Induced oxidation

Chemical Antioxidant Systems. The antioxidant activity of tea extracts and tea polyphenols have been determined using in vitro model systems which are based on hydroxyl-, peroxyl-, superoxide-, hydrogen peroxide-, and oxygen-induced oxidation reactions (109—113). The effectiveness of purified tea polyphenols and cmde tea extracts as antioxidants against the autoxidation of fats has been studied using the standard Rancimat system, an assay based on air oxidation of fats or oils. A direct correlation between the antioxidant index of a tea extract and the concentration of epigallocatechin gallate in the extract was found (107). 

Tea extracts and tea polyphenols inhibit copper- and peroxide-induced oxidation of LDL in vitro (116,123,124). The inhibitory concentration for 50% reduction (IC q) values for inhibition of copper-induced oxidation of LDL by some phenoHc antioxidants are Hsted in Table 7. The IC q for epigaHocatechin gaHate was found to be 0.075 p.mM, which was the most potent of all the phenoHc antioxidants tested (123,124). Similar results have been reported elsewhere (115,116,125,126). 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Fig. 15. Free radical induced oxidative degradation reactions.

Loikkanen, J., Naarala, J., and Savolainen, K. M. (1998). Modification of glutamate-induced oxidative stress by lead The role of extracellular calcium. Free Rad. Biol. Med., 24, 377-384,... 

What is the for the light-generated primary oxidant of photosystem II if the light-induced oxidation of water (which leads to Og evolution) proceeds with a AG° of —25 kj/mol ... 

In some cases the number of oxide layers can be related directly to the number of breaks in the curve and there is then no doubt that the acceleration derives from repetitive stress-induced oxide cracking. 

The induced oxidation of iodide reveals that some intermediate is more kinetically competent toward iodide oxidation than either HCr04 or VOJ. The stoichiometric coefficients in Eq. (5-2) suggest that chromium(V) is the species responsible. 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers37 38. They used the system T1m-H202 to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfmic radicals in this reaction either directly or by spin trapping experiments39-44. 

Huvaere, K., and Skibsted, L. H. (2009) Light-Induced Oxidation of Tryptophan and Histidine. Reactivity of Aromatic N-Heterocycles toward Triplet-Excited Flavins, Journal of American and Chemical Society, Vol. 131, (May 2009) pp. 8049-8060, ISSN 0002-7863. 

Vayalil PK, Mittal A, Kara Y, Elmets CA, Katiyar SK (2004) Green tea polyphenols prevent ultraviolet light-induced oxidative damage and matrix metal-loproteinases expression in mouse skin. J Invest Dermatol 122 1480-1487... 

Substituent groups on a polyphosphazene chain containing mobile hydrogen atoms (4-isopropylphenol [715,716], 4-benzylphenol [293,718], etc.) showed a completely different photochemical reactivity both in solution and in sohd state under accelerated conditions, based mostly on the fight-induced oxidation of these groups and radical formation reactions. 

McMillian M, Nie A, Parker JB, Leone A, Kemmerer M, Bryant S, et al. Drug-induced oxidative stress in rat liver from a toxicogenomics perspective. Toxicol Appl Pharmacol 2005 207 171-8. 

One advantage with semiconductor particulate systems is that light-induced oxidation/reduction is very often irreversible, unlike homogeneous solutions of... 

That is, the activated complex contains one Cr(V) atom and one Fe(II) atom. Espenson has shown, from a consideration of the induced oxidation of iodide ion, that the reaction between Cr(VI) and Fe(Il) requires one added proton. Consequently, the [H ] -dependence of the rate can be viewed as the addition of two protons in a pre-equilibrium followed by the addition of a further proton in the slow step, viz. 

Westheimer has also reviewed the induced oxidations by the Cr(VI)-As(III) couple of iodide, bromide and manganous ions vide supra). The induction factor of 0.5 for Mn(II) implies an intermediate tetravalent chromium species however, the factor of 2 for iodide points to a pentavalent chromium intermediate. Both... 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

This has the attraction of explaining other characteristic features of this reaction, namely (i) the sensitivity to oxygen of the rate and course of reaction (because of capture of O2 by CH(C02H)2), (11) acceleration by added acrylonitrile to produce a carboxyl group-containing polymer, and (///) powerful induced oxidations by the reaction mixture of molecules inert to Mn(II[) pyrophosphate, e.g. ethanol, diethyl ether. 

The latter effect is presumed to result from oxidation by CH(C02H)2. Of relevance also are the kinetics of the Mn(III) pyrophosphate oxidation of ethyl-and benzylmalonic acids the order in Mn(III) is approximately one, no retardation by Mn(II) is found and induced oxidations do not occur. This has been explained in terms of a lowering by R of the redox potential of RC(C02H) to a value insufficient to reoxidise Mn(II) pyrophosphate in reaction (58). 

Besides induced oxidation-reduction reactions we often speak of induced dissolution, induced precipitation, as well as of induced complex formation there is even a reference to an induced reaction caused by neutralization. It is only necessary to examine briefly the latter cases. 

Case a, i is well illustrated by the arsenite-induced oxidation of manganese(II) by chromic acid, studied by Lang and Zwerina. The overall equation of this induced reaction is... 

ARSENITE-INDUCED OXIDATION OF IODIDE BY DICHROMATE ACCORDING TO... 

Considering the limiting values of the induction factor it may be postulated that in the case of iodide and bromide the induced oxidation is caused by chromium(V), whereas for induced oxidation of manganese(II) chromium(lV) is the coupling intermediate. Therefore, one has to assume that in the course of reaction between arsenic(ril) and chromium(VI) both chromium(V) and chromium(IV) intermediates are involved. The mechanism below, proposed by Westheimer seems to be in agreement with experiment. 

It was found by DeLury that the overall rate of reduction of chromate is practically unaffected by the concentration of iodide, i.e. the sum of the rates of formation of iodine and aresenic(V) is constant and just equal to the rate at which chromate is reduced in a raction mixture containing no iodide (Fig. 1). The rate of oxidation of arsenite at a sufficiently high concentration of iodide decreases to i of its original value this is in accordance with the value of ci = 2 found. Fig. 1 well illustrates the general feature of coupled reactions, that the reaction of the inductor is always inhibited by the acceptor. The induced oxidation of iodide can... 

Fig. 1. Arsenic(III)-induced oxidation of iodide by dichromate. Data of DeLury .

At first sight the argument of Westheimer, according to which it is unwise to postulate a reaction in which two unstable intermediates are simultaneously produced, seems to be acceptable, especially if we take into consideration that these induced oxidations are not autocatalytic in nature most of mechanisms involving arsenic(IV), chromium(IV), and chromium(V) lead to the conclusion that the... 

In the presence of manganese(II) ions the rate of oxidation of H2R by chromic acid decreases . Under favourable experimental conditions (high concentration of alcohol and low concentration of chromate) the diminution of rate is about 50 % which is in accordance with results listed in Table 6, according to which ci = 0.5. The inhibiting effect of manganese(II) on the oxidation of H2R can be explained by reaction (23) followed by step (24). Therefore the induced oxidation of manganese(II) can be described by reactions (26), (23) and (24). 

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