Oxidation of hydroquinones by a nonheme iron(IV)-oxo species

The reactivity of an isolated, well-characterized nonheme iron(IV)-oxo complex, [(TMC)Fe^IVO]²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), has been examined in the oxidation of hydroquinones. This complex has been shown to oxidize a series of hydroquinones, resulting in the production of the corresponding quinone products. Kinetic studies performed with substituted hydroquinones revealed a good correlation between reaction rates of the iron(IV)-oxo complex and the electron-donating ability of the substituents and the bond dissociation energy (BDE) of substrate O-H bonds. These results with a kinetic isotope effect (KIE) of 2.7 led us to propose a hydrogen-atom abstraction mechanism for the oxidation of hydroquinones by the nonheme iron(IV)-oxo species.     

Kinetic studies of reactions of iron(IV)-oxo porphyrin radical cations with organic reductants

Iron(IV)-oxo porphyrin radical cations are observed intermediates in peroxidase and catalase enzymes, where they are known as Compound I species, and the putative oxidizing species in cytochrome P450 enzymes. In this work, we report kinetic studies of reactions of iron(IV)-oxo porphyrin radical cations that can be compared to reactions of other metal-oxo species. The iron(IV)-oxo radical cations studied were those produced from 5,10,15,20-tetramesitylporphryinato-iron(III) perchlorate (1), 5,10,15,20-tetramesitylporphryinato-iron(III) chloride (2), both in CH(3)CN solvent, and that from 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato-iron(III) perchlorate (3) in CH(2)Cl(2) solvent. The substrates studied were alkenes and activated hydrocarbons diphenylmethane and ethylbenzene. For a given organic reductant, various iron(IV)-oxo porphyrin radical cations react in a relatively narrow kinetic range; typically the second-order rate constants vary by less than 1 order of magnitude for the oxidants studied here and the related oxidant 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato-iron(IV)-oxo porphyrin radical cation in CH(3)CN solvent. Charge transfer in the transition states for epoxidation reactions of substituted styrenes by oxidants 1 and 2, rho(+) values of -1.9 and -0.9, respectively, mirrors results found previously for related species. Competition kinetic reactions with a catalytic amount of porphyrin iron(III) species and a terminal oxidant give relative rate constants for oxidations of competing substrates that are somewhat smaller than the ratios of absolute rate constants. Water in CH(3)CN solutions has an apparent modest stabilizing effect on oxidant 1 as indicated in slightly reduced rate constants for oxidation reactions. The iron(IV)-oxo porphyrin radical cations are orders of magnitude less reactive than porphyrin-manganese(V)-oxo cations and a corrole-iron(V)-oxo species. The small environment effects found here suggest that high energy demanding hydrocarbon oxidation reactions catalyzed by cytochrome P450 enzymes might require highly reactive iron(V)-oxo transients as oxidants instead of the more stable, isomeric iron(IV)-oxo porphyrin radical cations.     

How axial ligands control the reactivity of high-valent iron(IV)-oxo porphyrin pi-cation radicals in alkane hydroxylation: a computational study

The push effect of anionic axial ligands of high-valent iron(IV)-oxo porphyrin pi-cation radicals, (Porp)(+.)Fe(IV)(O)(X) (X=OH(-), AcO(-), Cl(-), and CF(3)SO(3)(-)), in alkane hydroxylation is investigated by B3LYP DFT calculations. The electron-donating ability of anionic axial ligands influences the activation energy for the alkane hydroxylation by the iron(IV)-oxo intermediates and the Fe-O bond distance of the iron-oxo species in transition state.     

Ferrous iron content of intravenous iron formulations

The observed biological differences in safety and efficacy of intravenous (IV) iron formulations are attributable to physicochemical differences. In addition to differences in carbohydrate shell, polarographic signatures due to ferric iron [Fe(III)] and ferrous iron [Fe(II)] differ among IV iron formulations. Intravenous iron contains Fe(II) and releases labile iron in the circulation. Fe(II) generates toxic free radicals and reactive oxygen species and binds to bacterial siderophores and other in vivo sequestering agents. To evaluate whether differences in Fe(II) content may account for some observed biological differences between IV iron formulations, samples from multiple lots of various IV iron formulations were dissolved in 12 M concentrated HCl to dissociate and release all iron and then diluted with water to achieve 0.1 M HCl concentration. Fe(II) was then directly measured using ferrozine reagent and ultraviolet spectroscopy at 562 nm. Total iron content was measured by adding an excess of ascorbic acid to reduce Fe(III) to Fe(II), and Fe(II) was then measured by ferrozine assay. The Fe(II) concentration as a proportion of total iron content [Fe(III) + Fe(II)] in different lots of IV iron formulations was as follows: iron gluconate, 1.4 and 1.8 %; ferumoxytol, 0.26 %; ferric carboxymaltose, 1.4 %; iron dextran, 0.8 %; and iron sucrose, 10.2, 15.5, and 11.0 % (average, 12.2 %). The average Fe(II) content in iron sucrose was, therefore, ≥7.5-fold higher than in the other IV iron formulations. Further studies are needed to investigate the relationship between Fe(II) content and increased risk of oxidative stress and infections with iron sucrose.     

