Peroxyl formation

The recombination of fragments stemming from one macromolecule, at times shorter than the diffusion time, prevents the linear increase in RD with the absorbed dose per pulse, as not all main-chain scissions result in the formation of fragments. The effect of molecular oxygen on RD in the case of PBS can be interpreted by formation of peroxyl radicals, e.g. 

A steady-state analysis of R13-R16 provides a means of understanding the role of peroxyl radicals such as HO2 in ozone formation ... 

The formation and reaction of peroxyl radicals derived by reaction of tervalent phosphorus compounds with oxygen have attracted interest. Photolysis of trialkyl phosphites in oxygenated solutions of aromatic hydrocarbons gives phenols. " Phosphorus trichloride reacts with 1,2-dichloroethylene, in the presence of oxygen, to give (17). It is tempting to suggest that both reactions occur via similar intermediates, e.g. (15) and (16). 

The protective effects of carotenoids against chronic diseases appear to be correlated to their antioxidant capacities. Indeed, oxidative stress and reactive oxygen species (ROS) formation are at the basis of oxidative processes occurring in cardiovascular incidents, cancers, and ocular diseases. Carotenoids are then able to scavenge free radicals such as singlet molecular oxygen ( O2) and peroxyl radicals particularly, and protect cellular systems from oxidation. 

However, peroxidation can also occur in extracellular lipid transport proteins, such as low-density lipoprotein (LDL), that are protected from oxidation only by antioxidants present in the lipoprotein itself or the exttacellular environment of the artery wall. It appeats that these antioxidants are not always adequate to protect LDL from oxidation in vivo, and extensive lipid peroxidation can occur in the artery wall and contribute to the pathogenesis of atherosclerosis (Palinski et al., 1989 Ester-bauer et al., 1990, 1993 Yla-Herttuala et al., 1990 Salonen et al., 1992). Once initiation occurs the formation of the peroxyl radical results in a chain reaction, which, in effect, greatly amplifies the severity of the initial oxidative insult. In this situation it is likely that the peroxidation reaction can proceed unchecked resulting in the formation of toxic lipid decomposition products such as aldehydes and the F2 isoprostanes (Esterbauer et al., 1991 Morrow et al., 1990). In support of this hypothesis, cytotoxic aldehydes such as 4-... 

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. 

Ascorbate is known to act as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, reduced ascorbate can react non-enzymatically with molecular oxygen to produce dehydroascorbate and hydrogen peroxide. Also, ascorbate in the presence of light, hydrogen peroxide and riboflavin, or transition metals (e.g. Fe, Cu " ), can give rise to hydroxyl radicals (Delaye and Tardieu, 1983 Ueno et al., 1987). These phenomena may also be important in oxidative damage to the lens and subsequent cataract formation. 

Kennedy, TA and Liebler, DC, 1991. Peroxyl radical oxidation of beta-carotene—Formation of beta-carotene epoxides. Chem Res Toxicol 4, 290-295. 

The elementary reaction step, which involves the formation of singlet oxygen, is a reaction of superoxide anion radicals CO ), which are the reaction intermediates of the above oxidation when performed in an alkaline medium. In the presence of water they may be converted to hydrogen peroxyl radicals HOO as follows ... 

For the rate of formation of alkyl (P ) and peroxyl (PO ) radicals we may obtain the following differential equations (the rate constants correspond to the reactions in Scheme 2)... 

A cellular antioxidant activity (CAA) assay for quantifying the antioxidant activity of phytochemicals, food extracts, and dietary supplements has been developed by Wolfe and Liu (2007). The method measures the ability of compounds to prevent the formation of dichlorofluorescein (DCF) by ABAP-generated peroxyl radicals in human hepatocarcinoma HepG2 cells. The decrease in cellular fluorescence when compared to the control cells indicates the antioxidant capacity of the compounds. The method... 

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... 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

Miller and Mayo studied the styrene oxidation and came to conclusion that this chain reaction occurs via addition of peroxyl radical to double bond of styrene with formation of polyperoxide as a product A.A. Miller and F.R. Mayo [54]... 

The formation of dihydroperoxides as the primary products of hydrocarbon oxidation is the result of peroxyl radical isomerization (see Chapter 2). 

The hydroperoxide group weakens the a-C—H bonds, and the peroxyl radical of the oxidized hydrocarbon attacks this group with aldehyde formation. 

Electron paramagnetic resonance (EPR) spectroscopy proves the formation of peroxyl radicals in oxidized hydrocarbons [12—15]. 

The values of enthalpies of peroxyl radical formation (A//r°) calculated from the enthalpies of hydroperoxide formation according to the thermochemical equation ... 

The enthalpy of peroxyl radical formation is related to A//f°(ROOH) by the following relationship ... 

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