The reaction between ferrous polyaminocarboxylate complexes and hydrogen peroxide: an investigation of the reaction intermediates by stopped flow spectrophotometry

The reactions of Fe(II)EDTA, Fe(II)DTPA, and Fe(II)HEDTA with hydrogen peroxide near neutral pH have been investigated. All these reactions have been assumed to proceed through an active intermediate, I1, (Formula: see text) where pac is one of the three polyaminocarboxylates mentioned above. I1, whether .OH radical or an iron complex, reacts with ethanol, formate, and other scavengers at rates relative to k2 that, with the exception of t-butanol and benzoate, are similar, but not identical, to those expected for the.OH radical. In contrast, at pH 3, in the absence of ligands the reaction of I1 with Fe2+ was inhibited by ethanol and t-butanol and the reactivity of I1 towards these two scavengers relative to ferrous ion is identical to that exhibited by the hydroxyl radical. When pac = HEDTA, the intermediate of the first reaction reacts with formate ion to form the ferrous HEDTA ligand radical complex, which is characterized by absorption maxima at 295 nm (epsilon = 2,640 M-1 cm-1) and 420 nm (epsilon = 620 M-1 cm-1). For the reaction of Fe(II)HEDTA with H2O2, the following mechanism is proposed: (Formula: see text) where k17 = 4.2 X 10(4) M-1 sec-1 and k19 = 5 +/- 0.2 sec-1.     

Oxidizing intermediates in the reaction of ferrous EDTA with hydrogen peroxide. Reactions with organic molecules and ferrocytochrome c

The reaction between hydrogen peroxide and ferrous EDTA generates an oxidizing intermediate (I1) which is not the hydroxyl radical. It oxidizes ferrocytochrome c and also reacts with hydrogen peroxide (k5 = 3.2 X 10(3) M-1 S-1) to form a second oxidizing transient (I2). I1 is not scavenged by t-butyl alcohol whereas I2 is. I1 is found to be significantly less reactive than the hydroxyl radical toward benzoate ion, t-butyl alcohol, acetate ion, arginine, and serine, but is scavenged by compounds with readily oxidizable functional groups such as ethanol and isopropyl alcohol. This indicates that I1 does not undergo the characteristic reactions of the hydroxyl radical but shows a pattern of reactivity more associated with a metal ion oxidant like a ferryl (FeO2+)-EDTA complex.     

Chemiluminescence study on the peroxidation of linoleic acid initiated by the reaction of ferrous iron with hydrogen peroxide

Linoleic acid was used as a model system to study lipid peroxidation initiated by the reaction of ferrous iron with hydrogen peroxide. Low-level chemiluminescence of the peroxidation was measured with a high-sensitivity single-photon counter. It was found that the luminescence primarily comes from the dimol reaction of singlet oxygen and that the peak intensity of emission is a quadratic function of the concentration of either Fe2+ or H2O2, provided that the other Fenton reagent is in great excess. Under the same conditions, analysis on reaction kinetics shows a linear relationship between the maximal level of the initiator formed by the Fenton reaction and the initial concentration of Fe2+ or H2O2. This implies that the peak intensity of the chemiluminescence may be a good index of the maximal level of the initiator.     

Effective DNA cleavage by bleomycin-vanadium(IV) complex plus hydrogen peroxide

It has been firstly found that the bleomycin-vanadyl(IV) complex is effectively capable of cleaving DNA in the presence of hydrogen peroxide. The 1:1 bleomycin-VO(IV) complex has been characterized by ESR and electronic absorption spectra, and its ESR parameters (go = 1.982 and Ao = 93.5 G) are indicative of VO(N5) coordination type for the metal-binding environment. The mode of nucleotide sequence cleavage induced by the present bleomycin-VO(IV)-H2O2 complex system was appreciably different from the corresponding Fe(III) complex system. Of special interest is the fact that the bleomycin-vanadium complex system more preferentially attacked G-A(5'----3') sequences than the bleomycin-iron complex system.     

Kinetic studies on the reaction of ethylenediaminetetraacetatochromium(III) complex with hydrogen peroxide

The rate of reaction of [Cr(III)Y]_aq (Y is EDTA anion) with hydrogen peroxide was studied in aqueous nitrate media [μ = 0.10 M (KNO₃)] at various temperatures. The general rate equation, Rate = $\text{k}{}_1\text{+}\text{k}{}_2\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}\text{1 +}\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}$ [Cr(III)Y]_aq[H₂O₂] holds over the pH range 5–9. The decomposition reaction of H₂O₂ is believed to proceed via two pathways where both the aquo and hydroxo-quinquedentate EDTA complexes are acting as the catalyst centres. Substitution-controlled mechanisms are suggested and the values of the second-order rate constants k₁ and k₂ were found to be 1.75 × 10^?2 M^?1 s^?1 and 0.174 M^?1 s^?1 at 303 K respectively, where k₂ is the rate constant for the aquo species and k₂ is that for the hydroxo complex. The respective activation enthalpies (ΔH^*₁ = 58.9 and ΔH^*₂ = 66.5 KJ mol^?1) and activation entropies (ΔS^*₁ = ?85 and ΔS^*₂ = ?40 J mol^?1 deg^?1) were calculated from a least-squares fit to the Eyring plot. The ionisation constant pK₁, was inferred from the kinetic data at 303 K to be 7.22. Beyond pH 9, the reaction is markedly retarded and ceases completely at pH ? 11. This inhibition was attributed in part to the continuous loss of the catalyst as a result of the simultaneous oxidation of Cr(III) to Cr(VI).     

