Outer-sphere Electron Transfer Mechanism Research Articles

Ionic strength effect on the ascorbate reduction of sperm whale and horse heart metmyoglobins

Kinetics of the reduction of sperm whale skeletal muscle and horse heart metmyoglobins (metMb) by ascorbic acid (H 2A) were studied in a nitrogen atmosphere at 25 °C, at different ionic strengths (NaCl) and between pH 7.29 and 8.29. The second order rate constants were determined for the reductions of metMb(H 2O) by ascorbate (HA − and A 2−). It was found that the ascorbate reduction is faster for horse heart metMb than for sperm whale metMb at a higher ionic strength, and that at a lower ionic strength similar reactivities are observed. The logarithm of the rate constants correlates to the square root of the ionic strength. This is explained by a simple outer-sphere electron transfer mechanism. It was found that the effective charge of sperm whale metMb is higher than that of horse heart metMb and that there is no specific interaction of ascorbate with metMb.

Light-induced electron transer and photoelectrocatalysis of chlorine evolution at FeS2 electrodes

Abstract Photo-induced electron transfer to FeS 2 ( E g = 0.9 eV) (which can be stabilized efficiently by iodide against corrosion at high photocurrent densities (1–2 A cm −2 )) was studied using rotating ring disc techniques. With decreasing efficiency I −1 , Br − and Cl − compete with water and OH − ions which form photocorrosion products (iron hydroxide, sulfate). I − is very effective in stabilizing n-FeS 2 against corrosion even at high photocurrent densities (100% at 1–2 A cm −2 ). The production efficiency for liberating chlorine (from a 3 M KCl solution at pH 0) is still 73%. Remarkably the electron transfer from Fe 2+ and Mn 2+ is more than one order of magnitude less efficient, suggesting that specific surface interaction between negative electron donors and iron d -states is involved. Evidently photo-induced electron transfer via a complex formation between surface transition metals and electron donating ligands has a clear advantage over the outer sphere electron transfer mechanism.

Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by oxalate. The catalytic role of alkali metal ions through outer-sphere bridging

The kinetics of reduction of dodecatungstocobaltate(III) by oxalate follow the rate laws kobs.= 2k1[HC2O4–] and kobs.= 2k2[C2O42–] at high and low hydrogen ion concentrations respectively. Both these apparent second-order rate constants (k1 and k2) depend on the concentration and type of cation present; k1 and k2 decrease in the order K+ > Na+∼ Li+ and K+ > Na+ > Li+ respectively. The K+ and Na+ assisted paths have been studied in detail. The rate constants for these catalysed paths along with those for uncatalysed ones have been evaluated. Thermodynamic parameters (ΔH‡ and ΔS‡) associated with k1 and k2 have been determined. A cation-bridged outer sphere electron transfer mechanism is most likely to take place, with simultaneous generation of free radicals (C2O4˙–).

Mechanism of electron transfer to coordinated dioxygen of oxyhemoglobins to yield peroxide and methemoglobin. Protein control of electron donation by aquopentacyanoferrate(II).

Reactions of human oxyhemoglobin A with iron(II) compounds have been investigated. Human oxyhemoglobin (HbO2) reacts with aquopentacyanoferrate(II), Fe(II)(CN)5H2O3-, to yield hydrogen peroxide, aquomethemoglobin and Fe(III)(CN)5H2O2-. The reaction follows a second order rate law, first order in the pentacyanide and in HbO2. Since reaction rates are lower in the presence of catalase, the H2O2 produced must promote metHb formation in reactions independent of pentacyanide. Changes in concentrations of effectors (e.g. H+, inositol hexaphosphate, Cl-, and Zn2+), alkylation of beta-93 cysteine with N-ethylmaleimide, and substitution at distal histidine (as in Hb Zurich with beta-63 His----Arg) in each case can markedly affect pentacyanide reaction rates demonstrating a fine control of rates by protein structure. Hexacyanoferrate(II) (ferrocyanide) reacts with HbO2 to produce cyano-metHb as well as aquo-metHb but the reaction with the hexacyanide is much slower than with the aquopentacyanide. Iron(II) EDTA converts HbO2 to deoxy-Hb with no evidence for formation of metHb as an intermediate. These findings support a mechanism in which the pentacyanide anion reacts directly with coordinated dioxygen. One-electron transfers to O2 from both pentacyanide iron(II) and heme iron(II) result in the formation of a mu-peroxo intermediate, HbFe(III)-O-O-Fe(III) (CN)5(3-). Hydrolysis of this intermediate yields metHb . H2O, H2O2, and FeIII(CN)5H2O2-. The reaction of HbO2 with Fe(CN)6(4-) must follow an outer sphere electron transfer mechanism. However, the very slow rate that is seen with Fe(CN)6(4-) could arise entirely from the pentacyanide produced from loss of one cyanide ligand from the hexacyanide. Fe(II)EDTA reacts rapidly with free O2 in solution but can not interact directly with the heme-bound O2 of HbAO2. The dynamic character of the O2 binding sites apparently permits access of the Fe2+ of the pentacyanide to coordinated dioxygen but the protein structure is not sufficiently flexible to allow the larger Fe2+EDTA molecule to react with bound O2. It is necessary for maintenance of the oxygen transport function of the red cell for reductants such as the methemoglobin reductase system, glutathione, and ascorbate to be able to reduce metHb to deoxy-Hb. It is also important for these reductants to be unable to donate an electron to HbO2 to yield H2O2 and metHb. Thus, a mechanistic requirement for the delivery of one-electron directly to the dioxygen ligand, if peroxide is to be produced, enables the protein to protect the oxygenated species from those electron donors normally present in the cell by denying these reductants steric access to coordinated O2.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics and mechanism of reduction of a nickel(IV) complex by tris(1,10-phenanthroline)cobalt(II); spectrophotometric detection of a nickel(III) intermediate

