Peroxyl radicals acids

The sulfenic acids have been found to be extremely active radical scavengers showing rate constants of at least 107 m"1 s 1 for the reactions with peroxyl radicals at 333 K17. It has also been suggested that the main inhibiting action of dialkyl sulfoxides or related compounds in the autoxidation of hydrocarbon derives from their ability to form the transient sulfenic acids on thermal decomposition, i.e.17... 

Several powerful oxidants are produced during the course of metabolism, in both blood cells and most other cells of the body. These include superoxide (02 ), hydrogen peroxide (H2O2), peroxyl radicals (ROO ), and hydroxyl radicals (OH ). The last is a particularly reactive molecule and can react with proteins, nucleic acids, lipids, and other molecules to alter their structure and produce tissue damage. The reactions listed in Table 52-4 play an important role in forming these oxidants and in disposing of them each of these reactions will now be considered in turn. 

The antioxidant property of ferulic acid and related compounds from rice bran was reported by Kikuzaki et al, (2002). Their results indicated that these compounds elicit their antioxidant function through radical scavenging activity and their affinity with lipid substrates. Another recent study reported by Butterfield et al, (2002) demonstrated that ferulic acid offers antioxidant protection against hydroxyl and peroxyl radical oxidation in synaptosomal and neuronal cell culture systems in vitro. The effect of ferulic acid on blood pressure (BP) was investigated in spontaneously hypertensive rats (SHR). After oral administration of ferulic acid the systolic blood pressure (SBP) decreased in a dose-dependent manner. There was a significant correlation between plasma ferulic acid and changes in the SBP of the tail artery, suggesting... 

The chain-breaking antioxidant must scavenge peroxyl radicals at a fester rate than they can react with another unsaturated fetty acid (reaction 2.10). The reverse reaction, whereby the antioxidant radical converts the lipid peroxide to a peroxyl radical, should also be slow (reaction 2.11 in Scheme 2.3). 

The antioxidant radical formed must react at a lower rate with unsaturated fetty acids than the peroxyl radical (Reaction 2.12 in Scheme 2.3). 

Oxidation of the fatty acids in an LDL particle shares many of the characteristics associated with lipid peroxidation in other biological or chemical systems. Once initiated peroxyl radicals are formed and this results in the oxidation of a-tocopherol to give the a-tocopheroyl radical (Kalyanaraman etal., 1990). This can be demonstrated by e.s.r. techniques that allow the direct observation of stable radicals such as the a-tocopheroyl radical. After the a-tocopheryl radical is consumed, lipid-derived peroxyl radicals can be detected after reaction with spin traps (Kalyanaraman etal., 1990, 1991). 

The reactions described so far do not require the involvement of the apo-B protein, neither would they necessarily result in a significant amount of protein modification. However, the peroxyl radical can attack the fatty acid to which it is attached to cause scission of the chain with the concomitant formation of aldehydes such as malondialdehyde and 4-hydroxynonenal (Esterbauer et al., 1991). Indeed, complex mixtures of aldehydes have been detected during the oxidation of LDL and it is clear that they are capable of reacting with lysine residues on the surface of the apo-B molecule to convert the molecule to a ligand for the scavenger receptor (Haberland etal., 1984 Steinbrecher et al., 1989). In addition, the lipid-derived radical may react directly with the protein to cause fragmentation and modification of amino acids. 

The accumulation of hydroperoxides and their subsequent decomposition to alkoxyl and peroxyl radicals can accelerate the chain reaction of polyunsaturated fatty-acid p>eroxidation leading to oxidative damage to cells and membranes as well as lipoproteins. It is well-recognized that transition metals or haem proteins, through their... 

The major lipid-soluble antioxidant primarily associated with lipid membranes is a-tocopherol (vitamin E). Circulating a-tocopherol is carried by chylomicrons, LDL and HDL and also has extracellular antioxidant capacities. As a chain-breaking antioxidant, it short circuits the propagation phase of lipid peroxidation because the peroxyl radical will react with a-tocopherol more rapidly than a polyunsaturated ffitty acid (Burton and Traber, 1990). The resulting a-tocopheryl radical reacts with a second peroxyl radical to form an inactive, nonradical complex. In vitro, ascorbate regenerates the tocopheryl radical into its native non-radical form (Burton and Traber, 1990). 

