Spectroscopic study of a water-soluble iron(III) meso-tetrakis(4-N-methylpyridiniumyl) porphyrin in aqueous solution: effects of pH and salt

The equilibrium behavior of cationic iron(III) meso-tetrakis(4-N-methyl-pyridiniumyl) porphyrin, Fe(III)TMPyP, in aqueous solution was studied as a function of pH by optical absorption, EPR and (1)H NMR spectroscopies. The presence of several Fe(III)TMPyP species in solution was unequivocally demonstrated: monomeric porphyrin species (a monoaqueous five-coordinated complex, a diaaqueous six-coordinated complex and a monoaqueous-hydroxo six-coordinated complex), a micro-oxo dimer and a bis-hydroxo complex. The addition of salt to the porphyrin solution leads to a simplification of the equilibrium as a function of pH. In this case, only three species were observed in solution: a monomeric porphyrin species, a micro-oxo dimer and a bis-hydroxo complex. Optical absorption, EPR and (1)H NMR spectra contributed to the characterization of these species. Four critical pH values (pK) for Fe(III)TMPyP were obtained in pure buffer and only three pK values were observed in the presence of NaCl. The addition of salt favors the presence of the dimeric species in solution and simplifies the equilibrium in the acidic pH range.     

Biofunctional nanocomposite of carbon nanofiber with water-soluble porphyrin for highly sensitive ethanol biosensing

A biofunctional hybrid nanocomposite of carbon nanofiber (CNF) with water-soluble iron(III) meso-tetrakis(N-methylpyridinum-4-yl) porphyrin (FeTMPyP) was designed via non-covalent interaction for preparation of highly sensitive ethanol biosensor. The prepared nanocomposite showed good dispersion in water and was characterized with steady-state electronic absorption spectroscopy and scanning electron microscope. The nanocomposite combined the good conductivity of CNF and the excellent catalytic activity of both CNF and FeTMPyP toward the reduction of dissolved oxygen, producing a method for amperometric detection of oxygen ranging from 6.5 nM to 6.4 microM at a low overpotential. The nanocomposite modified electrode was further used for assembly of alcohol oxidase to construct an amperometric biosensor for ethanol. The biosensor showed rapid and highly sensitive response to ethanol with a linear range from 2.0 microM to 112 microM. The immobilized alcohol oxidase also showed its direct electrochemistry. The biofunctional nanocomposite provides a new way to not only construct the highly sensitive biosensors but also mimic the catalytic activity of enzyme in the life process.     

Synthesis and in vitro antioxidant properties of manganese(III) beta-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin

Manganese(III) meso-tetrakis(4-carboxypheny)porphyrin (MnTBAP) is a readily available and widely used agent to scavenge reactive oxygen species. A major limitation of MnTBAP is its relatively weak potency due to its low metal centered redox potential. The goal of these studies was to prepare a more potent analog of MnTBAP by increasing its redox potential through beta-substitution on the porphyrin ring by bromination. Manganese(III) beta-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin (MnBr(8)TBAP) was prepared in three steps starting from the methyl ester of the free ligand meso-tetrakis(4-carboxyphenyl)porphyrin, with an overall yield of 50%. The superoxide dismutase (SOD)-like activity of MnBr(8)TBAP (IC(50)=0.7 microM) was the same as manganese(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MnTM-4-PyP(5+)), while the metal-centered redox potential of the first was considerably higher than the second (E(1/2)=+128 and 0 mV vs. normal hydrogen electrode, respectively). However, a number of these cationic Mn-porphyrins (such as MnTM-4-PyP(5+)) redox-cycle with cytochrome P450 reductase in the presence of oxygen and NADPH whereas MnTBAP and its halogenated analog, MnBr(8)TBAP do not. The enhanced ability of MnBr(8)TBAP to inhibit paraquat- and hypoxia-induced injuries in vitro is also reported. In these in vitro models, in which cationic Mn-porphyrins exhibit very low activity, MnBr(8)TBAP appears to be at least eightfold more active than the non-brominated analog MnTBAP.     

