Radical scavenger concentration

Four major factors influence the oxidation rate of organic pollutants (1) pH, (2) relative concentration of oxidants (03/H202), (3) photon flux in the UV/03 system, and (4) radical scavenger concentration. 

Siddiqi MA, Bothe E (1987) Single- and double-strand break formation in DNA irradiated in aqueous solution Dependence on dose and OH radical scavenger concentration. Radiat Res 112 449-463... 

In a recent review, Hayon (47) endeavored to show general dependence of Gr on pH and on radical scavenger concentration. He asserted that GR is a function of kR+s[S] and the difference of GR, when measured in the different systems, and he attributed this to dissimilar values of kR+Si [Si] in these systems. 

To summarize the effect of scavengers on the radical yields in the majority of cases reviewed most, if not all, of the observed increase of radical yield at high alkalinity, acidity or, radical scavenger concentration is an artifact. There is no real experimental evidence in most systems studied, that GR is affected to a large extent by adding high concentrations of solutes, although in a few systems the yield does increase with solute concentration. 

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

The rate of aqueous ozonation reactions is affected by various factors such as the pH, temperature, and concentration of ozone, substrate, and radical scavengers. Kinetic measurements have been carried out in dilute aqueous solution on a large number of organic compounds from different classes (56,57). Some of the chemistry discussed in the foUowing sections occurs more readily at high ozone and high substrate concentrations. 

A related mechanism of degradation involves the direct interaction of the radioactive emission with other tracer molecules in the preparation. This phenomenon is likely to occur in high specific activity compounds stored at high radiochemical concentrations in the absence of free-radical scavengers. 

Stabilization of Fuels and Lubricants. Gasoline and jet engine fuels contain unsaturated compounds that oxidize on storage, darken, and form gums and deposits. Radical scavengers such as 2,4-dimethyl-6-/ f2 butylphenol [1879-09-0] 2,6-di-/ f2 -butyl-/)-cresol (1), 2,6-di-/ f2 -butylphenol [128-39-2], and alkylated paraphenylene diamines ate used in concentrations of about 5—10 ppm as stabilizers. 

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. 

The potency of a chain-breaking antioxidant, which scavenges peroxyl radicals, will decrease as the concentration of lipid peroxides in the LDL particle increases (Scheme 2.2). This is illustrated in the experiment shown in Fig. 2.3 in which the antioxidant potency of a peroxyl radical scavenger (BHT) decreases as a function of added exogenous hpid hydroperoxide. If the endogenous lipid peroxide content of LDL were to vary between individuals, this could explain the observed diferences in the effectiveness of a-tocopherol in suppressing lipid peroxidation promoted by copper. 

It has long been recognized that ascorbate levels are low in patients with RA (Lunec and Blake, 1985) and ascorbate is predominantly found in the dehydro form. The presence of increased dehydroascorbate has been suggested to indicate its rapid oxidation by stimulated PMNs (Halliwell and Gutteridge, 1990). When ascorbate concentrations are lower than about 20 /tmol/1, as can occur in rheumatoid synovial fluid, the Fe(III) reducing effects of ascorbate outweigh its radical-scavenging effects. Ascorbate then causes increased OH formation and promotes lipid peroxidation (Blake et al., 1981). 

Pippenger, C.E., Meng, X., Stolfi, V, McGonagle, B. and Fazio, V.W. (1991). Free radical scavenging activities and trace element concentrations in erythrocytes and plasma of adult patients with inflammatory bowel disease. In Inflammatory Bowel Diseases. Progress in Basic Research and Clinical Implications (eds. H. Gocbell, K. Ewe, H. Malchow and Ch. Koelbel) p. 33. Kluwer Academic Publishers, Lancaster. 

Selden, C., Seymour, C.A. and Peters, T.J. (1980). Activities of some free radical scavenging enzymes and glutathione concentrations in human and rat liver and their relationship to the pathogenesis of tissue damage in iron overload. Clin. Sci. 58, 211-219. 

It should be remembered that some of the established antioxidants have other metabolic roles apart from free-radical scavenging. The finding of reduced antioxidant defences in diabetes, for example, may not be prima fascie evidence of increased oxidative stress, since alternative explanations may operate. For example, this may reflect a response to reduced free-radical activity as su ested by the results of a previous study (Collier et al., 1988). In the case of ascorbate, an alternative explanation has been proposed by Davis etal. (1983), who demonstrated competitive inhibition of ascorbate uptake by glucose into human lymphocytes. This view is supported by the similar molecular structure of glucose and ascorbic acid (see Fig. 12.4) and by a report of an inverse relationship between glycaemic control and ascorbate concentrations in experimental diabetes in rats. Other investigators, however, have not demonstrated this relationship (Som etal., 1981 Sinclair etal., 1991). 

Bioactive compounds, such as carotenoids have strong antioxidative properties and are used as efficient radical scavengers. In some natural sources several carotenoid isomers can be found, which differ in their biochemical activities such as bioavailability or antioxidation potency. Knowing the structure and concentration of each stereoisomer is crucial for an understanding of the effectiveness... 

Unlike the oxygen, Fe2+ scavenging process is due to ionic nature which is limited to bulk medium only. Fe2+ reacts mainly with H2O2, produced by recombination of OH radicals and liberated in the bulk system with transient collapse of the bubble. Experimentally [91] the average concentration of Fe2+ in the bulk medium was found to be almost constant possibly due to continuous regeneration, providing an effective radicals scavenging. 

An unsubstituted hydroxylamine is a powerful hydroperoxide decomposer and peroxyl radical scavenger, and could play an important role in photo-stabilization even if present at only a low concentration after dark intervals. 

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