Ferrous ion catalysis

Catalytic Decomposition of Hydrogen Peroxide by Ferrous Ions Catalysis by Transition Metal Ions and Complexes in Liquid-Phase Oxidation of Hydrocarbons and Aldehydes by Dioxygen... 

The accelerating effect of cupric ions on the ferric ion catalysis which was observed by Bohnson and Robertson is considered by Barb el al. to be due to reaction (5 ), as was the analogous effect of cupric salts on the ferrous ion catalysis. For conditions in which Scheme A applies reaction (4 ) is in effect catalyzed by (5 ). At high cupric ion concentrations (5 ) will eliminate (3), since effectively all the radical HO2 will react in (5 ). In these conditions the enhancement reaches a limit as was observed by Bohnson and Robertson. However (1) now becomes the operative chain-terminating step, and hence the kinetics of Scheme B should apply (Eq. g). Unfortunately no data on the peroxide dependence is available, but analysis of the data (43,66) shows the rate to be proportional to [Fe+++]w as required by (g). There is the same discrepancy in hydrogen ion dependence as was found in the simple ferric reaction. 

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalytically. Many ionic effects are known. The hydronium ion H3O and the hydroxyl ion OH catalyze hydrolyses such as those of esters ferrous ion catalyzes the decomposition of hydrogen peroxide decomposition of nitramide is catalyzed by acetate ion. Other instances are inversion of sucrose by HCl, halogenation of acetone by H and OH , hydration of isobutene by acids, hydrolysis of esters by acids, and others. 

The reaction is approximately first-order with respect to each reactant (the second-order rate coefficient increases with increase of substrate concentration), and catalysis by hydroxide ions is observed. Henderson and Winkler studied the ferrous ion-catalysed oxidation of thioglycolicacidto dithioglycolic acid. The rate is sensitive to traces of metal ions, and reproducible results could not be obtained in the absence of the catalyst. The oxidation is first-order with respect to both peroxodisulphate and ferrous ions, and zero-order with respect to the substrate. The second-order rate coefficient is approximately equal to that determined in the absence of the substrate, so Henderson and Winkler suggested that the ratedetermining step is the oxidation of ferrous to ferric ions, as in reaction (96), and that this is followed by reaction (97) and then rapid oxidation of thioglycolic acid by ferric ions. 

This catalysis seems to be related to that of ferrous ion which has been more widely investigated (see below). 

Polarographic catalytic currents" are well known, especially in inorganic electrochemistry, with classic examples, such as the reoxidation of ferrous ions—fonned cath-odically from ferric ions—by hydrogen peroxide or hydroxylamine. However, it appears obvious that the term catalysis is used too often quite ambiguously. Therefore it use in organic electrochemistry needs to be clarified a distinction should be made between the two main kinds of catalysis. This differentiation has been emphasized by Andrieux and Saveant [1,2]. 

Bacterial oxidation of ferrous ion in Acidithiobacillus ferrooxidans occurs by the catalysis of a system consisted of several enzymes and proteins. In the oxidation of ferrous ion by A. ferrooxidans Fel (JCM 7811), electrons are first pulled out of the ion by the catalysis of Fe(II)-cytochrome c oxidoreductase. Then, electrons are transferred to ferricytochrome c-552 (native cytochrome c of the bacterium), fer-rocytochrome c-552 formed is oxidized with oxygen by the catalysis of cytochrome c oxidase (Yamanaka and Fukumori, 1995). However, the mechanism of the oxidation of ferrous ion appears to be a little different between the strains of A. ferrooxidans. Thus, in certain strains of the bacterium the oxidation of ferrous ion is catalyzed by Fe(II)-rusticyanin oxidoreductase (Blake and Shute, 1994). However, as will be pointed out below, the enzyme should be carefully checked. Moreover, in a moderately thermophilic iron-oxidizing bacterium, the oxidation of ferrous ion is reported to be catalyzed by an iron oxidase containing heme A (Takai et al., 1999, 2001). 

The Fe(II)-cytochrome c oxidoreductase catalyzes the reduction of cytochrome c-552 with ferrous ion but does not catalyze the direct reduction of rusticyanin. This agrees with the finding of Hazra et al. (1992) that rusticyanin is reduced via cytochrome c. Blake and Shuts (1994) have claimed that the oxidation of ferrous ion occurs by the catalysis of Fe(II)-rusticyanin oxidoreductase. However, as their enzyme preparation probably contained cytochrome c as a contaminant, its catalysis could be the reduction of rusticyanin by Fe(II)-cytochrome c oxidoreductase mediated by cytochrome c. It has been reported that rusticyanin is not present in Leptospirillum ferrooxidans (Blake et al., 1993). This may mean that rusticyanin is not necessarily required for the oxidation of ferrous ion by the iron-oxidizing bacteria. 

