Reactivity of a new class of P450 enzyme models

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

Fe(NO3)3·9H2O-catalyzed aerobic oxidative deoximation of ketoximes and aldoximes under mild conditions

A mild, simple process for the effective aerobic oxidative deoximation of a wide range of ketoximes and aldoximes has been developed that utilizes Fe(NO3)3·9H2O as the single catalyst and molecular oxygen as the green oxidant. The environmentally benign protocol provides moderate to excellent yield and broad functional groups tolerance and is a valuable synthetic method for practical applications. According the relevant verification experiment, a plausible mechanism has been proposed.

Synthesis, Characterisation and Magnetic Behaviour of Ionic Metalloporphyrins: Metal–Tetrakis(N-Octyl-4-Pyridinium)–Porphyrins with Tetrabromoferrate(III) Anions

A series of magnetic, ionic-substituted tetrapyridyl metalloporphyrins, [tetrakis(N-octyl-4-pyridinium)–metal–porphy-rin][tetrabromoferrate(III)]4 (metal=iron, cobalt, manganese, copper or zinc), have been synthesised. All compounds show weak ferromagnetic behaviour at room temperature and respond to an external neodymium magnet.

Characterization of electrodes chemically modified with Mn(III) porphyrin/polypyrrole films as catalytic surfaces for an azo dye

In this study we describe the electrochemical behavior of 5,10,15,20-tetrakis(2'-aminophenylporphyrin)manganese(III) chloride supported on a glassy carbon electrode, as well as the electrochemical preparation and characterization of thin films based on pyrrole-3-carboxylic acid. The electrocatalytic action of the electrode modified with the Mn(III) porphyrin toward an azo dye was tested, and the characteristic strong interaction between the incorporated metalloporphyrin and RR120 dye was verified.

Sulfur versus iron oxidation in an iron-thiolate model complex

In the absence of base, the reaction of [Fe(II)(TMCS)]PF6 (1, TMCS = 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV) complex (2, [Fe(IV)(O)(TMCS)]PF6), as previously observed in the presence of strong base (KO(t)Bu). Instead, the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored by UV-vis, ESI-MS, and Mössbauer spectroscopies. ESI-MS showed 3 to be formulated as [Fe(II)(TMCS) + 2O](+), while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O = 1.95 Å, Fe-S = 3.26 Å). The addition of a third equivalent of peracid resulted in the formation of yet another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species. Mössbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (Fe═O = 1.64 Å) and resonance Raman (ν(Fe═O) = 831 cm(-1)) showed that indeed an oxoiron(IV) unit had been generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a metal-bound sulfinate moiety (ν(s)(SO2) ≈ 1000 cm (-1), ν(as)(SO2) ≈ 1150 cm (-1)). Investigations into the reactivity of 1 and 2 toward H(+) and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.

Mechanistic insight into formation of oxo-iron(IV) porphyrin pi-cation radicals from enzyme mimics of cytochrome P450 in organic solvents

Two new models for cytochrome P450 in which the thiolate axial ligand is replaced by a RSO(3)(-) group, form oxo-iron(IV) porphyrin pi-cation radicals as sole oxidation products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar) and electronic nature of the porphyrin rings. Although the properties of the solvent and push-pull effects from the porphyrin rings do not affect the mode of the O-O bond cleavage (heterolytic or homolytic) in these models, they strongly affect the rate and mechanism of each reaction step leading to the formation of the high-valent iron intermediates. This article reports the results of mechanistic studies involving the measurements of the rate of oxo-iron(IV) porphyrin pi-cation radical formation from the enzyme mimics of P450 for different oxidant concentration, temperature and pressure in selected organic solvents. Extraction of the appropriate rate constants and activation parameters for the reactions studied enable a detailed discussion of the effects of solvent and electronic nature of the porphyrin rings on the position of the first pre-equilibrium involving formation of the acylperoxo-iron(III) porphyrin intermediate, as well as on the rate of heterolytic O-O bond cleavage leading to the formation of the high-valent iron species. Furthermore, an unusual effect of solvent on the kinetics of oxo-iron(IV) porphyrin pi-cation radical formation in methanol is demonstrated and discussed in the present work.

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.

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.

Thermodynamic and Kinetic Studies on the Binding of Nitric Oxide to a New Enzyme Mimic of Cytochrome P450

A new model for the P450 enzyme carrying a SO(3)(-) ligand coordinated to iron(III) (complex 2) reversibly binds NO to yield the nitrosyl adduct. The rate constant for NO binding to 2 in toluene is of the same order of magnitude as that found for the nitrosylation of the native, substrate-bound form of P450(cam) (E.S-P450(cam)). Large and negative activation entropy and activation volume values for the binding of NO to complex 2 support a mechanism that is dominated by bond formation with concomitant iron spin change from S = (5)/(2) to S = 0, as proposed for the reaction between NO and E.S-P450(cam). In contrast, the dissociation of NO from 2(NO) was found to be several orders of magnitude faster than the corresponding reaction for the E.S-P450(cam)/NO system. In a coordinating solvent such as methanol, the alcohol coordinates to iron(III) of 2 at the distal position, generating a six-coordinate, high-spin species 5. The reaction of NO with 5 in methanol was found to be much slower in comparison to the nitrosylation reaction of 2 in toluene. This behavior can be explained in terms of a mechanism in which methanol must be displaced during Fe-NO bond formation. The thermodynamic and kinetic data for NO binding to the new model complexes of P450 (2 and 5) are discussed in reference to earlier results obtained for closely related nitrosylation reactions of cytochrome P450(cam) (in the presence and in the absence of the substrate) and a thiolate-ligated iron(III) model complex.