Reduction reaction kinetics iron porphyrins

Though unconventional reversible 0—0 is indicated by experiments which demonstrated that the isotope composition of the unreacted H202 was dramatically altered in 180-enriched water. The proposed mechanism has implications for the interpretation of the kinetic parameters for the enzymatic reaction,79 suggesting that kcat as well as /ccat/KM(H202) is determined by an irreversible step after 0—0 heterolysis. One possibility is the reduction of the iron(IV) oxo porphyrin + by the cosubstrate 2-methoxyphenol, as shown in Figure 9.13. 

On this ground, one reason for the increased activity observed when the iron porphyrins are subsequently anchored to the surface (through the NHj function), is the recovery of Oj reduction activity. This purely kinetic cause does not explain the formation of hydroi lated species in the TiOj-sil-porphyrin/hydrocarbon systems. Clearly, either intermediate species have a different reactivity or entirely different intermediates are formed. We envisage two possible reaction pathways me reaction of hydrocarbon radical species with Oj or with the superoxide coordinated to the iron-porphyrin. 

The temperature dependence of the catalyst activity of an iron fluoro-porphyrin-coated graphite electrode was studied by RDE coupled with the surface cyclic voltammetry. The purpose was to investigate the surface adsorption and reaction, O2 reduction catalysis kinetics, and especially the temperature effect on the catalyst activity. Figure 7.11(A) shows the surface CVs of 5,10,15,20-Tetrakis(pentafluorophenyl)-21H,23H-porphine iron (III) chloride (abbreviated as Fe TPFPP)-coated graphite electrode, recorded in a pH 1.0 Ar-saturated solution at different potential scan rates. The 1-electron reversible redox peak of approximately 0.35 V can be seen, which has a peak current increased linearly with increasing the potential scan rate, indicating the electrochemical behavior of this peak follows the feature of a reversible redox reaction of an adsorbed species on the electrode surface. 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with adsorbed iron or cobalt phthalocyanines or porphyrins and when certain metals are under potential deposited (upd). 

Benzaldehyde and isobutyraldehyde have been used as co-reductants in aerobic BV oxidation of cyclohexanone, catalysed by iron(III) porphyrins. The dramatic difference in the yield of e-caprolactone (96% for benzaldehyde and 11% for isobutyraldehyde) has been investigated kinetically, leading to elucidation of a mechanistic difference. The reaction with benzaldehyde involves a high-valent iron porphyrin, whereas the isobutyraldehyde version proceeds via peroxy isobutyric acid. 

The first step of the cycle [Sch. 7, (1)] involves the removal of two electrons from the protein, one coming from the metal ion and the other usually coming from the porphyrin moiety, which contains the iron [257]. The intermediate thus generated is known as Compound I. This species that contains two oxidizing equivalents of the protein can oxidize two substrate molecules. The first step, which is described in Eq. (2) of Sch. 7, involves formation of Compound II via reduction of the porphyrin jr-cation radical, while the second step leads to the resting state of the protein via a reaction between Compound II and a second substrate molecule [Sch. 7, (3)]. Both Compounds I and II are sufficiently stable so that several spectral methods have been used to determine the electronic structure of these species [258-262]. Kinetic studies for formation of these compounds have also been carried out [263-269]. The rate-limiting step under steady state conditions is most often the reduction of Compound II [266-271], a result that is accounted for by an easier reduction of the porphyrin radical as compared with the Fe(IV) center in Compound II. Traditional substrates for peroxidases are phenols and other aromatic dyes, although in cytochrome c peroxidase the substrate is ferro-cytochrome c [272, 273]. 

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