Iron complex, electron-transfer reaction

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

The three rate constants for Eq. (98) correspond to the acid-catalyzed, the acid-independent and the hydrolytic paths of the dimer-monomer equilibrium, respectively, and were evaluated independently (107). The results clearly demonstrate that the complexity of the kinetic processes is due to the interplay of the hydrolytic and the complex-formation steps and is not a consequence of electron transfer reactions. In fact, the first-order decomposition of the FeS03 complex is the only redox step which contributes to the overall kinetic profiles, because subsequent reactions with the sulfite ion radical and other intermediates are considerably faster. The presence of dioxygen did not affect the kinetic traces when a large excess of the metal ion is present, confirming that either the formation of the SO5 radical (Eq. (91)) is suppressed by reaction (101), or the reactions of Fe(II) with SO and HSO5 are preferred over those of HSO3 as was predicted by Warneck and Ziajka (86). Recently, first-order formation of iron(II) was confirmed in this system (108), which supports the first possibility cited, though the other alternative can also be feasible under certain circumstances. 

A values have been obtained for oxidation of benzenediols by [Fe(bipy)(CN)4], including the effect of pH, i.e., of protonation of the iron(III) complex, and the kinetics of [Fe(phen)(CN)4] oxidation of catechol and of 4-butylcatechol reported. Redox potentials of [Fe(bipy)2(CFQ7] and of [Fe(bipy)(CN)4] are available. The self-exchange rate constant for [Fe(phen)2(CN)2] has been estimated from kinetic data for electron transfer reactions involving, inter alios, catechol and hydroquinone as 2.8 2.5 x 10 dm moF s (in dimethyl sulfoxide). 

Fe +aq reacts with chloranilic acid to give iron(II) chloranilate. " Fe " aq reacts with promazine, (204), to give the promazine radical cation complex of Fe. The volume profile for this combined substitution and electron transfer reaction has been established. The activation volumes for the forward and reverse reactions are —6.2cm mol and — 12.5cm mol the respective activa-... 

A condition where metal ions within a coordination complex or cluster are present in more than one oxidation state. In such systems, there is often complete delocalization of the valence electrons over the entire complex or cluster, and this is thought to facilitate electron-transfer reactions. Mixed valency has been observed in iron-sulfur proteins. Other terms for this behavior include mixed oxidation state and nonintegral oxidation state. 

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

Rajagukguk S, Yang S, Yu C-A, Yu L, Durham B, Millett F. Effect of mutations in the cytochrome b of loop on the electron-transfer reactions of the Rieske iron-sulfur protein in the cytochrome bcl complex. Biochemistry 2007 46 1791-8. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

The first observation was reported in the electron transfer reaction between spinach plastocyanin with optically active iron(II) complexes, [Fe(S,S)-alamp] (A-form) and its (R,R)-isomer (A-form), where alamp is 2,6-bis[3-(S)- or 3-(R)-carboxy-2-azabutylpyridine (see Scheme 25) [56]. 

Spinach ferredoxine was applied to the stereoselective electron transfer reaction with optically active cobalt(III) complexes that are similar to the iron complexes that were applied to the stereoselective electron transfer reaction of... 

There has been considerable interest in the chemistry and electronic structures of cobalt and iron complexes of a s-l,2-disub-stituted ethene-1,2-dithiol1 2 and their Lewis base adducts.3-5 The complexes, and their Lewis base adducts which may contain pyridine, phosphines, NO, dipyridyl (2,2/-bipyridine), etc., are capable of undergoing reversible one-electron transfer reactions, thereby generating a series of complex ions differing from each other by only one unit of electric charge. 

Tian, H., White, S., Yu, L., and Yu, C. A., 1999, Evidence for the head domain movement of the rieske iron-sulfur protein in electron transfer reaction of the cytochrome bcl complex, J. Biol. Chem. 274 7146n7152. 

Ferredoxins and Rieske proteins employ a (Fe )2/Fe Fe redox couple for biological electron transfer reactions. Within the protein, the two iron atoms are rendered inequivalent, even in the hilly oxidized (Fe )2 state, by the surrounding protein environment Within a synthetic cluster, however, both iron atoms are typically equivalent, as may be expected from the symmetry of the overall complex. Table 4 shows reduction potentials for selected analog clusters. 

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