Iron catalyzed autoxidation

In a mechanism for the iron catalyzed autoxidation of sulfite, this would be a chain branching step. We have some evidence that the reaction of Fe2+ with HSO, in acid, does lead to free radicals, but it is somewhat more complex than written above (24). 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Fig. 4. Absorbance-time traces for the iron(III) catalyzed autoxidation of sulfur(IV) oxides (a) [02] =0m (b) [02] = 7.5xlO 4M. Experimental conditions [Fe(III)] = 5.0 x 1(T6 M [S(IV)] = 5.0 x 1(T3 M ionic strength = 0.5 M T= 25 °C pH = 2.5 A = 390 nm absorbance scale is in V (10 V = 1 absorbance unit). Reprinted with permission from Brandt, C. Fabian, I. van Eldik, R. Inorg. Chem. 1994, 33, 687. Copyright (2002) American Chemical Society.

Recent studies demonstrated that the composition of the reaction mixture, and in particular the pH have significant effects on the kinetics of iron(III)-catalyzed autoxidation of sulfur(IV) oxides. When the reaction was triggered at pH 6.1, the typical pH profile as a function of time exhibited a distinct induction period after which the pH sharply decreased (98).The S-shaped kinetic traces were interpreted by assuming that the buffer capacity of the HSO3 / SO3- system efficiently reduces the acidifying effect of the oxidation process. The activity of the... 

The work of Fallab and his collaborators has shown how the coordination act may bring the reactants together in autoxidation reactions. In several instances coordination furnishes a catalytic path for these reactions. Specific examples include the autoxidation of Fe+2 in the presence of sulfosalicylic acid (28), the autoxidation of 1-hydrazinophthalazine by iron (II) (27, 83), and the autoxidation of a formazyl-zinc complex (11). It is probable that the importance of this kind of a mechanism will be more widely realized as more and more detailed kinetic studies are made on metal-catalyzed autoxidation reactions. Some other... 

The effect of the neutralization reaction on the rate of metal-catalyzed degradation can be assessed in isolation from its effect on the acidic hydrolysis component by a comparison of the relative stability values before and after the sodium bicarbonate treatment. In the iron-catalyzed system, the relative stability values observed after the bicarbonate treatment are appreciably higher, especially at higher metal concentrations. The catalytic effect of iron for the autoxidative process may be reduced. However, the iron species adsorbed from the sulfate solution may itself be acidic. In such a case, neutralization of this acidic moiety would reduce the rate of acidic hydrolysis. Some metal salts, such as alum, are known to increase the acidic content of paper and accelerate its hydrolytic degradation (36). 

Edetic acid and edetates are primarily used as antioxidant synergists, sequestering trace amounts of metal ions, particularly copper, iron, and manganese, that might otherwise catalyze autoxidation reactions. Edetic acid and edetates may be used alone or in combination with true antioxidants the usual concentration employed being in the range 0.005-0.1% w/v. Edetates have been used to stabilize ascorbic acid ... 

From 2, it was concluded that the ferryl complex is the catalytically active species. Observation 1 suggested that 80% of the epoxide product in the aerobic reaction is derived from a carbon-based radical, which is quenched by O2 (autoxidation), and this is known to produce epoxide in reactions with cyclooc-tene (325). Methanol (observation 3) is known to quench radicals. The fact that the diols formed are a mixture of cis and trans products (observation 1 this is very unusual in iron-catalyzed olefin oxidations) suggested that the diol results from the capture of OH radicals by the putative carbon-based radical. 

The enhanced chemiluminescence associated with the autoxidation of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence of trace amounts of iron(II) is being used extensively for selective determination of Fe(II) under natural conditions (149-152). The specificity of the reaction is that iron(II) induces chemiluminescence with 02, but not with H202, which was utilized as an oxidizing agent in the determination of other trace metals. The oxidation of luminol by 02 is often referred to as an iron(II)-catalyzed process but it is not a catalytic reaction in reality because iron(II) is not involved in a redox cycle, rather it is oxidized to iron(III). In other words, the lower oxidation state metal ion should be regarded as a co-substrate in this system. Nevertheless, the reaction deserves attention because it is one of the few cases where a metal ion significantly affects the autoxidation kinetics of a substrate without actually forming a complex with it. 

Transition-metal-ion-free solutions of ascorbate autoxidize (i.e., react with 02) only slowly (e.g., carefully demetalized with a chelating resin such as Chelex 100 1.25 x 10-4mol dm"3 solutions lose only 0.05% ascorbate/15min (Buettner 1988 for a review giving valuable information how to deal with ascorbate solutions, see Buettner and Jurkiewicz 1995). The stability of ascorbate solutions is dramatically reduced in the presence of EDTA which apparently catalyzes the degradation by chelating adventitious iron ions (Buettner and Jurkiewicz 1995). 