The kinetics of reduction of a nickel(IV) di(oxime–imine) complex, [NiIVL]2+(H2L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by [Co(phen)3]2+(phen = 1, 10-phenanthroline) are reported. The reaction is biphasic. An initial, rapid, pH-independent reaction between [NiIVL]2+ and [Co(phen)3]2+ has a second-order rate constant of 3.2 × 105 dm3 mol–1 s–1 at 25.0 °C and I= 0.10 mol dm–3(Na[NO3]) and leads to formation of a nickel(III) intermediate, [NiIIIL]+. Subsequent reductions of this intermediate and its protonated form [NiIIILH]2+ by [Co(phen)3]2+ with second-order rate constants 8.3 × 102 and 4.1 × 104 dm3 mol–1 s–1 respectively give the nickel(II) product. Both nickel(IV) and nickel(III) reactions proceed by outer-sphere electron transfer mechanisms. The nickel(III) intermediates are generated electrochemically and both forms are characterised. Data previously published for the ascorbate reduction of the nickel(IV) complex are reinterpreted.

Kinetics of the quenching of *RuL2+3 emission by monochloro and dichloro complexes of copper(II) in aqueous solution

The quenching of charge-transfer excited states of RuL2+3 complexes (L = 2,2′-bypyridine, 1,10-phenanthroline or related derivatives) by aqueous Cu2+ ions has been shown to proceed via an outer-sphere electron-transfer mechanism. However, the dependence of the rate constant on the free energy of reaction is much weaker than expected from a simple theoretical treatment for this series of reactions. In this paper we examine the effects of chloride ion catalysis on the rates of these reactions and deduce the rate constants for quenching by CuCl(2-n)+n, with n= 0–2. We find similar, weak free-energy dependences for the rate constants for all three species, although the efficiency of quenching increases with n. The rate constants obtained are discussed within the framework of Marcus' theory for adiabatic outer-sphere electron-transfer reactions.

Reduction of hexachloroiridate (IV) by benzene‐1,2‐diol in aqueous perchloric acid

AbstractThe kinetics of the reaction between benzene‐1,2‐diol(catechol) and hexachloroiridate (IV) have been measured in aqueous acidic perchlorate solutions by the stopped‐flow method. The reaction is second order overall, and first order in each reactant. A reverse reaction also occurs, but it is much slower than the forward process. Observed rate constants are dependent on acidity, but the variation can be attributed to activity rather than mechanistic effects. The reaction appears to proceed predominantly by an outer sphere electron transfer mechanism, yielding o‐benzoquinone and hexachloroiridate (III), although monoaquopentachloroiridate (III) is formed also at the higher [catechol]/[IrCl62−]ratios.

Preparation of ruthenium(II) and ruthenium(III) myoglobin and the reaction of dioxygen, and carbon monoxide, with ruthenium(II) myoglobin.

Ruthenium myoglobins have been prepared by the reconstitution of horse heart apomyoglobin with either ruthenium(II) or ruthenium(III) mesoporphyrin IX (MpIX) derivatives. The ruthenium(II) and -(III) myo globins (RuMb and RuMb+, respectively) contain one ruthenium porphyrin/heme binding site; the species are readily interconverted using dithionite for reduction and bromine for oxidation. RuMb binds carbon monoxide to give the known carbonyl complex. Reversible oxygenation occurs readily with protein-free RuII(MpIX) species in dimethylformamide, but RuMb in phosphate buffer is irreversibly oxidized by dioxygen to give RuMb+ via an outer sphere electron transfer mechanism.

