Peroxidation reaction

In its native state, the iron-III species is coordinated equatorially by a heme unit and axially by a histidine residue and is therefore very similar to cytochrome P 450 [1311]. The first step in the reaction involves oxidation of the Fe to form an iron-oxo derivative called Compound I. The latter contains a Fe =0 moiety and 

A class of peroxidases specializes in the (per)oxidation of halides (d , Br , T but not FO, thus creating reactive halogenating species (such as hypohalite), which in turn form haloorganic compounds [1320, 1321]. These reactions are described in Sect. 2.7.1. 

From a synthetic viewpoint, selective oxygen transfer (path 3) is the most interesting peroxidation reaction. The transformations are comparable to those catalyzed by monooxygenases with one significant advantage - they are independent of redox cofactors, such as NAD(P)H. 

Among the various types of reactions - C-H bond oxidation, epoxidation of alkenes and heteroatom oxidation - the most useful transformations are described below. 

Epoxidation of Alkenes. Due to the fact that the asymmetric epoxidation of alkenes using monooxygenase systems is impeded by the requirement for NADPH-recycling and the toxicity of epoxides to microbial cells, the use of H202-depending peroxidases represents a valuable alternative. 

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Divalent copper, cobalt, nickel, and vanadyl ions promote chemiluminescence from the luminol—hydrogen peroxide reaction, which can be used to determine these metals to concentrations of 1—10 ppb (272,273). The light intensity is generally linear with metal concentration of 10 to 10 M range (272). Manganese(II) can also be determined when an amine is added to increase its reduction potential by stabili2ing Mn (ITT) (272). Since all of these ions are active, ion exchange must be used for deterrnination of a particular metal in mixtures (274). 

Luminol chemiluminescence has also been recommended for measuring bacteria populations (304,305). The luminol—hydrogen peroxide reaction is catalyzed by the iron porphyrins contained in bacteria, and the light intensity is proportional to the bacterial concentration. The method is rapid, especially compared to the two-day period required by the microbiological plate-count method, and it correlates weU with the latter when used to determine bacteria... 

M. M. Cmtchfeld, in J. O. Edwards, ed.. Peroxide Reaction Mechanisms, Wiley-Interscience, New York, 1962. 

Peracid Processes. Peracids, derived from hydrogen peroxide reaction with the corresponding carboxyUc acids in the presence of sulfuric acid and water, react with propylene in the presence of a chlorinated organic solvent to yield propylene oxide and carboxyUc acid (194—196). 

Again, according to Kirk and Othmer (Ref 14a), the systematic study of the chemistry of organic peroxides began in the 1930 s. In recent years the amount of research in the field has increased considerably, and not only have many new organic peroxides been isolated but many of the peroxide reaction mechanisms have been elucidated. The development of safer, easily handled and more efficient polymerization initiators is a major goal... 

Many reactions with complicated rate laws proceed by bimolecular steps. The complexity often arises from attendant equilibria. Several instances have been cited where no clear-cut choice could be made between algebraically compatible alternatives. Thus, do Cr2+, Fe3+, and Cl- react via CrCl+ and Fe3+ orCr2+ and FeCl2+ Does the first term in Eq. (6-33) correspond to CrOH+ and Fe3+ or Cr2+ and FeOH2+ Does the iodide-peroxide reaction necessarily imply that H302+ reacts with I- could not H202 and HI be responsible The answers to these questions will not be found strictly from the kinetics. Other experiments must be devised. Some have been mentioned previously, and two more will be cited here. 

In order to optimize the chemiluminescence response, we have investigated the mechanism of the complex reactions leading to chemical generation of chemiluminescence. A new peroxyoxalate-hydrogen peroxide reaction mechanism has emerged from our preliminary studies on the five contributing factors listed above. Two kinetic models are discussed, one for the... 

Table 4.2 Examples of phase transfer catalysed hydrogen peroxide reactions...

In principle there is a competition for the HO2 radical between peroxydisulphate and hydrogen peroxide [reactions (63) and (86)] however, when the stoichiometry is 1 1 reaction (86) can be neglected. Assuming that the chain length is large, with the usual steady-state approximation, we obtain the following rate equation ... 

As has already been mentioned, during the iron(II)-hydrogen peroxide reaction a number of organic compounds which do not react, or react only slowly with hydrogen peroxide, are readily oxidizable. In the induced oxidation of organic compounds, hydrogen peroxide plays the role of the actor and iron(II) is the inductor. 

