Cytochrome P-450 Model Compounds Research Articles

A siloxyl bis-pocket thiolate-tailed Fe(III) porphyrin complex

We report the synthesis and characterization of a bis-pocket iron(III) porphyrin complex that has a covalently attached siloxy thiolate group as the axial ligand. As a new cytochrome P450 model compound, this siloxyl bis-pocket thiolate-tailed iron(III) porphyrin is easily synthesized and is surprisingly stable under air due to the extreme steric protection of the siloxy pockets on both faces of the porphyrin. A single-crystal XRD structure has been determined; the Fe–S bond distance is 2.237 (7) Å with Fe–N bond distances of 2.100 (8) and the Fe is 0.5 Å out of the mean porphyrin plane. The Fe–S bond distance in the siloxy thiolate-tailed iron(III) porphyrin is very similar to that in cytochrome P450 and this structure represents a very rare crystallographically-characterized five-coordinate high spin alkylthiolate-tailed ferric porphyrin. EPR spectrum of this compound showed g values and an E/D ratio very similar to those of cytochrome P450 (CYP450) enzyme, demonstrating the importance of using a very basic thiolate group as the axial ligand in P450 model studies. UV-vis studies of its reduced form with carbon monoxide shows a hyper spectrum, which is characteristic of carbonyl complexes of Fe(II)porphyrin thiolates.

A Systematic Account on Aromatic Hydroxylation by a Cytochrome P450 Model Compound I: A Low-Pressure Mass Spectrometry and Computational Study.

Cytochrome P450 enzymes are heme-containing mono-oxygenases that mainly react through oxygen-atom transfer. Specific features of substrate and oxidant that determine the reaction rate constant for oxygen atom transfer are still poorly understood and therefore, we did a systematic gas-phase study on reactions by iron(IV)-oxo porphyrin cation radical structures with arenes. We present herein the first results obtained by using Fourier transform-ion cyclotron resonance mass spectrometry and provide rate constants and product distributions for the assayed reactions. Product distributions and kinetic isotope effect studies implicate a rate-determining aromatic hydroxylation reaction that correlates with the ionization energy of the substrate and no evidence of aliphatic hydroxylation products is observed. To further understand the details of the reaction mechanism, a computational study on a model complex was performed. These studies confirm the experimental hypothesis of dominant aromatic over aliphatic hydroxylation and show that the lack of an axial ligand affects the aliphatic pathways. Moreover, a two-parabola valence bond model is used to rationalize the rate constant and identify key properties of the oxidant and substrate that drive the reaction. In particular, the work shows that aromatic hydroxylation rates correlate with the ionization energy of the substrate as well as with the electron affinity of the oxidant.

A mononuclear nonheme iron(iv)-oxo complex which is more reactive than cytochrome P450 model compound I

A highly reactive mononuclear nonheme iron(IV)-oxo complex with a low-spin (S = 1) triplet ground state in both C–H bond activation and oxo transfer reactions is reported; this nonheme iron(IV)-oxo complex is more reactive than an iron(IV)-oxo porphyrin π-cation radical (i.e., a model of cytochrome P450 compound I) and is the most reactive species in kinetic studies among nonheme iron(IV)-oxo complexes reported so far. DFT calculations support the experimental results with extremely low activation barriers in the C–H bond activation of cyclohexane and 1,4-cyclohexadiene. The DFT calculations reveal that the S = 1 state is set up to easily lead to the highly reactive S = 2 high-spin iron(IV)-oxo species.

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.

Cytochrome P-450 model compound catalyzed selective hydroxylation of C–H bonds: Dramatic solvent effect

Selective hydroxylation of cyclohexane and cyclohexene by t-BuOOH in presence of F2oTPPFe(III)Cl as the catalyst has been achieved at room temparature in high yields.

Multistate Reactivity in Styrene Epoxidation by Compound I of Cytochrome P450: Mechanisms of Products and Side Products Formation

Density functional theoretical calculations are used to elucidate the epoxidation mechanism of styrene with a cytochrome P450 model Compound I, and the formation of side products. The reaction features multistate reactivity (MSR) with different spin states (doublet and quartet) and different electromeric situations having carbon radicals and cations, as well as iron(III) and iron(IV) oxidation states. The mechanisms involve state-specific product formation, as follows: a) The low-spin pathways lead to epoxide formation in effectively concerted mechanisms. b) The high-spin pathways have finite barriers for ring-closure and may have a sufficiently long lifetime to undergo rearrangement and lead to side products. c) The high-spin radical intermediate, (4)2(rad)-IV, has a ring closure barrier as small as the C--C rotation barrier. This intermediate will therefore lose stereochemistry and lead to a mixture of cis and trans epoxides. The barriers for the production of aldehyde and suicidal complexes are too high for this intermediate. d) The high-spin radical intermediate, (4)2(rad)-III, has a substantial ring closure barrier and may survive long enough time to lead to suicidal, phenacetaldehyde and 2-hydroxostyrene side products. e) The phenacetaldehyde and 2-hydroxostyrene products both originate from crossover from the (4)2(rad)-III radical intermediate to the cationic state, (4)2(cat,z(2) ). The process involves an N-protonated porphyrin intermediate that re-shuttles the proton back to the substrate to form either phenacetaldehyde or 2-hydroxostyrene products. This resembles the internally mediated NIH-shift observed during benzene hydroxylation.

