Radical scavengers, and the

As mentioned above, in contrast to classic antioxidant vitamins E and C, flavonoids are able to inhibit free radical formation as free radical scavengers and the chelators of transition metals. As far as chelators are concerned their inhibitory activity is a consequence of the formation of transition metal complexes incapable of catalyzing the formation of hydroxyl radicals by the Fenton reaction. In addition, as shown below, some of these complexes, for example, iron- and copper-rutin complexes, may acquire additional antioxidant activity. 

The final products of Fenton oxidation are dependent on several factors. The amount of hydroxyl radical is clearly important. In addition, the amount of hydroxyl radical scavengers and the amount of pollutant present will affect the array of final products. The concentrations of Fe2 + and Fe3 +, as well as those of other metals, also affect product distributions because these species are involved in some degradation pathways. The presence of... 

Simple chromones (Phe ] y-pyran-4-one) include the glucoside biflorin (a cAMP phosphodiesterase inhibitor and free radical scavenger) and the 2-phenoxychromone capillarisin (an aldose reductase inhibitor) as well as a number of variously cytotoxic and antimicrobial compounds. 

Primary alcohols are selectively oxidized to aldehydes by the RuCl2(TPP)3/ tert-butylhydroperoxide (TBHP) system. The reaction takes place at room temperature using catalytic amounts of the mthenium complex. Specific conditions are found leading to improved efficiency in the TBHP decomposition and in reduction of the level of secondary products originating from the aldehyde post-reactions. The effect of some free-radical scavengers and the stracture of the mthenium conplex will also be presented. 

The observation of an induction period, the inhibiting effect of radical scavengers, and the ease of rupture of cyclooctasulfur (Sg ) to a catena-octasulfur () biradical 7,8) argue in favor of a radical initiated mechanism for the reaction of all but the p-amino and p-nitrothiophenols studied. The rate law described in Equation 5 is overall fifth order indicating that the mechanism is complex, involving several steps, some of which may be pre-rate determining equilibria. The second order dependence on thiol concentration is not siuprising since the final product ArS rAr requires the combination of two initial reactants. The third order dependence on sulfur, however, is accounted for less easily in mechanistic terms. Equations 7 and 8 represent an overall mechanism consistent with the facts considered above. 

In the present study, the mechanism of photolytic ozonation was investigated in the laboratory by measuring ozone, hydrogen peroxide, and total oxidant levels in purified water as a function of time in a two-liter stirred tank reactor in which concurrent ozone sparging and UV irradiation were taking place. Experiments were run in the presence and absence of various radical scavengers and the results Interpreted in terms of reaction rate constants taken from the literature. This paper will discuss the mechanism of photolytic ozonation, show its relationship to other ozonation processes, and... 

A great many polymers can be better stabilized by combined use of two or more AOs than with one alone. The most effective AO nuxtures are those in which one component acts as a radical scavenger and the other as a peroxide decomposer. When the two components regenerate each other, this is termed synergism. Although AOs are required in the formulae of nearly all polymers, only about 85 % of consumption is attributed to ABS, PP, LDPE, HOPE, and PS. 

Ginkgo biloba extract or zinc on Upid peroxide, some free radical scavengers and the cardiovascular system of aged rats. Pharmacol Res 1998 38 65-72. 

Initiation by light, retardation by radical scavengers, and the cyclization of intermediates indicate that the reaction of primary and secondary alkyl-mercury halides with the salts of secondary nitroalkanes [equation (53)]... 

While in pure alkanes the lifetime of the ions is very short, this is not necessarily the situation in the presence of solutes. In some cases, solute anions and cations formed by electron capture and charge transfer, respectively, may have considerably long lifetimes. If this occurs, it is possible that ionic reactions take place and therefore the free radical mechanism of the chain reaction has to be established. This can be done by various methods such as the addition of electron, charge and radical scavengers, and the determination of the effect of these additives on the chain reaction. Another method is to show that the same reaction can also be initiated thermally or photochemically. 

Table II showed the radical-scavenging and the OSI values of the extract and fractions obtained from cuny leaves. The dichloromethane extract and the ethyl acetate-soluble fraction showed both high radical scavenging and...

Holler, T.P. and Hopkins, P.B. (1990) Ovothiols as free-radical scavengers and the mechanism of ovofhiol-promoted NAD(P)H-02 oxidoreductase activity. Biochemistry, 29,1953-1961. 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

Vitamin E can also act as an antioxidant (qv) in animals and humans alone or in combination with vitamin C (qv). Both are good free-radical scavengers with the vitamin C acting to preserve the levels of vitamin E (35). Vitamin E in turn can preserve the levels of vitamin A in animals (13). It has been shown that vitamin E reduces the incidence of cardiovascular disease (36—39). This most likely results from the antioxidant property of the vitamin which inhibits the oxidation of low density Hpoproteins (LDLs) (40—42). The formation of the oxidized LDLs is considered important in decreasing the incidence of cardiovascular disease (43). 

Perhaps the most powerful variant of this approach is the use of labelled starting materials or additives and analyzing the products for isotopic distribution. This technique is valuable in assessing the importance of molecular detachment processes. Moreover, unique distributions result from proton, hydride, and H2 transfer particularly in. the presence of radical scavengers and these have been studied extensively (4, 6). 

Nonaka, A., Manabe, T. and Tobe, T. (1991). Effect of a new synthetic ascorbic acid derivative as a free radical scavenger on the development of acute pancreatitis in mice. Gut 32, 528-532. 

The antiulcer agent rebamipide ((2-(4-chlorobenzoy-lamino)-3-[2(lH)-quinolinon-4-yl]propionic acid) dose-dependently decreased hydroxyl radical signal generated by the Fenton reaction in an e.s.r. study. Rebamipide is active as a hydroxyl radical scavenger and inhibitor of superoxide production by neutrophils (Yoshikawa etal., 1993). 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

---