Kinetic data for redox reactions of cytochrome c with Fe(CN)5X complexes and the question of association prior to electron transfer

Mechanistic studies on the oxidation of ascorbic acid and hydroquinone by a {Mn4O6}4+ core in aqueous media

Described in this work is the kinetics of oxidation of ascorbic acid and hydroquinone by a tetranuclear Mn(IV) oxidant, [Mn(4)(μ-O)(6)(bipy)(6)](4+) (1(4+), bipy =2,2(/)-bipyridine), in aqueous solution over a wide pH range 1.5-6.0. In particular, below pH 3.0, protonation on the oxo-bridge of 1(4+) results in the formation of [Mn(4)(μ-O)(5)(μ-OH)(bipy)(6)](5+) (1H(5+)) as an additional oxidant over 1(4+). Both ascorbic acid and ascorbate whereas only hydroquinone and none of its protolytic species were found to be reactive reducing agents in these reactions. Analysis of the rate data clearly established that the oxo-bridge protonated oxidant 1H(5+) is kinetically far more superior to 1(4+) in oxidizing ascorbic acid and hydroquinone. Rates of these reactions are substantially lowered in D(2)O-enriched media in comparison to that in H(2)O media. An initial one electron one proton transfer electroprotic rate step could be mechanistically conceived. DFT studies established that among the two sets of terminal and central Mn(IV) atoms in the tetranuclear oxidant, one of the two terminal Mn(IV) is reduced to Mn(III) at the rate step that we can intuitively predict considering the probable positive charge distribution on the Mn(IV) atoms.

Comparative Kinetic Analysis of Reversible Intermolecular Electron-Transfer Reactions between a Series of Pentaammineruthenium Complexes and Cytochrome c

In this kinetic and thermodynamic study, the reversible outer-sphere electron-transfer reactions between a series of Ru(NH(3))(5)L(3+/2+) complexes (L = etpy, py, lut) (etpy = 4-ethylpyridine; py = pyridine; lut = 3,5-lutidine) and cytochrome c were investigated as a function of ionic strength, buffer, pH, temperature, and pressure. Due to the low driving forces of these systems, it was possible to study all the reactions in both redox directions. The observed rate constants for various L are correlated on the basis of the ability of ligands on the ruthenium complex to penetrate the heme groove on cytochrome c. The measurements as a function of pressure enabled the construction of volume profiles for all investigated systems. The activation volumes for all of these processes are very similar: between -14.9 and -17.8 cm(3) mol(-)(1) for the reduction and between +14.7 and +17.8 cm(3) mol(-)(1) for the oxidation of the protein by Ru(NH(3))(5)L(2+/)(3+), respectively. The overall reaction volume varies between 27 and 35 cm(3) mol(-)(1), from which it follows that the transition state lies exactly halfway between reactant and product states on a volume basis in all cases. There is good agreement throughout between kinetic and thermodynamic data.

Electron transfer between cytochrome c and metal hexacyanide complexes. Effect of thermodynamic driving force on the electron transfer rate

The electron transfer reactions between ferrocytochrome c and three isomorphic hexacyanide complexes, [Fe(CN)6]3-, [Os(CN)6]3- and [Ru(CN)6]3-, have been studied using the method of photoexcitation. The transfer rates for [Os(CN)6]3- and [Ru(CN)6]3- are, respectively, about 45- and 200-times higher than that of [Fe(CN)6]3-. A reorganization energy of approx. 0.8 eV was found for the cytochrome c-hexacyanide system when the data were analyzed according to the theory of Marcus.

Kinetics of electron transfer between cytochrome c and iron hexacyanides. Evidence for two electron-transfer sites

The electron-transfer reaction between ferrocytochrome c and ferricyanide has been studied by the method of photoexcitation. The observed transfer rate shows saturation behaviour at high ferricyanide concentration. Data analysis indicates that there are two binding sites of vastly different affinities at which electron transfer occurs. The binding constant for the strong binding site decreases from 1600 M-1 to 80 M-1 as the ionic strength increases from 15 mM to 140 mM. At 20 degrees C, the intramolecular electron-transfer rate for this site is 4.65 X 10(4) s-1, which gives an electron-transfer distance of approx. 9.7 A according to Hopfield's model.