The reactions of octaethylporphyrin and its iron (II) and iron (III) complexes with free radicals

The reactions of dilute solutions of octaethylporphyrin and its iron (II) and iron (III) complexes with methyl, 2-cyanopropyl, t-butoxy, and benzoyloxy radicals are described. The results are summarized: (i) The reactivity of the porphyrin and its high-spin iron (II) and iron (III) complexes toward alkyl and t-butoxy radicals stands in the order: Fe^II > Fe^III ? free porphyrin. For benzoyloxy radicals the order is Fe^II > Porp > Fe^III. (ii) The exclusive path of reaction of high-spin iron (II) porphyrin with radicals is the rapid reduction of the radical and generation of an iron (III) porphyrin. The dominant path of reaction of high-spin iron (III) porphyrin with alkyl and (presumably) t-butoxy radicals is a rapid axial inner sphere reduction of the porphyrin. An axial ligand of iron is transferred to the radical. (iv) The reaction of benzoyloxy radicals with high or low-spin iron (III) porphyrins occurs primarily at the meso position. With the low-spin dipyridyl complex in pyridine the attendant reduction to iron (II) can be observed spectrally. Methyl radicals also reduce this complex by adding to the meso position. (v) The reaction of a radical with either an iron (II) or an iron (III) porphyrin results in the generation of the other valence state of iron and consequently oxidation and reduction products emanating from both iron species are obtained. (vi) No evidence for an iron (IV) is intermediate is apparent. (vii) Iron (II) porphyrins in solvents that impart either spin state are easily oxidized by diacyl peroxides. The occurrence of both axial and peripheral redox reactions with the iron complexes supports an underlying premise of a recent theory of hemeprotein reactivity. The relevance of the work to bioelectron transfer and heme catabolism is noted.     

Nonheme iron(II) complexes of macrocyclic ligands in the generation of oxoiron(IV) complexes and the catalytic epoxidation of olefins

Mononuclear nonheme oxoiron(IV) complexes bearing 15-membered macrocyclic ligands were generated from the reactions of their corresponding iron(II) complexes and iodosylbenzene (PhIO) in CH(3)CN. The oxoiron(IV) species were characterized with various spectroscopic techniques such as UV-vis spectrophotometer, electron paramagnetic resonance, electrospray ionization mass spectrometer, and resonance Raman spectroscopy. The oxoiron(IV) complexes were inactive in olefin epoxidation. In contrast, when iron(II) or oxoiron(IV) complexes were combined with PhIO in the presence of olefins, high yields of epoxide products were obtained. These results indicate that in addition to the oxoiron(IV) species, there must be at least one more active oxidant (e.g., Fe(IV)-OIPh adduct or oxoiron(V) species) that effects the olefin epoxidation. We have also demonstrated that the ligand environment of iron catalysts is an important factor in controlling the catalytic activity as well as the product selectivity in the epoxidation of olefins by PhIO.     

Dioxygen activation by copper, heme and non-heme iron enzymes: comparison of electronic structures and reactivities

Enzymes containing heme, non-heme iron and copper active sites play important roles in the activation of dioxygen for substrate oxidation. One key reaction step is CH bond cleavage through H-atom abstraction. On the basis of the ligand environment and the redox properties of the metal, these enzymes employ different methods of dioxygen activation. Heme enzymes are able to stabilize the very reactive iron(IV)-oxo porphyrin-radical intermediate. This is generally not accessible for non-heme iron systems, which can instead use low-spin ferric-hydroperoxo and iron(IV)-oxo species as reactive oxidants. Copper enzymes employ still a different strategy and achieve H-atom abstraction potentially through a superoxo intermediate. This review compares and contrasts the electronic structures and reactivities of these various oxygen intermediates.     