There is evidence from a number of in vitro studies that the vitamin E peroxyl radical formed during fatty-acid degradation may be converted to vitamin E plus nonradical through the actions of vitamin C (Burton et al., 1985). RA patients have reduced serum ascorbate levels (Situnayake et al., 1991) and potentially a reduced capacity for the regeneration of vitamin E. In vitro studies suggest that vitamin E becomes a pro-oxidant when ascorbate levels are low (Bowry and Stocker, 1993). 

The human lens is rich in ascorbate, which is required for normal collagen synthesis and acts as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, ascorbic acid can undergo auto-oxidation and, at certain concentrations, can form hydroxyl radicals with hydrogen peroxide in the presence of light and riboflavin as described above (Delaye and Tardieu, 1983 Ueno et al., 1987). 

Forni, L.G., Packer, J.E., Slater, T.F. and Willson, KL. (1983). Reactions of the trichloromethyl and halothane derived peroxyl radicals with unsaturated fatty acids a pulse radiolysis study. Chem. Biol. Interact. 45, 171-177. 

R02 represents any peroxyl radical formed from iPP, which will take part in a termination reaction with the primary peroxyl radical II). From a comparison with acid fluorides prepared from model... 

The most probable elementary step from which chemiluminescence is emitted is disproportionation of secondary peroxyl radicals, which may be formed only on carbon 6 of the glucopyranosyl unit. According to various authors, who accept the Russel Scheme 5 of disproportionation of secondary peroxyl radicals as a possibility leading to the light emission, it may be deduced that carboxylic acid,... 

Peroxyl radicals are the species that propagate autoxidation of the unsaturated fatty acid residues of phospholipids (50). In addition, peroxyl radicals are intermediates in the metabolism of certain drugs such as phenylbutazone (51). Epoxidation of BP-7,8-dihydrodiol has been detected during lipid peroxidation induced in rat liver microsomes by ascorbate or NADPH and during the peroxidatic oxidation of phenylbutazone (52,53). These findings suggest that peroxyl radical-mediated epoxidation of BP-7,8-dihydrodiol is general and may serve as the prototype for similar epoxidations of other olefins in a variety of biochemical systems. In addition, peroxyl radical-dependent epoxidation of BP-7,8-dihydrodiol exhibits the same stereochemistry as the arachidonic acid-stimulated epoxidation by ram seminal vesicle microsomes. This not only provides additional... 

Figure 8. Proposed mechanism of the generation of peroxyl radicals by reaction of hematin with unsaturated fatty acid hydroperoxides. ...

This method was first reported by Winston and others (1998) and it is based on the oxidation of alpha-kclo-y-mcthiolbutyric acid (KMBA) to ethylene by peroxyl radicals produced from AAPH. The ethylene formation, which is partially inhibited... 

The accumulation of hydroxyl-containing products, such as hydroperoxides, alcohols, acids, and water, also reduce the total activity of peroxyl radicals due to the hydrogen bonding with R02 [150], When acting together, these factors cause self-inhibition of autoxidation at conversion levels of 40-50% [3],... 

The study of benzaldehyde and cyclohaxanone co-oxidation showed the formation of s-caprolactone as the main product of cyclohexanone oxidation [5]. Cyclohexanone was found not to react practically with peroxyl radicals under mild conditions. The oxidation of benzaldehyde produces perbenzoic acid. The latter oxidizes the benzaldehyde to benzoic acid and cyclohexanone to s-caprolactone. 

OXIDATIVE DECARBOXYLATION OF CARBOXYLIC ACIDS 8.4.1 Attack of Peroxyl Radicals on C—H Bonds... 

Peroxyl radicals attack all groups of acid Me(CH2) COOH and preferencially the a-CH2 group. The more the number of CH2 groups in carboxylic acid, the higher is the rate constant (see Table 8.20). 

The attack of peroxyl radicals on 0-CH2 groups produces the same functional groups (hydroperoxyl, hydroxy, oxo) as in the case of subsequent hydrocarbon oxidation. The oxidation of unsaturated acids proceeds similarly to the oxidation of olefins [4,7]. 

The decarboxylation via the peroxyl radical reaction with the carboxylic group was the main channel of C02 production (78-82%). The attack of the peroxyl radical on the CH2 group adds 18-22% of C02. However, in the case of isobutyric acid with a weak tertiary C—H bond, the attack on the C—H bond appeared to be the main reaction of decarboxylation. 

As a result, the carboxyl groups are blocked, and the peroxyl radicals react only with the C—H bonds of the dicarboxylic acid. The results of the experimental study of acid decarboxylation in reactions with cumylperoxyl radicals are presented below (393 K, [acid] = 0.8 mol L-1, cumene [CuOOH] = 0.1 mol LA1 [106]). 

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