Syntheses and Catalytic Activities of New Cytochrome P-450 Model Compounds. Effect of Peptide Chains

Abstract As cytochrome P-450 model compounds with structures similar to heme proteins, we designed and synthesized novel porphyrin iron (III) chloride complexes, 11 and 12, which have three peptide chains (3PCs) and four peptide chains (4PCs), and evaluated their catalytic activities. The asymmetric porphyrin complexes are derived from etioporphyrin and peptide chains equivalent to proteins were provided by ring-opening polymerization of N-carboxy l-amino acid anhydride (l-Phe-NCA, γ-BLG-NCA, and Nε-benzyloxycarbonyl-l-Lys-NCA) initiated by amino groups on meso positions. The new asymmetric porphyrin complex, 11 or 12, was used for asymmetric epoxidations of styrene as is done by cytochrome P-450. The iron complex achieved the induction of asymmetry, although it has no special conformation, and gave S-styrene oxide in excess (ca. 60%ee). It was found, furthermore, that asymmetric induction was affected by the kind of amino acid residue, the number of peptide chains, and length of peptide chains.

Suicide inactivation of cytochrome P-450 model compounds by terminal olefins. 1. A mechanistic study of heme N-alkylation and epoxidation

Les conclusions de cette etude permettent d'approfondir le mecanisme de l'inactivation du cytochrome P450 par les alcenes-1

Suicide inactivation of cytochrome P-450 model compounds by terminal olefins. 2. Steric and electronic effects in heme N-alkylation and epoxidation

On examine les effets steriques et electroniques des styrenes substitues reactifs, et des divers catalyseurs (chloro[tetraarylporphyrine] fer)

Manganese “Picnic Basket” Porphyrins: Cytochrome P-450 Active Site Analogues

Abstract This paper describes studies of a new class of cytochrome P-450 model compounds called the “picnic basket porphyrins.” The picnic basket porphyrins are unsymmetrical porphyrins with one face sterically protected by a superstructure and the other unhindered. The manganese(III) porphyrins have been investigated as shape selective catalysts using oxygen atom donors in the presence of a sterically bulky imidazole. This paper presents ligand binding studies and competitive olefin epoxidations which lead to the surprising conclusion that catalysis by the manganese picnic basket porphyrins occurs exclusively on the sterically unhindered face indicating reaction by the minor catalyst component and not the selective major catalyst component.

Epoxidation of olefins by cytochrome P-450 model compounds: kinetics and stereochemistry of oxygen atom transfer and origin of shape selectivity

Epoxidation of olefins by cytochrome P-450 model compounds: kinetics and stereochemistry of oxygen atom transfer and origin of shape selectivity

Epoxidation of olefins by cytochrome P-450 model compounds: mechanism of oxygen atom transfer.

The mechanism of the Mn(III) porphyrin-catalyzed epoxidation of olefins by lithium hypochlorite is examined. The active oxidant is thought to be a high-valent manganese-oxo complex. It is shown that a relatively stable intermediate is reversibly formed upon interaction of the olefin and the oxo complex. The decomposition of this intermediate to Mn(III) porphyrin and epoxide is the rate-determining step of the catalytic cycle. Some analogies to the biochemical epoxidation of olefins catalyzed by cytochrome P-450 are discussed.

Oxygenation of hydrocarbons by cytochrome P-450 model compounds: modification of reactivity by axial ligands.

The rate of olefin oxygenation catalyzed by synthetic metalloporphyrins is examined, employing sodium hypochlorite as the oxygen atom source. The rate of epoxidation and the stability of the catalyst are shown to be dependent on the nature of the axial ligand employed. A rationale for this effect is presented and analogy is made to the role of the thiolate ligand in cytochrome P-450.

Letter: Electronic structure and electric field gradients in oxyhemoglobin and -cytochrome P-450 model compounds.

Letter: Electronic structure and electric field gradients in oxyhemoglobin and -cytochrome P-450 model compounds.