP119 Compound I apparently is much less reactive than the oxidizing species formed in the P450 cam reaction cycle. Studies of competition kinetics employing CYP119 activated by hydrogen peroxide indicated that the same oxidizing transient is formed in the photochemical reaction and in the hydrogen peroxide shunt reaction.

Laser flash photolysis generation of high-valent transition metal-oxo species: insights from kinetic studies in real time

High-valenttransition metal-oxo species are active oxidizing species in many metal-catalyzed oxidation reactions in both Nature and the laboratory. In homogeneous catalytic oxidations, a transition metal catalyst is oxidized to a metal-oxo species by a sacrificial oxidant, and the activated transition metal-oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseconds and direct kinetic studies of their reactions with substrates by laser flash photolysis (LFP) methods. This Account describes studies of high-valent manganese- and iron-oxo intermediates. Irradiation of porphyrin-manganese(III) nitrates and chlorates or corrole-manganese(IV) chlorates resulted in homolytic cleavage of the O-X bonds in the ligands, whereas irradiation of porphyrin-manganese(III) perchlorates resulted in heterolytic cleavage of O-Cl bonds to give porphyrin-manganese(V)-oxo cations. Similar reactions of corrole- and porphyrin-iron(IV) complexes gave highly reactive transients that were tentatively identified as macrocyclic ligand-iron(V)-oxo species. Kinetic studies demonstrated high reactivity of the manganese(V)-oxo species, and even higher reactivities of the putative iron(V)-oxo transients. For example, second-order rate constants for oxidations of cis-cyclooctene at room temperature were 6 x 10(3) M(-1) s(-1) for a corrole-iron(V)-oxo species and 1.6 x 10(6) M(-1) s(-1) for the putative tetramesitylporphyrin-iron(V)-oxo perchlorate species. The latter rate constant is 25,000 times larger than that for oxidation of cis-cyclooctene by iron(IV)-oxo perchlorate tetramesitylporphyrin radical cation, which is the thermodynamically favored electronic isomer of the putative iron(V)-oxo species. The LFP-determined rate constants can be used to implicate the transient oxidants in catalytic reactions under turnover conditions where high-valent species are not observable. Similarly, the observed reactivities of the putative porphyrin-iron(V)-oxo species might explain the unusually high reactivity of oxidants produced in the cytochrome P450 enzymes, heme-thiolate enzymes that are capable of oxidizing unactivated carbon-hydrogen bonds in substrates so rapidly that iron-oxo intermediates have not been detected under physiological conditions.

X-ray absorption spectroscopic characterization of a cytochrome P450 compound II derivative

The cytochrome P450 enzyme CYP119, its compound II derivative, and its nitrosyl complex were studied by iron K-edge x-ray absorption spectroscopy. The compound II derivative was prepared by reaction of the resting enzyme with peroxynitrite and had a lifetime of approximately 10 s at 23 degrees C. The CYP119 nitrosyl complex was prepared by reaction of the enzyme with nitrogen monoxide gas or with a nitrosyl donor and was stable at 23 degrees C for hours. Samples of CYP119 and its derivatives were studied by x-ray absorption spectroscopy at temperatures below 140 (K) at the Advanced Photon Source of Argonne National Laboratory. The x-ray absorption near-edge structure spectra displayed shifts in edge and pre-edge energies consistent with increasing effective positive charge on iron in the series native CYP119 < CYP119 nitrosyl complex < CYP119 compound II derivative. Extended x-ray absorption fine structure spectra were simulated with good fits for k = 12 A(-1) for native CYP119 and k = 13 A(-1) for both the nitrosyl complex and the compound II derivative. The important structural features for the compound II derivative were an iron-oxygen bond length of 1.82 A and an iron-sulfur bond length of 2.24 A, both of which indicate an iron-oxygen single bond in a ferryl-hydroxide, Fe(IV)OH, moiety.

Low-Temperature Rapid-Scan Detection of Reactive Intermediates in Epoxidation Reactions Catalyzed by a New Enzyme Mimic of Cytochrome P450

The use of synthetic iron(III) porphyrins as models for heme-type catalysts in biomimetic cytochrome P450 research has provided valuable information on the nature and reactivity of intermediates produced in the "peroxide shunt" pathway. This article reports spectroscopic detection of reactive intermediates formed in the epoxidation reaction of cis-stilbene with m-chloroperoxybenzoic acid catalyzed by a new mimic of cytochrome P450 with a substituted RSO3- group (1). The application of low-temperature rapid-scan stopped-flow techniques enabled the determination of equilibrium and rate constants for the formation and decay of all intermediates in the catalytic cycle of 1, including the rate constant for the formation (1*+)FeIV=O and for oxygen transfer to the substrate. Noteworthy, the reaction of (1*+)FeIV=O with cis-stilbene leads to an almost complete re-formation (95%) of the starting complex 1. The results show that complex 1 is a valuable catalyst with promising properties for further applications in a biomimetic approach toward mimicking oxygenation reactions of cytochrome P450.

Kinetics and mechanism of oxidation reactions of porphyrin-iron(IV)-oxo intermediates

The kinetics of the reactions of three porphyrin-iron(IV)-oxo derivatives with alkenes and benzylic alcohols were measured. The iron-oxo systems studied were 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (2a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(IV)-oxo (2c). Species 2 were stable for hours at room temperature as dilute solutions in acetonitrile and reacted hundreds to thousands of times faster in the presence of high concentrations of substrates. Typical second-order rate constants determined from pseudo-first-order kinetic studies are 1-2 x 10(-2) M(-1) s(-1) for reactions with styrene and 3 x 10(-2) M(-1) s(-1) for reactions with benzyl alcohol. The reactivity order for the iron-oxo species was 2a > 2b > 2c, which is inverted from that expected on the basis of the electron demand of the porphyrin macrocycles, and the oxidation reaction was suppressed when excess porphyrin-iron(III) complex was added to reaction mixtures. These observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be an iron(IV)-oxo porphyrin radical cation, that is the true oxidant in the reactions. Analyses of the kinetics of oxidations of a series of para-substituted benzylic alcohols with Hammett sigma+ -substituent constants and with a dual-parameter method developed by Jiang (Jiang, X. K. Acc. Chem. Res. 1997, 30, 283) indicated that considerable positive charge developed on the benzylic carbons in the oxidation reactions, as expected for electrophilic oxidants, and also that substantial radical character developed on the benzyl carbon in the transition states.

Photochemical production of a highly reactive porphyrin-iron-oxo species

Oxidation of 5,10,15,20-tetramesitylporphyrinatoiron(III) perchlorate, (TMP)FeIII(ClO4), with ferric perchlorate in acetonitrile gave a metastable species identified as (TMP)FeIV(ClO4)2 that decayed within seconds to the known isomeric species (TMP*+)FeIII(ClO4)2. Irradiation of the metastable species with 355 nm laser light gave a highly reactive transient that reacts with simple organic reductants (alkenes and arylalkanes) 5 orders of magnitude faster than known Compound I analogues, (TMP*+)FeIV(O)(X-).