Steady-state and transient kinetic analyses of taurine/alpha-ketoglutarate dioxygenase: effects of oxygen concentration, alternative sulfonates, and active-site variants on the FeIV-oxo intermediate

Taurine/alpha-ketoglutarate (alphaKG) dioxygenase (TauD), an archetype alphaKG-dependent hydroxylase, is a non-heme mononuclear Fe(II) enzyme that couples the oxidative decarboxylation of alphaKG with the conversion of taurine to aminoacetaldehyde and sulfite. The crystal structure of taurine-alphaKG-Fe(II)TauD is known, and spectroscopic studies have kinetically defined the early steps in catalysis and identified a high-spin Fe(IV)-oxo reaction intermediate. The present analysis extends our understanding of TauD catalysis by investigating the steady-state and transient kinetics of wild-type and variant forms of the enzyme with taurine and alternative sulfonates. TauD proteins substituted at residues surrounding the active site were shown to fold properly based on their abilities to form a diagnostic chromophore associated with the anaerobic Fe(II)-alphaKG chelate complex and to generate a tyrosyl radical upon subsequent reaction with oxygen. Steady-state studies of mutant proteins confirmed the importance of His 70 and Arg 270 in binding the sulfonate moiety of taurine and indicated the participation of Asn 95 in recognizing the substrate amine group. The N97A and S158A variants are likely to undergo an increase in hydrophobicity and expansion of the substrate-binding pocket, thus accounting for their decreased K(m) toward pentanesulfonic acid compared to wild-type TauD. Stopped-flow UV-visible spectroscopic examination of the reaction of oxygen with taurine-alphaKG-Fe(II)TauD confirmed a minimal three-step sequence of reactions attributed to Fe(IV)-oxo formation (k(1)), bleaching to the Fe(II) state upon substrate hydroxylation (k(2)), rebinding of excess substrates (k(3)), and indicated that none of the steps exhibit detectable solvent k(H)/k(D) isotope effects. This demonstrates that no protons are involved in the rate-determining step of Fe(IV)-oxo formation, in contrast to heme iron oxygenases. The Fe(IV)-oxo species is likely to be utilized in conversion of the alternative substrates pentanesulfonic acid and 3-N-morpholinopropanesulfonic acid; however, this spectroscopic intermediate was not detected because of the decreased k(1)/k(2) ratio. With taurine, k(1) was shown to depend on the oxygen concentration allowing calculation of a second-order rate constant of 1.58 x 10(5) M(-)(1) s(-)(1) for this irreversible reaction. Stopped-flow analyses of TauD variants provided several insights into how the protein environment influences the rates of Fe(IV)-oxo formation and decay. The Fe(IV)-oxo species was not detected in the N95D or N95A variants because of a reduced k(1)/k(2) ratio, likely related to a decreased substrate-dependent conversion of the six-coordinate to five-coordinate metal site.     

A nonheme iron(II) complex that models the redox cycle of lipoxygenase

The air-stable complex [Fe(6-Me3-TPA) (O2CAr)]+ [1; 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine] has been synthesized as a model for the iron(II) site of lipoxygenase. This iron(II) complex reacts with 0.5 equiv ROOH to form a yellow species, which has been formulated as [FeIII(OH)(6-Me3-TPA) (O2CAr)]+ (2) by electrospray mass spectrometry. Addition of more ROOH converts 2 into a purple species, which is characterized by electrospray ionization mass spectrometry and resonance Raman spectroscopy as [FeIII(OOR)(6-Me3-TPA)(O2CAr)]+. The purple species is metastable and decomposes via Fe-O bond homolysis to regenerate the starting iron(II) complex. These metal-centered transformations parallel the changes observed for lipoxygenase in its reaction with its product hydroperoxide.     

The first direct characterization of a high-valent iron intermediate in the reaction of an alpha-ketoglutarate-dependent dioxygenase: a high-spin FeIV complex in taurine/alpha-ketoglutarate dioxygenase (TauD) from Escherichia coli