Hydroperoxides and superoxides in microsomal oxidations

The following scheme summarizes our present knowledge on the mechanism of the reactions carried out by cytochrome P450: X δ+ can be a protein radical or a porphyrin radical cation. Donor can be: (i) reduced cytochrome b 5 or NADPH:cytochrome c reductase; (ii) tetramethylphenylenediamine, dimethylphenylenediamine, diaminobenzidine; (iii) dimethylaniline or aminopyrine and resulting in N-dealkylation (Griffin, 1977); (iv) ROOH resulting in ROO . production and thereby increasing cytochrome P450 destruction, lipid peroxidation, singlet oxygen formation and oxidation of peroxidase donors; (v) unsaturated lipids resulting in lipid peroxidation and singlet oxygen formation; and (vi) various antioxidants. S substrate can be amines, drugs, steroids, carcinogens, antioxidants R·CH 2OH alcohols (from mechanism by Chance and Schonbaum, 1976). Hydroperoxides (e.g. ethyl hydroperoxide) that form alcohol substrates on reduction would be expected to be less effective in catalyzing hydroxylation reactions (Nordblom et al., 1976). R·CHO Aldehyde product from alcohol oxidation ROOH Primary, secondary or tertiary hydroperoxides The scheme explains the following three pathways involved in the formation of the oxenoid species: 1. (1) Organic hydroperoxide catalyzed. This involves first the formation of an enzyme-peroxide complex (Reaction 1) followed by a fast rearrangement by an outer sphere electron transfer mechanism (Chance and Schonbaum, 1976). Some of this complex may be dissociated to the corresponding aldehyde and the original enzyme. The complex may also be hydrated to compound I, the active hydroxylating species, and release the alcohol. The hydroperoxide catalyzed alcohol oxidation (Rahimtula and O'Brien, 1976) can be explained by the reversal of these changes from compound I to the complex and dissociation of this complex (Reaction 4). The hydroperoxide catalyzed substrate hydroxylation involves the transfer of activated oxygen from compound I to the substrate (Reaction 5). It is also possible that alcohol oxidation proceeds by a hydroxylation mechanism, followed by the rearrangement of the ‘hydroxylated’ intermediate to an aldehyde. The hydroperoxide catalyzed oxidation of hydrogen or electron donors, unsaturated lipids or antioxidants involves the protein or porphyrin free radical of compound I and the ferryl iron of compound II. In competition with these donors, ROOH can also convert compound I to compound II (Chance, 1952; Reaction 6) and the resulting peroxy radicals can also oxidize these donors. In the absence of these donors, cytochrome P450 destruction readily occurs as a result of the peroxy radicals or protein or porphyrin radicals. 2. (2) H 2O 2. The relatively high concentration of H 2O 2 required compared with that needed with the alkyl hydroperoxides, and the alkaline pH dependence, suggest that the hydroperoxide ion HO 2 − forms a reversible enzyme peroxide complex (Nordblom et al., 1976; Reaction 2). Further protonation and subsequent loss of a molecule of water forms compound I. 3. (3) NADPH- reductase catalyzed. Binding of a P450 drug substrate greatly enhances the reduction of P450 (Guengerich et al., 1975, 1976) with formation of the ferrous protein (Peterson et al., 1977). Phospholipid enhances this reduction (Guengerich and Coon, 1975). Binding of oxygen is very rapid (Guengerich et al., 1976) and protonation generates the same tertiary peroxide complex formed with H 2O 2.

Ligand effects upon the reactions of Ni(II) with sodium tetrahydroborate: Ni(I) complexes of bipyridyl and 1,10-phenanthroline

The reactions between NaBH4 and Ni(II) have been studied in the presence of a variety of ligands in an effort to determine (a) the conditions under which reduction occurs, (b) the extent of the reduction (e.g. to Ni(I), Ni(0) complexes, or to Ni metal or boride), and (c) whether intermediate Ni complexes can be isolated.With ligands having no π-bonding capabilities (NH3, ethylenediamine, edta, citrate), reduction depends upon the Ni:ligand ratio and, in the presence of an excess of the ligand, reduction of Ni(II) is very slow. When vacant coordination sites exist on the metal through dissociation of, for example, Ni(NH3)62+, allowing for the interaction of the BH4 group with the metal, then rapid reduction to the metal (or boride) takes place.With the π-bonding N-donor ligands 1,10-phenanthroline (phen) and 2,2′-bipyridyl (bipy) reduction of the stable ML32+ complexes probably occurs via an outer-sphere electron transfer mechanism but, in these cases, new species of the type NiL2X (L = phen, bipy; X = BH4, PF6, BPh4), which formally contain Ni(I), have been isolated.

Electron transfer oxidations of organic compounds. Part IV. The kinetics of oxidations of some difunctional ketonic substrates by the hexachloroiridate(IV) anion, the oxidation of 2-chlorocyclohexanone by hexachloro- and hexabromo-iridate(IV), and a survey of the reactivity of hexachloroiridate(IV) towards organic compounds

The reactions between hexachlororidiate(IV) in dilute mineral acid and the substrates 2-chloro- and 2-hydroxycyclohexanone have been studied kinetically in acid solution and found to be dependent on the rate of enolisation of the substrate. Hexabromoiridate(IV) behaved similarly with the former compound. The substrates 2-oxocyclohexanone and acetylacetone, which have a high enol content, are however not found to react exceptionally rapidly, since their enols are relatively inert and the reactions proceed mainly by oxidation of the enol anions. These reactions all appear to proceed by outer-sphere electron-transfer mechanisms. Oxalic acid, pyruvic acid, and mandelic acid are only attacked slowly by hexachloroiridate(IV), while cyclohexanol, 2,3-dimethylbutane-2,3-diol, and acetaldehyde do not react detectably. The last compounds, though normally readily oxidised by one-equivalent oxidants with which they can complex, are not susceptible to electron-transfer oxidation by hexachloroiridate(IV).