Gangliosides mediate association of tetanus toxin with neural cells in culture

The effects of human serum albumin (HSA) on the rate of dithionite reduction of iron(III)deuteroporphyrin (iron(III)Dp) have been investigated in order to further characterize the porphyrin binding site and the changes manifested in this site under various conditions. These studies were performed under pseudo-first-order conditions, and in the presence of carbon monoxide as a "trapping agent" for the reduced iron(II)porphyrin. The rate of reduction of the free iron(III)Dp in phosphate buffer at pH 7.4 follows second-order kinetics with a rate constant (4.2 X 10(9) M-1 s-1) suggestive of a diffusion-controlled process. A six-orders of magnitude decrease in the rate of reduction was observed with iron(III)Dp was complexed with HSA. This result is consistent with HSA-bound porphyrin being less accessible to the aqueous environment. Additional studies demonstrated that both pH and anions induce various alterations in the complex that are reflected in the rate of reduction of iron(III)porphyrin.     

Mössbauer studies on tetra(p-sulphophenyl)porphine iron(III) solutions

The Mössbauer spectra of frozen aqueous solutions of iron(III) over the pH range 1.9 to 12 are reported. The spectra show that tetra(p-sulphophenyl)porphine iron(III) exists principally in two forms, monomeric and as μ-oxo-oligomers, depending on the pH of the solution. Above pH 4.0 only the μ-oxo-oligomer exists and at very low pHs (2.0 or less) only monomers are present. The quadrupole splitting of the monomer is the largest yet recorded for a five coordinate high spin iron(III) porphyrin.     

Binding of the Mn(III) complex of meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin to DNA. Effect of ionic strength

Interactions of the water-soluble Mn(III) complex of meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin (Mn(III)TMPyP) with DNA in aqueous solutions at low (0.01 M) and high (0.2 M) ionic strengths have been studied by optical absorption, resonance light scattering (RLS) and 1H NMR spectroscopies. Optical absorption and RLS measurements have demonstrated that in DNA solutions at low ionic strength the Mn(III)TMPyP form aggregates, which are decomposed at DNA excess. At high ionic strength the aggregation was not observed. We explain this effect by assuming that upon increase in ionic strength, Mn(III) TMPyP dislocates from the DNA sites, which produces better conditions for the porphyrin aggregation, to sites where the aggregation is hindered. The 1H NMR data demonstrated that the aggregation observed at low ionic strength reduces the paramagnetism of Mn(III)TMPyP. This phenomenon was not observed at the high ionic strength in the absence of aggregation.     

Air-stable,heme-like water-soluble iron(II) porphyrin: in situ preparation and characterization

Preparation of the water-soluble, kinetically labile, high-spin iron(II) tetrakis(4-sulfonatophenyl)porphyrin, Fe(II)TPPS⁴⁻, has been realized in neutral or weakly acidic solutions containing acetate buffer. The buffer played a double role in these systems: it was used for both adjusting pH and, via formation of an acetato complex, trapping trace amounts of iron(III) ions, which would convert the iron(II) porphyrins to the corresponding iron(III) species. Fe(II)TPPS⁴⁻ proved to be stable in these solutions even after saturation with air or oxygen. In the absence of acetate ions, however, iron(II) ions play a catalytic role in the formation of iron(III) porphyrins. While the kinetically inert iron(III) porphyrin, Fe(III)TPPS³⁻, is a regular one with no emission and photoredox properties, the corresponding iron(II) porphyrin displays photoinduced features which are typical of sitting-atop complexes (redshifted Soret absorption and blueshifted emission and Q absorption bands, photoinduced porphyrin ligand-to-metal charge transfer, LMCT, reaction). In the photolysis of Fe(II)TPPS⁴⁻ the LMCT process is followed by detachment of the reduced metal center and an irreversible ring-opening of the porphyrin ligand, resulting in the degradation of the complex. Possible oxygen-binding ability of Fe(II)TPPS⁴⁻ (as a heme model) has been studied as well. Density functional theory calculations revealed that in solutions with high acetate concentration there is very little chance for iron(II) porpyrin to bind and release O₂, deviating from heme in a hydrophobic microenvironment in hemoglobin. In the presence of an iron(III)-trapping additive that is much less strongly coordinated to the iron(II) center than the acetate ion, Fe(II)TPPS⁴⁻ may function as a heme model.     