Results demonstrate that Mb er se is not the catalyst of lipid oxidation in cooked meat. However, cooking destroys part of the Mb, releasing Fe2+ which then catalyzes the development of WOF. Although the role of grinding in development of WOF was not studied, it seems likely that it also releases Fe2+. It has recently been shown that solubilization of iron from grinding equipment can increase the free iron content of fish meal, which could also be a factor in autoxidation of fresh ground meat (TS). 

Anionic iron bis(dithiolene) complexes have been reported to catalyze the autoxidation of phosphine, arsine, and cumene (92). No evidence for formation of molecular oxygen complexes has been found. The catalytic activity has been proposed to be a consequence of the redox activity of the complex. On this... 

Rao (1991) showed that hematin catalyzed the autoxidation of hydroquinone or 1,2,4-benzenetriol in vitro, producing reduced oxygen species that may be responsible for protein or DNA binding after benzene exposure. Further work along this line of research has provided evidence that in vitro, chelates of iron and hydroquinone or 1,2,4-benzenetriol are potent DNA cleaving agents (Rao 1996 Singh et al. 1994), and that 1,2,4-benzenetriol, but not hydroquinone, causes the release of iron from ferritin (Ahmad et al. 1995). 

Termination of the autoxidation process occurs as peroxyl radicals couple to produce nonradical products. Additional sources of free radicals to initiate the free radical chain process include ultraviolet (UV) light and heavy metals (copper, iron, cobalt, manganese, and nickel) which catalyze oxidation by shortening the induction period and promoting free radical formation. 

One aspect which sets oxidation apart from other reactions, e.g. hydrogenation and carbonylation is the fact that there is almost always a reaction (free radical chain autoxidation) in the absence of the catalyst (Reactions 1-3). Moreover, (transition) metal ions which readily imdergo a reversible one-electron valence change, e.g. manganese, cobalt, iron, chromium, and copper, catalyze this process by generating alkoxy and alkylperoxy radicals from RO2H (Reactions 4-6). 

As to the mechanistic pathway of aerobic oxidations of this type Httle evidence of any details has emerged as yet [ 18]. However, it seems quite reasonable to assume the intermediacy of peracids being formed by autoxidation of the aldehydes [ 19,20]. Metals can be involved in various stages of oxidation processes like the described nickel(II)- or iron(III)-catalyzed reactions [21] acyl radicals may be produced by metals from aldehydes which then participate in the autoxidation of the aldehydes. Metals can direct the oxygen insertion itself, too, or promote other catalytic pathways as well as even inhibit catalytic turnover. 

Ions of transition metals (homogeneously or in some cases supported on polymers [5]) also effectively catalyze the autoxidation. Salts of cobalt, manganese, iron, copper, chromium, lead, and nickel are used as catalysts that allow the reactions to be carried out at lower temperatures, therefore increasing the selectivity of the oxidation (see, for example, [6]). However, it is more important that the catalyst itself may regulate the selectivity of the process, leading to the formation of a particular product. The studies of the mechanism of the transition metal salt involvement have shown their role to consist, in most cases, of enhancing the formation of free radicals in the interaction with the initial and intermediate species. 

The basic chemistry of enzyme catalyzed oxidation of food lipids such as in cereal products, or in many fruits, and vegetables is the same as for autoxidation, but the enzyme lipoxygenase (LPX) is very specific for the substrate and for the method of oxidation." Lipoxygenases are globulins with molecular weights ranging from 0.6-1 x 10 Da, containing one iron atom per molecule at the active site. 

There already exist examples of significant chemical reactions which demonstrate the unusual reaction characteristic resulting from the presence of two metal centers.Here we examine two, neither of which involves phosphine bridging ligands. The autoxidation of triphenylphosphine catalyzed by four-coordinate iron(II) porphyrins in a noncoordinating solvent like toluene occurs by the mechanism shown in Figure Two... 

To test the dominance of electrostatic effects in the mineralization model, a mutant of CCMV was constructed (subE) in which all the basic residues on the N-terminus of the coat protein were substituted for glutamic acid (E), thus dramatically altering the electrostatic character of the interior of the assembled protein cage." This mutant was able to catalyze the oxidative hydrolysis of Fe(II) to form an iron oxide nanoparticle encapsulated within the protein cage of the modified virus. High-rcsolution spectral imaging allowed the elemental composition of a protein-mineral composite material to be resolved (1 nm spatial resolution, Fig. 3). This clearly showed that the mineral nanoparticle was completely encapsulated within the protein cage structure. This mutant is able to bind Fe(lT), facilitate its autoxidation... 

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