Iron complexes self-exchange reactions

The overall rate of an electron transfer reaction is controlled by the rearrangement of the solvent molecules in the precursor complex. This can be appreciated by considering the self-exchange of an electron from a donor ferrous iron ion to an acceptor ferric iron ion. The ions in self-exchange reactions like this one can be distinguished using radioactive tracers (Silverman and Dodson, 1952). 

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. 

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). 

To compare the obtained value we first note that i 1 is the same for all three reactions because the reducing agent is the same and that we approximated yj 2 to be 1. Thus, the differences lie in the reduction potential differences (essentially Kn with all reactions being one electron processes) and 22 values. Although the iron aqua complexes have a slower self-exchange rate than Ru hexamine ones, the difference (and hence larger K 2 value) for... 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

The electron self-exchange in iron porphyrins follows a pattern close to the one of iron-tiisphenanthroline or bipyiidine complexes. The self-exchange in cytochrome c has a higher AG in part due to the nature of the Fe-S bond in the transition state, but its distance dependence is similar to that observed in other proteins. The free-energy relationships observed in porphyrin-cytochrome c systems depend on the nature of the reactant electronic state and, relative to the reactants, may exhibit a more relaxed transition state than the reactions that occur in soludoa... 

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