Anomalous excitonic CD of meso-tetrakis(3-N-methylpyridiniumyl)porphyrin bound to poly[d(A-T)(2)

When meso-tetrakis(3-N-methylpyridiniumyl)porphyrin (m-TMPyP) formed a complex with poly[d(A-T)(2)], an intense bisignate excitonic CD in the Soret absorption region was observed. The excitonic CD of the m-TMPyP-poly[d(A-T)(2)] complex is unique in that no other combination of the related porphyrin, namely, meso-tetrakis(n-N-methylpyridiniumyl)porphyrin (where n = 2, 4), and polynucleotide including calf thymus DNA, poly[d(G-C)(2)], poly[d(I-C)(2)], and poly(dA).poly(dT), exhibits a comparable CD spectrum. From the [drug]/[DNA] ratio-dependence of the intensity and the shape of the CD spectrum, this porphyrin species is assigned to an extensively aggregated form. The extensively aggregated porphyrin disperses in 1 h after mixing to form moderately stacked porphyrin at a low mixing ratio. The magnitude of linear dichroism of the extensively aggregated porphyrin was small and the sign was negative in the Soret band, which indicated that the molecular plane of porphyrin in the complex is strongly tilted. On the other hand, the molecular plane of porphyrin is almost parallel to the DNA base plane (perpendicular to the DNA helix axis) in the moderately stacked form.     

Nitric oxide transfer from the NO-donor S-nitroso-N-acetylpenicillamine to monomers and dimers of water-soluble iron-porphyrins

The nitrosylation of two water-soluble iron-porphyrins, the anionic Fe(III)-meso-tetrakis(p-sulfonatophenyl)porphyrin (FeTPPS(4)) and the cationic Fe(III)-meso-tetrakis(4-N-methylpyridiniumyl)porphyrin (FeTMPyP), by the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) was studied using optical absorption spectroscopy. The influence of ionic and non-ionic micelles on rates of nitric oxide transfer was investigated. Initially, the effect of the micelles on the pH-dependent equilibrium between monomeric and micro-oxo-dimeric species of the iron-porphyrins was examined. It is not affected in micelle-porphyrin systems with electric charges identical in sign. The non-ionic micelles of polidocanol induce a small negative pK shift. In contrast, the dimerization equilibrium of porphyrins in oppositely charged micellar phases is displaced to lower pH by approximately 2 units. Nitric oxide binding to monomers and micro-oxo-dimers was examined at pH 5.0 and 8.0, respectively. Contrary to nitrosylation by NO gas, SNAP induces reductive nitrosylation. There was no observed NO-Fe(III)porphyrin intermediate. Nitrosylation rates were obtained and compared in aqueous buffer and in micellar systems. Monomers nitrosylate much faster than micro-oxo-dimers. Oppositely charged micelles prevent nitrosylation of the iron-porphyrins or considerably enhance nitrosylation times. Nitrosylation rates are comparable to transnitrosylation rates between several S-nitrosothiols and thiol-containing proteins, suggesting biological relevance for the process.     

Modeling the haloperoxidases: reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

The manganese meso-dimethylimidazolium porphyrin complex Mn(III)[TDMImP] reacted with HOBr/OBr(-) to generate the corresponding oxo-Mn(V)[TDMImP] species. The rate of this process accelerated with increasing pH. A forward rate constant, k(for), of 1.65x10(6)M(-1)s(-1) was determined at pH 8. Under these conditions, the oxo-Mn(V) species is short-lived and is transformed into the corresponding oxo-Mn(IV) complex. A first-order rate constant, k(obs), of 0.66 s(-1) was found for this reduction process at pH 8. The mechanism of this reduction process, which was dependent on bromide ion, appeared to proceed via an intermediate Mn(III)-O-Br complex. Thus, both a fast, reversible Mn(III)-O-Br bond heterolysis and a slower homolytic pathway occur in parallel in this system. The reverse oxidation reaction between oxo-Mn(V)[TDMImP] and bromide was investigated as a function of pH. The rate of this oxo-transfer reaction (k(rev)=1.4x10(3)M(-1)s(-1) at pH 8) markedly accelerated as the pH was lowered. The observed first-order dependence of the rate on [H(+)] indicates that the reactive species responsible for bromide oxidation is a protonated oxo-hydroxo complex and the stable species present in solution at high pH is dioxo-Mn(V)[TDMImP], [O=Mn(V)=O](-). The oxo-Mn(V) species retains nearly all of the oxidative driving force of the hypohalite. The equilibrium constant K(equi)=k(for)/k(rev) for the reversible process was determined at three different pH values (K(equi)=1.15x10(3) at pH 8) allowing the measurement of the redox potentials E of oxo-Mn(V)/Mn(III) (E=1.01 V at pH 8). The redox potential for this couple was extrapolated over the entire pH scale using the Nernst relationship and compared to those of the manganese 2- and 4-meso-N-methylpyridinium porphyrin couples oxo-Mn(V)[2-TMPyP]/Mn(III)[2-TMPyP], oxo-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP], OBr(-)/Br(-) and H(2)O(2)/H(2)O. Notably, the redox potential of oxo-Mn(V)/Mn(III) for the imidazolium porphyrin approaches that of H(2)O(2)/H(2)O at low pH.     