The Fe(II)- and alpha-ketoglutarate(alphaKG)-dependent dioxygenases have roles in synthesis of collagen and sensing of oxygen in mammals, in acquisition of nutrients and synthesis of antibiotics in microbes, and in repair of alkylated DNA in both. A consensus mechanism for these enzymes, involving (i) addition of O(2) to a five-coordinate, (His)(2)(Asp)-facially coordinated Fe(II) center to which alphaKG is also bound via its C-1 carboxylate and ketone oxygen; (ii) attack of the uncoordinated oxygen of the bound O(2) on the ketone carbonyl of alphaKG to form a bicyclic Fe(IV)-peroxyhemiketal complex; (iii) decarboxylation of this complex concomitantly with formation of an oxo-ferryl (Fe(IV)=O(2)(-)) intermediate; and (iv) hydroxylation of the substrate by the Fe(IV)=O(2)(-) complex via a substrate radical intermediate, has repeatedly been proposed, but none of the postulated intermediates occurring after addition of O(2) has ever been detected. In this work, an oxidized Fe intermediate in the reaction of one of these enzymes, taurine/alpha-ketoglutarate dioxygenase (TauD) from Escherichia coli, has been directly demonstrated by rapid kinetic and spectroscopic methods. Characterization of the intermediate and its one-electron-reduced form (obtained by low-temperature gamma-radiolysis of the trapped intermediate) by M?ssbauer and electron paramagnetic resonance spectroscopies establishes that it is a high-spin, formally Fe(IV) complex. Its M?ssbauer isomer shift is, however, significantly greater than those of other known Fe(IV) complexes, suggesting that the iron ligands in the TauD intermediate confer significant Fe(III) character to the high-valent site by strong electron donation. The properties of the complex and previous results on related alphaKG-dependent dioxygenases and other non-heme-Fe(II)-dependent, O(2)-activating enzymes suggest that the TauD intermediate is most probably either the Fe(IV)-peroxyhemiketal complex or the taurine-hydroxylating Fe(IV)=O(2)(-) species. The detection of this intermediate sets the stage for a more detailed dissection of the TauD reaction mechanism than has previously been reported for any other member of this important enzyme family.     

Substrate activation by iron superoxo intermediates

A growing number of non-heme-iron oxygenases and oxidases catalyze reactions for which the well-established mechanistic paradigm involving a single C-H-bond-cleaving intermediate of the Fe(IV)-oxo (ferryl) type [1(?)] is insufficient to explain the chemistry. It is becoming clear that, in several of these cases, Fe(III)-superoxide complexes formed by simple addition of O(2) to the reduced [Fe(II)] cofactor initiate substrate oxidation by abstracting hydrogen [2,3(?)]. This substrate-oxidizing entry route into high-valent-iron intermediates makes possible an array of complex and elegant oxidation reactions without the consumption of valuable reducing equivalents. Examples of this novel mechanistic strategy are discussed with the goal of bringing forth unifying principles.     

A strongly bound high-spin iron(II) coordinates cysteine and homocysteine in cysteine dioxygenase

The first experimental evidence of a tight binding iron(II)-CDO complex is presented. These data enabled the relationship between iron bound and activity to be explicitly proven. Cysteine dioxygenase (CDO) from Rattus norvegicus has been expressed and purified with ~0.17 Fe/polypeptide chain. Following addition of exogenous iron, iron determination using the ferrozine assay supported a very tight stoichiometric binding of iron with an extremely slow rate of dissociation, k(off) ~ 1.7 × 10(-6) s(-1). Dioxygenase activity was directly proportional to the concentration of iron. A rate of cysteine binding to iron(III)-CDO was also measured. M?ssbauer spectra show that in its resting state CDO binds the iron as high-spin iron(II). This iron(II) active site binds cysteine with a dissociation constant of ~10 mM but is also able to bind homocysteine, which has previously been shown to inhibit the enzyme.     

Inhibition of the bovine branched-chain 2-oxo acid dehydrogenase complex and its kinase by arylidenepyruvates

A novel class of inhibitors for the branched-chain 2-oxo acid dehydrogenase (BCOAD) complex has been synthesized and studied. The sodium salts of arylidenepyruvates: e.g., furfurylidenepyruvate (compound I), 4-(3-thienyl)-2-oxo-3-butenoate (compound II), cinnamalpyruvate (compound III) and 4-(2-thienyl)-2-oxo-3-butenoate (compound IV) inhibit the overall and kinase reactions of the BCOAD complex from bovine liver. Inhibitions of the overall reaction occur at the decarboxylase (E1) step as determined by a spectrophotometric assay with 2,6-dichlorophenolindophenol as an electron acceptor. Inhibition of the E1 reaction by compound I (Ki = 0.5 microM) is competitive, whereas inhibitions by compounds II (Ki = 150 microM) and III (Ki = 500 microM) are non-competitive with respect to the substrate 2-oxoisovalerate. The Km value for 2-oxoisovalerate is 6.7 microM as measured by the E1 assay. Inhibition of the E1 step by compounds I, II and III are reversible at low inhibitor concentrations based on the Michaelis-Menten kinetics observed. By comparison, compound I does not significantly inhibit pyruvate and 2-oxoglutarate dehydrogenase complexes. The arylidenepyruvates (compounds I, II and IV) inhibit the BCOAD kinase reaction in a manner similar to the substrate 2-oxo acids. The inhibition of the kinase reaction by compound I is non-competitive with respect to ATP, with an apparent Ki value of 4.5 mM. The results suggest that arylidenepyruvates may be useful probes for elucidating the reaction mechanisms of the BCOAD complex and its kinase.     