The reaction of copper-2,9-dimethyl-1,10-phenanthroline with hydrogen peroxide

The copper complex of 2,9-dimethyl-1,10-phenanthroline(2,9-dmp) is accumulated by a variety of organisms and is highly toxic. Bioaccumulation depends on reduction of copper(II) to (I), since only the copper(I)-2,9-dmp complex is lipophilic. In the case of the marine diatom, Nitzschia closterium, it is proposed that hydrogen peroxide, produced by the algae during photosynthesis, is the in vivo reductant. Hydrogen peroxide rapidly reduces copper(II)-2,9-dmp, but an excess of H₂O₂ leads to destruction of the yellow copper(I) complex. Rate constants for the formation and degradation of the yellow complex are reported. Oxygen, light, and a hydroxylating agent are released during the degradation reaction. A reaction mechanism is proposed for the destruction of copper-2,9-dmp by excess H₂O₂, involving attack on the 5, 6 positions of the phenanthroline ring by hydroxyl radical, then further oxidation by singlet oxygen and H₂O₂. These in vivo degradation reactions are believed to be the cause of the extreme toxicity of the complex.     

Reaction of ferrous lactoperoxidase with hydrogen peroxide and dioxygen: an anaerobic stopped-flow study

Lactoperoxidase (LPO) is found in mucosal surfaces and exocrine secretions including milk, tears, and saliva and has physiological significance in antimicrobial defense which involves (pseudo-)halide oxidation. LPO compound III (a ferrous-dioxygen complex) is known to be formed rapidly by an excess of hydrogen peroxide and could participate in the observed catalase-like activity of LPO. The present anaerobic stopped-flow kinetic analysis was performed in order to elucidate the catalytic mechanism of LPO and the kinetics of compound III formation by probing the reactivity of ferrous LPO with hydrogen peroxide and molecular oxygen. It is shown that ferrous LPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl LPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (7.2+/-0.3) x 10(4) M(-1) s(-1) at pH 7.0 and 25 degrees C. The H2O2-mediated conversion of compound II to compound III follows also second-order kinetics (220 M(-1) s(-1) at pH 7.0 and 25 degrees C). Alternatively, compound III is also formed by dioxygen binding to ferrous LPO at an apparent bimolecular rate constant of (1.8+/-0.2) x 10(5) M(-1) s(-1). Dioxygen binding is reversible and at pH 7.0 the dissociation constant (K(D)) of the oxyferrous form is 6 microM. The rate constant of dioxygen dissociation from compound III is higher than conversion of compound III to ferric LPO, which is not affected by the oxygen concentration and follows a biphasic kinetics. A reaction cycle including the redox intermediates compound II, compound III, and ferrous LPO is proposed, which explains the observed (pseudo-)catalase activity of LPO in the absence of one-electron donors. The relevance of these findings in LPO catalysis is discussed.     

Reaction of hydrogen peroxide with hexaaquacobalt(III) perchlorate

The reaction of with H₂O₂ in 1.0 M HClO₄/LiClO₄ was found to be first-order in both reactants and the [H⁺] dependence of the second-order rate constant is given by k_2obs = b/[H⁺], b at 25 °C is 26.4 ± 0.5 s⁻¹. The rate law shows a simple inverse dependence on [H⁺] that is consistent with a rapidly maintained equilibrium between and its hydrolyzed form Co(H₂O)₅(OH)²⁺, followed by the rate controlling step, i.e. oxidation of H₂O₂ by Co(H₂O)₅(OH)²⁺.     

Kinetics of the decomposition of hydrogen peroxide in the presence of ethylenediaminetetraacetatocerium(IV) complex

The rate of reaction of [Ce(EDTA)(OH)_nⁿ⁻] with H₂O₂ in 0.10 M KNO₃ solution was investigated at various temperatures. The presence of a peroxy intermediate is inferred from spectrophotometric measurements. The general rate equation,

is valid for pH 7-9 with n= 1 and 2 complexes involved. The rate constants k_l and k₂ were determined at 25 °C to be 0.054 and 0.171 M⁻¹ s⁻¹ respectively. The corresponding activation enthalpies, as calculated from Arrhenius plots, were δH₁^#= 51.3 ± 14.8 and δH₂^#= 41.8 ± 5.3 kJ m⁻¹ and the activation entropies were δS₁^#=-97 ± 47 and ΔS₂^#=−119±17 J K⁻¹ m⁻¹.