Differential patterns of peroxynitrite mediated apoptosis in proximal tubular epithelial cells following ATP depletion recovery

Ischemia-reperfusion injury (IRI) is characterized by ATP depletion in the ischemic phase, followed by a rapid increase in reactive oxygen species, including peroxynitrite in the reperfusion phase. In this study, we examined the role of peroxynitrite on cytotoxicity and apoptosis in an in vitro model of ATP depletion-recovery. Porcine proximal tubular epithelial (LLC-PK₁) cells were ATP depleted for either 2 h (2/2) or 4 h (4/2) followed by recovery in serum free medium for 2 h. A subset of cells was treated with 100 μM of the peroxynitrite scavenger, iron (III) tetrakis (N-methyl-4′pyridyl) porphyrin pentachloride (FeTMPyP) 30 min prior to and during treatment/recovery. Treatment with FeTMPyP reduced cytotoxicity and superoxide levels at both the 2/2 and 4/2 time points, however FeTMPyP decreased nitric oxide only at the 2/2 time point. FeTMPyP also partially blocked caspase-3 and caspase-8 activation at both 2/2 and 4/2 time points. At the 4/2 time point, FeTMPyP also partially inhibited the ATP depletion mediated increase in tumor necrosis factor alpha (TNF-α) and decreased Bax and FasL gene expression. These data show that peroxynitrite induces apoptosis by activation of multiple pathways depending on length and severity of insult following ATP depletion-recovery.     

Kinetic simulation studies on the transient formation of the oxo‐iron(IV) porphyrin radical cation during the reaction of iron(III) tetrakis‐5,10,15,20‐(N‐methyl‐4‐pyridyl)‐porphyrin with hydrogen peroxide in aqueous solution

High‐valent oxo‐iron(IV) species are commonly proposed as the key intermediates in the catalytic mechanisms of iron enzymes. Water‐soluble iron(III) tetrakis‐5,10,15,20‐(N‐methyl‐4‐pyridyl)porphyrin (Fe(III)TMPyP) has been used as a model of heme‐enzyme to catalyse the hydrogen peroxide (H₂O₂) oxidation of various organic compounds. However, the mechanism of the reaction of Fe(III)TMPyP with H₂O₂ has not been fully established. In this study, we have explored the kinetic simulation of the reaction of Fe(III)TMPyP with H₂O₂ and of the catalytic reactivity of FeTMPyP in the luminescent peroxidation of luminol. According to the mechanism that has been established in this work, Fe(III)TMPyP is oxidized by H₂O₂ to produce (TMPyP)^·+Fe(IV)=O (k1 = 4.5 × 10⁴/mol/L/s) as a precursor of TMPyPFe(IV)=O. The intermediate, (TMPyP)^·+Fe(IV)=O, represented nearly 2% of Fe(III)TMPyP but it does not accumulate in suf?cient concentration to be detected because its decay rate is too fast. Kinetic simulations showed that the proposed scheme is capable of reproducing the observed time courses of FeTMPyP in various oxidation states and the decay pro?les of the luminol chemiluminescence. It also shows that (TMPyP)^·+Fe(IV)=O is 100 times more reactive than TMPyPFe(IV)=O in most of the reactions. These two species are responsible for the initial sharp and the sustained luminol emissions, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

A spectroscopic and electrochemical study of the interaction between copper (II) glycylglycyl-L-histidine-N-methylamide and iron (III) tetra-p-sulfophenylporphine

A binuclear complex has been produced by the reaction of an iron porphyrin (sodium tetra-p-sulfophenylporphine iron (III)-FeTPPS) with a copper metallo-tripeptide (copper (II) glycylglycyl-L-histidine-N-methylamide-CuGGH) in aqueous solution. The system has been characterized by electron spin resonance (ESR) spectroscopy, optical absorption spectroscopy, and electrochemical methods. Room-temperature ESR spectra of the copper complex and low-temperature ESR spectra of the iron porphine provide evidence for the formation of a binuclear complex. These findings are supported by absorption spectroscopy and electrochemical studies, and lead to a value of ca. 2 X 10(-3) M-1 (at room temperature) for the equilibrium constant for complex formation. The relevance of this system to the enzymic active site of mammalian cytochrome c oxidase is discussed.     