Functional models for mononuclear nonheme iron enzymes

Increasing interest in mononuclear nonheme iron enzymes that activate dioxygen has resulted in an explosion of information on such enzymes in recent years. Concomitantly, efforts to model the active sites of these enzymes have produced synthetic complexes capable of mimicking some aspect of the reactivity of the metal center in several enzymes. These functional models carry out oxidative transformations analogous to those catalyzed by the enzymes and in some cases allow iron(III)-peroxo or iron(IV)-oxo intermediates to be trapped and characterized.     

DNA and RNA damage by Cu(II)-amikacin complex.

The oxidation-promoting reactivity of copper(II) complex of aminoglycosidic antibiotic amikacin [Cu(II)-Ami] in the presence of hydrogen peroxide, was studied at pH 7.4, using 2'-deoxyguanosine (dG), pBR322 plasmid DNA and yeast tRNAPhe as target molecules. The mixtures of complex with H2O2 were found to be efficient oxidants, converting dG to its 8-oxo derivative, generating strand breaks in plasmid DNA and multiple cleavages in tRNAPhe. The complex underwent autooxidation as well, with amikacin hydroperoxides as likely major products. This reactivity pattern was found to be due to a combination of metal-bound and free hydroxyl radicals.     

Interaction between iron(II) and hydroxamic acids: oxidation of iron(II) to iron(III) by desferrioxamine B under anaerobic conditions

Interaction between iron(II) and acetohydroxamic acid (Aha), alpha-alaninehydroxamic acid (alpha-Alaha), beta-alaninehydroxamic acid (beta-Alaha), hexanedioic acid bis(3-hydroxycarbamoyl-methyl)amide (Dha) or desferrioxamine B (DFB) under anaerobic conditions was studied by pH-metric and UV-Visible spectrophotometric methods. The stability constants of complexes formed with Aha, alpha-Alaha, beta-Alaha and Dha were calculated and turned out to be much lower than those of the corresponding iron(II) complexes. Stability constants of the iron(II)-hydroxamate complexes are compared with those of other divalent 3d-block metal ions and the Irving-Williams series of stabilities was found to be observed. Above pH 4, in the reactions between iron(II) and desferrioxamine B, the oxidation of the metal ion to iron(III) by the ligand was found. The overall reaction that resulted in the formation of the tris-hydroxamato complex [Fe(HDFB)]+ and monoamide derivative of DFB at pH 6 is: 2Fe2+ + 3H4DFB+ = 2[Fe(HDFB)]+ + H3DFB-monoamide+ + H2O + 4H+. Based on these results, the conclusion is that desferrioxamine B can uptake iron in iron(III) form under anaerobic conditions.     

Human complex II (succinate-ubiquinone oxidoreductase): cDNA cloning of iron sulfur (Ip) subunit of liver mitochondria

Complex II (succinate-ubiquinone oxidoreductase) is an important enzyme complex of both the tricarboxylic acid cycle and of the aerobic respiratory chains of mitochondria in eukaryotic cell and prokaryotic organisms. In this study, the amino acid sequence of iron sulfur-subunit in human liver mitochondria was deduced from cDNA which was isolated by immunoscreening a human liver lambda gtll cDNA library. An isolated clone contains an open reading frame of 786 nucleotides and encodes a mature protein of 252 amino acids with a molecular weight of 28,804. The amino acid sequence was highly homologous with that of bovine heart (94.1%) which has been determined from the purified peptide and that of Escherichia coli sdh B product (50.8%). Striking sequence conservation was found around the three cysteine-rich clusters which have been thought to comprise the iron-sulfur centers of the enzyme. This is the first report on the cDNA sequence of mitochondrial complex II.     