Although the reduced forms of cytochromes c-552(m) (22.3 kDa) (or cytochrome c4) and c-550(m) (51 kDa) are also oxidized with molecular oxygen by the catalysis of cytochrome c oxidase, it has not yet been verified whether these cytochromes are reduced with ferrous ion by the catalysis of Fe(II)-cytochrome c oxidoreductase. On the basis of the studies of DNA that encodes the redox proteins of A. ferrooxidans, it is suggested that cytochrome c4 is the direct electron donor for cytochrome c oxidase in vivo (Appia-Ayme et al., 1999). However, as already mentioned, the reactivity with cytochrome c4 of cytochrome c oxidase is much lower than that of cytochrome c-552(s) (14 kDa) and is depressed with sulfate, while the enzymatic reaction of cytochrome c-552(s) (14 kDa) is stimulated by the salt. So cytochrome c-552(s) (14 kDa) seems to function as the real electron donor for the oxidase, if cytochrome c oxidase catalyzes the reduction of molecular oxygen at the outside of the plasma membrane as already mentioned (see also Fig. 5.2). 

A considerable body of results accumulated during earlier decades from activity studies of hCp now awaits a more meaningful analysis using the available 3D structure. Catalysis of amine oxidation by hCp, in particular biogenic ones present in plasma, cerebral, spinal, and intestinal fluids as well as of ferrous ions, which is probably physiologically relevant, has been studied extensively (68, 71). The mechanism of dioxygen reduction by hCp at the trinuclear center is of particular interest, as the presence of three distinct Tl sites raises the question of which centers are involved in internal ET to the single O2 reduction site. This mechanistic question prompted us to initiate ET studies by PR. 

Thus in the absence of any catalysis, ferrous ion is oxidized according to reaction I above and n = 0.5. The evolution of oxygen indicates the occurrence of reaction II and leads to values of n greater than 0.5. 

The above compensating reactions are attractive because of the success of similar schemes in the halide catalysis, but proof in this case is more difficult. Thus it was possible to show in the halide systems that halogen and halide are present simultaneously. Evidence for the presence of ferrous ion in the ferric catalysis would support a similar interpretation. Manchot and Lehmann (44) claimed to have proved that ferrous ion is formed from ferric ion in the presence of peroxide since the addition of <, < -dipyridyl to the mixture resulted in the slow formation of the red ferrous tris-dipyridyl ion Fe(Dipy)3++. However, later work (65,66), which will be discussed when these systems are considered in more detail (IV,6), indicates that the ferrous complex ion may be formed by reduction not of the ferric ion, but of a ferric dipyridyl complex. Similar conclusions on the presence of ferrous ion were drawn by Simon and Haufe (67) from the observation that on addition of ferri-cyanide to the system Prussian blue is formed. This again is ambiguous, since peroxide is known to reduce ferricyanide to ferrocyanide and the latter with ferric ion will of course give Prussian blue (53). 

The ferric ion catalysis has been considered by Haber and Weiss (4) in terms of the reactions of radicals along the lines adopted to explain the ferrous ion reaction. They proposed the following mechanism ... 

Barb et al. have also considered the ferric ion catalysis kinetics at low values of R2 where, as stated above, deviations from, the von Bertalan equation (c) occur. They conclude that with decreasing R2 reactions (3) and (1) will become of comparable importance as chain-terminating reactions, since peroxide will no longer be of such a concentration as to eliminate the ferrous reaction in the competition for the hydroxyl radical. At... 

The catalytic activity of these systems was interpreted by Haber and Weiss (4) in terms of their reaction mechanism for the ferric ion catalysis. They consider that the reaction of ferrous ion with the base to form the complex ion lowers the stationary concentration of ferrous ion and hence leads to longer reaction chains by decreasing the rate of the termination reaction... 

The experimental data of Lamb and Elder (96), however, are not in agreement with this predicted rate expression for they find the initial rate proportional to the square of the ferrous ion concentration and directly proportional to the oxygen pressure. This has recently been confirmed by the author (97), and it would appear that either the autoxidation is subject to a true catalysis by trace impurities (an induced reaction is excluded by the total ferrous ion oxidized being large, about M/20) or the actual mechanism is different from that suggested by Weiss. 

The ability of adenine nucleotides to aid catalysis of the hydrogen peroxide-dependent formation of hydroxyl radicals by ferrous ions has been investigated. 

Aldoses and alditols can be rapidly oxidized by hydrogen peroxide under certain conditions leading to the formation of formic acid with evolution of oxygen, and a mechanism was proposed involving catalysis by superoxide radicals generated from traces of transition metal salts in Fenton s reaction ferrous ion is considered to play an analogous catalytic role. ... 

Ferrous ions transferred into the blood are oxidised to ferric ions and then enter into the molecule of a transport protein under the catalysis of ferro oxidase ceruloplasmin, the main plasma metaUoprotein containing copper. Transferrin ensures the transfer of iron to all tissues. Under normal conditions, about 30% of plasma transferrin is saturated with iron, and the remainder is apotransferrin. The target tissue captures transferrin by specific receptors and iron is immediately available for the synthesis of proteins and other haem metalloproteins or temporarily stored in ferritin. A substantial portion of the transported iron from transferrin is taken away in the bone marrow for the production of erythrocytes. New erythrocytes absorb the whole transferrin molecules. The release of iron from transferrin occurs due to a lower pH compared with the pH in the extracellular space. Apotransferrin is then released from the erythrocytes and iron is built into the... 

---