Initiation of oxidation

Our conclusion then for the oxygen interactions with metals is that because of the specific association of cluster ion intensities with particular types of oxygen rather than total coverage, the technique is not suitable for monitoring coverages or kinetics in an independent manner. Once it is established which type of oxygen a particular cluster ion is representative of, then that ion may, in favorable circumstances, be used for quantification. In the case of Ni, it seems that the negative ions are very sensitive to the initiation of oxide nucleation. In the case of W(IOO), the WOj, WO+ and WOj ions may fill a similar role. 

One important point should be stressed here the efficiency of any stabilizing system depends very much on the removal or depletion of the defect structures in the polymer. An example is shown in Figure 1 where the case 5 mechanism vide supra) of a synergistic mixture is theoretically depicted [7]. The line 2 shows that the effect of the synergistic mixture of two antioxidants on thermo-oxidation stability is negligible, if there occurs a relatively significant initiation of oxidation reaction in a way independent from the route taking place via hydroperoxides. 

Addition of RNA or DNA prior to oxidation of BP by PGH synthase results in substantial nucleic acid binding (17,21). Addition of RNA five minutes after initiation of oxidation leads to no covalent binding (17). This implies that the quinones do not bind to nucleic acid but rather a short-lived intermediate in their formation does. Arachidonic acid oxygenation in ram seminal vesicle microsomes is complete within two min, which suggests that the reactive intermediate is generated concurrently with PGH2 The structures of the nucleic acid adducts have not been elucidated so the identity of the reactive intermediate is unknown. 

As already noted (see Chapter 4), autoxidation is a degenerate branching chain reaction with a positive feedback via hydroperoxide the oxidation of RH produces ROOH that acts as an initiator of oxidation. The characteristic features of inhibited autoxidation, which are primarily due to this feedback, are the following [18,21,23,26,31-33] ... 

On the other hand, they react with hydroperoxide to produce free radicals and, hence, contribute to the initiation of oxidation. 

A series of substituted diaryselenides were examined in three lipid peroxidation model systems isolated rat liver microsomes treated with Fe(II)/(ADP)/ascorbate and isolated rat hepatocytes treated with two different initiators of oxidation. In rat hepatocytes, all of the tellurides performed more effectively than the selenides. Particularly for the rat liver microsome system, the substituent effects on lipid peroxidation were consistent with what would be expected Electron-donating groups give more active compounds, while electron-withdrawing groups give poorer antioxidants. The same trends were seen for substituted diaryItellurides in inhibition of linoleic acid peroxidation in a two-phase model, where the dimethylamino... 

In our experiments described above, we added the NO 1 min after the initiation of oxidation with Fe ". However, we realized that the sequence of addition of the initiator of oxidation and antioxidant NO might be aitical, so in additional studies we examined the importance of this sequence by adding the NO prior to the Fe ". There was an antioxidant effect when the... 

The health impairing and toxic elfects of oxidation of lipids are due to loss of vitamins, polyenoic fatty acids, and other nutritionally essential components formation of radicals, hydroperoxides, aldehydes, epoxides, dimers, and polymers and participation of the secondary products in initiation of oxidation of proteins and in the Maillard reaction. Dilferent oxysterols have been shown in vitro and in vivo to have atherogenic, mutagenic, carcinogenic, angiotoxic, and cytotoxic properties, as well as the ability to inhibit cholesterol synthesis (Tai et ah, 1999 Wpsowicz, 2002). 

The data apparently require a free radical chain mechanism for the oxidation of benzhydrol. Potassium superoxide filtered from a completed oxidation completely removed the induction period for a fresh oxidation. Thus, potassium superoxide must either serve as an initiation of oxidation... 

It now seems that cytosolic calcium may not play a central role in the initiation of oxidative injury as changes in calcium homeostasis occur well after the appearance of other indications of cellular injury. However, mitochondrial lesions do occur early on in the time course of oxidative cellular damage, and calcium may indeed play a role in these. Changes in concentrations of calcium will, however, result in the activation of signaling mechanisms and alterations in cellular structure and in gene expression. Such alterations may in some instances play a critical role in cellular toxicity. For example, increases in cytosolic Ca2+ inhibit mitochondrial function. 

Autoxidation is a low-temperature oxidation of a substance (usually a liquid) by the atmosphere without the aid of other oxidizing agents, but requiring, in many cases, the presence of inductors for initiation of oxidation... 

Automobile and Hydrocarbon Emissions. The oxidation of carbon monoxide and hydrocarbons is catalyzed by platinum/palladium/rhodium on alumina. If catalyst poisons such as lead and phosphorus are not present, the major problems become initiation of oxidation at low temperature, thermal stability at high temperature, resistance to thermal schock, and a high external surface area catalyst configuration. 

Supported Initiators of Oxidations with O2 and a Sacrificial Aldehyde... 

In initiation of oxidation, the important role may also be played by reactive bonds in a polymer. With polyethylene which is the most simple structurally, such reactive sites may be small concentrations of double bonds or more numerous sites of branching of a main chain to side alkyls. Investigation of the oxidation reaction of different types of polyethylene has. however, revealed that the degree of polyethylene branching from 0 to 20 branches for each 1000 carbon atoms of the main chain does not affect the induction period of oxidation [13]. 

The formation of oxygen anion radicals and molecules of ozone also should be counted with at the ionization initiation of oxidation [26]. Initiation reaction caused by oxygen anion radicals may play an important role within the polymer bulk while the effect of ozone forming in the surrounding air atmosphere will include only the formation of radicals on the polymer surface. The latent effect of ionization initiation on polymer oxidation which is very distinct may be documented on a relatively fast increase of concentration of carbonyl groups, observed over 1 year after irradiation crosslinking of polyethylene [27]. 

Taking into account the effect of radiation on initiation of oxidation, one thing should be born in mind, namely that the cosmic radiation present everywhere may be one of the reasons of oxidation which occurs more or less slowly in any organic material. 

Korycka-Dahl, M., Richardson, T. 1980. Initiation of oxidative changes in foods. J. Dairy Sci. 63, 1181-1198. 

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