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.     

Reduction of Fe(III) porphyrin hydroxides by heterocyclic aromatic amines

The reduction of iron(III) porphyrin hydroxides by the heterocyclic aromatic amines, pyridine, 1-methylimidazole and derivatives, occurs in toluene to give the bisamine iron(II) porphyrin complexes. The reaction has not been fully characterized but is found to proceed through a different mechanism from that reported for the similar reductions by 1° and 2° amines in the absence of hydroxide ion. Preliminary data indicate that the first step in the reduction is formation of the bisamine Fe(III) porphyrin complex from the hydroxide. Nucleophilic attack by hydroxide ion on the aromatic ring of an axially ligated pyridine or methylimidazole of the Fe(III) complex followed by homolytic cleavage of the FeN bond is proposed.     

1'-carbon hydroxylation of deoxyribose as the mechanism of cleavage of poly(dA) by manganese porphyrin combined with potassium monopersulfate

Polyadenylic acid, poly(dA), was readily cleaved at neutral pH by meso-tetrakis (4-N-methylpyridyl) porphyrinatomangangeseIII pentaacetate after being activated by potassium monopersulfate. Spontaneously free adenine was released, and after heating an unstable sugar degradation product identified as 5-methylene-2-furanone was evidenced, indicating that C'1-H is the main target of the porphyrin high-valent metal-oxo species.     

Oxidative degradation of cationic metalloporphyrins in the presence of nucleic acids: a way to binding constants?

Mn(III) and Fe(III) complexes of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (M-TMePyP) and related hybrid molecules ("metalloporphyrin-ellipticine") were activated by potassium monopersulfate in the presence of variable calf thymus (CT) DNA and NaCl concentrations. Monitored by visible spectroscopy (Soret band), fast degradation of the free metalloprophyrin was observed while the DNA-bound form appeared protected. This direct quantitation of free versus bound metalloporphyrin ratios allowed determination of binding constants: Mn- and Fe-TMePyP respectively bind to CT DNA (5 mM phosphate buffer, 0.1 M NaCl, pH 7) with K = 3 X 10(4) and 1.2 X 10(4) M-1. Mn-TMePyP showed a greater affinity for poly[d(A-T)] (K = 1.2 X 10(5) M-1) than for poly[d(G-C)] (K = 0.2 X 10(4) M-1). This method allowed us access to the intrinsic DNA affinity of the metalloporphyrin moiety of the hybrid molecules "metalloporphyrin-ellipticine".     

Biosorption of iron(III)–cyanide complex anions to Rhizopus arrhizus: application of adsorption isotherms

The biosorption of iron(III)–cyanide complex anions to Rhizopus arrhizus was investigated. The iron(III)–cyanide complex ion binding capacity of the biosorbent was a function of initial pH, initial iron(III)–cyanide complex ion and biosorbent concentration. These results indicated that a significant reduction of iron(III)–cyanide complex ions was achieved at pH 13, a highly alkaline condition. The maximum loading capacity of biosorbent was 612·2 mg g⁻¹ at 1996·2 mg litre⁻¹ initial iron(III)–cyanide complex ion concentration at this pH. The Freundlich, Langmuir and Redlich–Peterson adsorption models were fitted to the equilibrium data at pH 3·0, 7·0 and 13·0. The equilibrium data could be best fitted to by all the adsorption models over the entire concentration range (50–2000 mg litre⁻¹) at pH 13.     

High-performance liquid chromatography with spectrophotometric and electrochemical detection of a series of manganese(III) cationic porphyrins

Recent studies have revealed potent pharmacological activities of manganese-containing cationic porphyrins. An analytical method employing high-performance liquid chromatography with spectrophotometric and electrochemical detection (HPLC-UV/EC) suitable for in vivo applications is described for a series of manganese(III) cationic porphyrins with good separation and resolution. In particular, this method resolved the four atropisomers of manganese(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP5+ or AEOL-10113), verified by mass spectrometry. Electrochemical and spectrophotometric methods of detection were compared using manganese(III) meso-tetrakis(1,3-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP5+ or AEOL-10150), the lead catalytic antioxidant of this series. Both methods of detection were quantitative, but electrochemical detection, although less specific for in vivo applications, appears to be considerably more sensitive than spectrophotometric detection.     

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.