Redox inactive

The enzymatic reactions of peroxidases and oxygenases involve a two-electron oxidation of iron(III) and the formation of highly reactive [Fe O] " species with a formal oxidation state of +V. Direct (spectroscopic) evidence of the formation of a genuine iron(V) compound is elusive because of the short life times of the reactive intermediates [173, 174]. These species have been safely inferred from enzymatic considerations as the active oxidants for several oxidation reactions catalyzed by nonheme iron centers with innocent, that is, redox-inactive, ligands [175]. This conclusion is different from those known for heme peroxidases and oxygenases... 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Zinc is also used in biological studies to gain information about non-zinc containing systems. It can be a convenient redox inactive replacement for the study of complex systems with multiple redox centers. For example, the mechanism of quenching the triplet state of zinc cytochrome c by iron(II) and iron(III) cytochrome c has been studied. Zinc insertion has been used to get around the difficulty of studying two heme proteins with the same absorption spectra and provides rate constants for iron and iron-free cytochrome c quenching.991... 

Amperometric sensors for redox-inactive cations and electroactive compounds... 

AMPEROMETRIC SENSORS FOR REDOX-INACTIVE CATIONS AND ELECTROACTIVE COMPOUNDS... 

Redox-inactive cations attract a particular interest for analytical chemists because of their importance in environmental control, industry, and medicine. For instance, in clinical diagnostics, tests for blood electrolytes (Na+, K+) are routine, because deviation of cation content from their normal values indicates a number of pathologies. 

Figure 8. The blocking effect (25 °C) of redox inactive complexes on the reaction of parsley plastocyanin PCu(I) + Co(phen)s3 Rate constants were determined at pH 7.5 (for U and m) and pH 5.8 (for A) [I = 0.10 M (NaCl)].

Figure 9. Behavior expected if there are two separate functional sites on the protein for electron transfer. One site (Site 1) is blocked directly by redox inactive 3 < 4 < 5 complexes, the other (Site 2) is not blocked.

The effect is of similar magnitude to that observed for blocking by the 3+ redox inactive Co (1 3)53+ ( 30% decrease). Use of Cr(III) modified protein has no effect on the reaction with Fe(CN)53- as oxidant. These observations (21) support the belief that positive and negative redox partners utilize different functional sites on the protein for electron transfer. 

Other Studies. Experiments in which rate constant pH profiles, blocking effects of redox inactive complexes as well as the effect of Cr(III) modification should now be possible enabling sites on plastocyanin used by cytochrome f and P700 to be specified (25,26). 

Similar approaches to those described herein are already underway with [2Fe-2S] ferredoxins. From NMR line-broadening studies Markley has demonstrated that redox inactive Cr(1 3)53+ preferentially associates at a specific site close to Tyr 83 (13,27). This particular group is a relatively long way from the X2Fe-2S] cluster and the result is surprising since conserved or partially conserved negative patches at 67-69 and 94-96 are nearer to the active site and might have been expected... 

Upon increasing the pH, first this species is present in increasing concentrations, but eventually the non-participating ligand is replaced with another cysteine molecule and the redox inactive Cu(RS)2 is formed. Consequently, the reaction rate decreases sharply The following reaction sequence was proposed for the redox reaction ... 

Conversely, the analogous rhodium complex [Rh6(CO)i5]2 is redox inactive. 

Fig. 8. Competitive inhibition of redox inactive complexes (I) on the [Co(phen)j] oxidation of parsley plastocyanin PCu(I) [98]. Second-order rate constants (25 °C), shown as relative values, were determined at pH 5.8 (Mes), 1=0.10 MfNaO) with [Pt(NH3)6] ( ), [(NH3)5CoNHj(NH3)5]s] [Co (III)4]a (A) and [CoflllUg ( ). Full formulae of the latter two complexes are as indicated above...

Table 7. Partitioning of electron transfer between adjacent (k ) and remote (ke) binding sites on spinach plastocyanin PCu(I) at 25°C, pH7.5, I=0.10M(NaCl), using redox inactive [(NH3)5CoNHjCo(NH3)5]5+ [100, 117]...

Very recently, a major revision of the chemical pathways operating in the photocatalytic conversion of organic substrates was undertaken. Tliese studies were carried out through very careful product analysis [58,59], by using selected substrates with similar chemical properties but different sorption behavior [60], or by surface modification through complexation by redox inactive species [61,62]. 

The dismutation reaction of a mixture of flavoquinone and flavohydroquinone in DMF was studied kinetically in the presence and absence of redox-inactive metal ions This system shows a very complex kinetic behaviour with the formation... 

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