Oxidase reaction of cytochrome cd(1) from Paracoccus pantotrophus

Cytochrome cd(1) (cd(1)NIR) from Paracoccus pantotrophus, which is both a nitrite reductase and an oxidase, was reduced by ascorbate plus hexaamineruthenium(III) chloride on a relatively slow time scale (hours required for complete reduction). Visible absorption spectroscopy showed that mixing of ascorbate-reduced enzyme with oxygen at pH = 6.0 resulted in the rapid oxidation of both types of heme center in the enzyme with a linear dependence on oxygen concentration. Subsequent changes on a longer time scale reflected the formation and decay of partially reduced oxygen species bound to the d(1) heme iron. Parallel freeze-quench experiments allowed the X-band electron paramagnetic resonance (EPR) spectrum of the enzyme to be recorded at various times after mixing with oxygen. On the same millisecond time scale that simultaneous oxidation of both heme centers was seen in the optical experiments, two new EPR signals were observed. Both of these are assigned to oxidized heme c and resemble signals from the cytochrome c domain of a "semi-apo" form of the enzyme for which histidine/methionine coordination was demonstrated spectroscopically. These observations suggests that structural changes take around the heme c center that lead to either histidine/methionine axial ligation or a different stereochemistry of bis-histidine axial ligation than that found in the as prepared enzyme. At this stage in the reaction no EPR signal could be ascribed to Fe(III) d(1) heme. Rather, a radical species, which is tentatively assigned to an amino acid radical proximal to the d(1) heme iron in the Fe(IV)-oxo state, was seen. The kinetics of decay of this radical species match the generation of a new form of the Fe(III) d(1) heme, probably representing an OH(-)-bound species. This sequence of events is interpreted in terms of a concerted two-electron reduction of oxygen to bound peroxide, which is immediately cleaved to yield water and an Fe(IV)-oxo species plus the radical. Two electrons from ascorbate are subsequently transferred to the d(1) heme active site via heme c to reduce both the radical and the Fe(IV)-oxo species to Fe(III)-OH(-) for completion of a catalytic cycle.     

Chemical properties of water-soluble porphyrins. 4. The reaction of a 'picket-fence-like' iron (III) complex with the superoxide oxygen couple

Solution properties of the iron-(III) 'picket-fence-like' porphyrin, Fe(III)-alpha,alpha,alpha, beta-tetra-ortho (N-methyl-isonicotinamidophenyl) porphyrin, (Fe(III)PFP) were investigated. These were acid/base properties of the aquo complex with pKa of 3.9 and its aggregation (formation of dimer with K = 1 X 10(-10) dm3 mol-1), complex formation with cyanide ions and 1-methyl imidazole (1-MeIm), spectral properties of the three iron complexes in their ferric and ferrous form and the one-electron reduction potential of these complexes. Knowing these properties, the reaction of the ferric complexes, aquo, dicyano and bis (1-MeIm), with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O2- with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7.6 X 10(7) dm3 mol-1 s-1, two orders of magnitude faster when compared to the reaction of each of the other complexes. The reduction by other radicals with all iron (III) complexes had similar second-order rate constants (10(9) to 10(10) dm3 mol-1 s-1). The reduction reaction in all cases produced Fe(II)PEP and no intermediate was found. The oxidation reaction of Fe(II)PEP by O2- was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O2- toward the three iron (III) porphyrin complexes follows their reduction potentials, it is suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The reactions of the Fe(II)PFP complexes with dioxygen were also studied. The aquo complex was found to be first order in O2 and second order in Fe(II)PFP, suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The intermediate formation was corroborated by evidence of the rapid CO binding reaction to the aquo complex of Fe(II)PFP. The two other complexes reacted very slowly with O2 as well as with CO.     

Potentiometric study of vitamin D3 complexes with manganese(II), iron(II), iron(III) and zinc(II) in water-ethanol medium.

Vitamin D3 (LH) complexes with manganese(II), iron(II), iron(III) and zinc(II) were identified in water-ethanol medium (30/70). Their stability constants were determined at 298 K and at a constant ionic strength of 0.100 M using potentiometric methods. The computerisation of the experimental data showed the presence of ML (M = metal, L = deprotonated vitamin D3) and ML2 species in all cases; in addition, the ML3 iron(III) complex was detected. The calculated overall stability constants beta for MnIIL, FeIIL, FeIIIL and ZnIIL are, respectively, in logarithms, 12.4, 16.5, 28.5 and 16.5. Under the experimental conditions, the only protonated species MLH detected was with iron(III).