Great promise exists in the use of graphitic carbons in the electrochemical synthesis of hydrogen peroxide [reaction (15.21)] and in the electrochemical reduction of carbon dioxide to various organic products. Considering the diversity in structures and surface forms of carbonaceous materials, it is difficult to formulate generalizations as to the influence of their chemical and electron structure on the kinetics and mechanism of electrochemical reactions occurring at carbon electrodes. 

Figure 19. Illustration of the crystal surface control on the hydrogen peroxide reaction pathway. (Reprinted with permission of B. Zhou, Headwaters, Inc.)...

With chlorotrifluoroethylene the following peroxidation reaction was iderrtified ... 

However, peroxidation can also occur in extracellular lipid transport proteins, such as low-density lipoprotein (LDL), that are protected from oxidation only by antioxidants present in the lipoprotein itself or the exttacellular environment of the artery wall. It appeats that these antioxidants are not always adequate to protect LDL from oxidation in vivo, and extensive lipid peroxidation can occur in the artery wall and contribute to the pathogenesis of atherosclerosis (Palinski et al., 1989 Ester-bauer et al., 1990, 1993 Yla-Herttuala et al., 1990 Salonen et al., 1992). Once initiation occurs the formation of the peroxyl radical results in a chain reaction, which, in effect, greatly amplifies the severity of the initial oxidative insult. In this situation it is likely that the peroxidation reaction can proceed unchecked resulting in the formation of toxic lipid decomposition products such as aldehydes and the F2 isoprostanes (Esterbauer et al., 1991 Morrow et al., 1990). In support of this hypothesis, cytotoxic aldehydes such as 4-... 

While it is generally acknowledged that lipid peroxidation reactions can be extremely complex, involving many components and reaction products, the most important reactions are readily classified according to Scheme 2.1. 

In this section, the general principles of lipid peroxidation reactions, which are well established, are discussed first and specific mechanisms, which may be relevant in vivo, are considered later. 

The efficiency of the antioxidant will depend on the ratio of the rates of Reaaion 2.10 to those for Reactions 2.11 and 2.12. A compound that is capable of reducing the antioxidant radical (A ) back to the parent compound (AH) will compete with Reactions 2.11 and 2.12, and so increase the efficiency of peroxyl radical scavenging (Reaction 2.10). In addition, the steady-state concentration of the antioxidant wiU be maintained at its initial concentration for a longer period and this should also result in more efficient suppression of the peroxidation reaction. The net result of these effects will be a synergistic enhancement of antioxidant activity. 

When stearamide (SA) was present in the DCP—MAH mixture added to EPR at 180 C, the amount of cyclohexane-insoluble EPR-g—MAH decreased, analogous to the effect of SA in reducing crossliriking in the PE-MAH-peroxide reaction (8,9). The of the cyclohexane-soluble EPR-g-MAH increased when SA was present in the reaction mixture, analogous to the effect of SA in reducing degradation in the PP-MAH-peroxide reaction (Table III). 

Different mechanisms to explain the disinfection ability of photocatalysts have been proposed [136]. One of the first studies of Escherichia coli inactivation by photocatalytic Ti02 action suggested the lipid peroxidation reaction as the mechanism of bacterial death [137]. A recent study indicated that both degradation of formaldehyde and inactivation of E. coli depended on the amount of reactive oxygen species formed under irradiation [138]. The action with which viruses and bacteria are inactivated by Ti02 photocatalysts seems to involve various species, namely free hydroxyl radicals in the bulk solution for the former and free and surface-bound hydroxyl radicals and other oxygen reactive species for the latter [139]. Different factors were taken into account in a study of E. coli inactivation in addition to the presence of the photocatalyst treatment with H202, which enhanced the inactivation... 

A large volume (11.25 m3) of mixed fatty acids was to be bleached by treatment with successive portions of 50 wt% hydrogen peroxide. 2-Propanol (450 1) was added to the acids (to improve the mutual solubility of the reactants). The first 20 1 portion of peroxide (at 51°C) was added, followed after 1 min by a second portion. Shortly afterwards an explosion occurred, which was attributed to spontaneous ignition of a 2-propanol vapour-oxygen mixture formed above the surface of the liquid. Oxygen is almost invariably evolved from hydrogen peroxide reactions, and volatile flammable solvents are therefore incompatible components in peroxide systems. 

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