Kinetics of electron transfer between mitochondrial cytochrome c and iron hexacyanides

The reduction of horse and Candida krusei cytochromes c by ferrocyanide has been studied by 1H NMR spectroscopy and the reaction found to involve a precursor complex of ferrocyanide bound to ferricytochrome c (pH* 7.4, 2H2O, I = 0.12, and 25 degrees C). The electron transfer rate constants for the reduction of the two ferricytochromes by associated ferrocyanide were found to be the same at 780 +/- 80 sec-1 but the association constants for binding of ferrocyanide to ferricytochrome c were significantly different: horse, 90 +/- 20 M-1 and Candida, 285 +/- 30 M-1. The different association constants partly accounts for the previously observed reactivity difference between horse and Candida cytochromes c. Comparison of the NMR data with data obtained by other kinetic methods has allowed the electron transfer rate constant for the oxidation of ferrocytochrome c by associated ferricyanide to be determined. This was found to be 4.6 +/- 1 X 10(4) sec-1.

Association constants for metal hexacyanide binding to cytochrome c

The binding of [Co(CN)6]3-, and that of [Fe(CN)6]3- and [Ru(CN)6]4- using a competitive method, to horse cytochrome c has been studied by 59Co NMR spectroscopy. At I = 0.07 M, without added salt and in 2H2O at pH* 7.3 (measured in 2H2O) and 25 degrees C, there are at least two binding sites on ferricytochrome c and ferrocytochrome c for [Co(CN)6]3-. Association constants were determined to be 2.0 +/- 0.6 X 10(3) M-1 and 1.5 +/- 0.5 X 10(2) M-1, respectively, with no effect of the oxidation state of the cytochrome. At higher ionic strength (I = 0.12 M) adjusted with KCl the binding markedly decreased, and, although it was not possible to determine the precise binding stoichiometry and magnitude of association constants, it is clear that the association constants are less than or equal to 1.5 X 10(2) M-1. The binding of [Ru(CN)6]4- at I = 0.07, without added salt and in 2H2O at pH* 7.3 and 23 degrees C, was not precisely defined, but its binding strength relative to that of [Fe(CN)6]3- was determined. Extrapolating this to I = 0.12 (KCl) suggests that under these conditions the association constant for [Ru(CN)6]4- binding to ferricytochrome c is less than or equal to 3 X 10(2) M-1.

The effect of iron-hexacyanide binding on the determination of redox potentials of cytochromes and copper proteins

The midpoint redox potentials of Pseudomonas aeruginosa cytochrome c-551 and Rhodopseudomonas viridis cytochrome c2 were measured as a function of pH in the presence of Euglena cytochrome c-558 and the results compared with those obtained in the presence of ferro-ferricyanide. The pattern of pH dependence observed for the two bacterial cytochromes was the same whether it was measured by equilibrium with another redox protein or with the inorganic redox couple. Thus, the pH dependence of redox potential is not a consequence of pH-dependent ligand binding. The midpoint potential of Ps. aeruginosa azurin was measured as a function of pH using both ferro-ferricyanide mixtures and redox equilibrium with horse cytochrome c or Rhodopseudomonas capsulata cytochrome c2. In this case also the pattern of pH dependence obtained did not vary with the redox system used and it closely resembled that of Ps. aeruginosa cytochrome c-551. This is consistent with the observation that the equilibrium between cytochrome c-551 and azurin is relatively independent of pH. An equation was derived which described ph-dependent ligand binding and which can produce theoretical curves to fit the experimental pH dependence of redox potential for both cytochrome and azurin. However, the pronounced effect on such curves produced by varying the ligand association constants, and the insensitivity of the experimental data to changes in ionic strength, suggest that ligand binding effects do not account for the pH dependence of redox potential.

1H NMR studies of the electron exchange between cytochrome c and iron hexacyanides. Definition of the iron hexacyanide binding sites on cytochrome c

Binding of [Fe(CN)6]3-, [Cr(CN)6]3-, [Co(CN)6]3- and [Cr(C2O4)3]3- to horse, tuna and Candida krusei cytochromes c has been studied by high-resolution 1H NMR spectroscopy. All the reagents bind at the same sites. There are at least two binding sites, and probably three, on horse cytochrome c at pH 7. One of the sites is only a weak binding site and is far from the haem group, whereas the other site(s) is(are) at the haem crevice. Ka for binding of [Fe(CN)6]3- to trimethyllysine-72 of C. krusei ferricytochrome c is 140 +/- 15 M-1 at 27 degrees C and pH 7.