Sources of Free Radicals

Free radical reactions are necessary for the normal operation of a number of biological processes. The catalytic action of many enzymes such as xanthine oxidase and electron transport processes involves one-electron transfers that yield free radical intermediates. Because of the ubiquity of molecular oxygen in aerobic organisms and 

The oxidant-generating enzyme of activated neutrophils (and other phagocytes) is a membrane-associated, electron-transporting NADPH oxidase, which consists of a flavoprotein and low-potential cytochrome, i.e., cytochrome b 245 (Segal, 1989). This oxidase is present in the latent form and is activated during exposure of the cells to a variety of diverse signals such as leukoattractants and leukotrienes (Babior, 1984 Weiss, 1989). 

The interaction of neutrophils with leukoattractants and especially opsonized particles is accompanied by degranulation with the consequent release of lysosomal enzymes and an oxidative burst. A consequence of the respiratory burst is the virtually immediate generation of superoxide anion (Babior, 1984) by the neutrophils and alveolar macrophages. Other oxygen species can also be released by these cells (Hoidal et al., 1979). This is primarily due to the activation of membranous NADPH oxidase which catalyzes the following reaction  

The superoxide (O2 ) free radical may spontaneously or enzymatically be converted to oxygen (O2) and hydrogen peroxide (H2O2). The enzyme that catalyzes this reaction is superoxide dismutase (SOD). 

H2O2 is a potent oxidant and must be rapidly removed from the tissue. Two mechanisms exist that enable phagocytes to transform H2O2 to particularly potent reactive 

There are several reactions that are frequently used to generate free radicals, both for the study of radical structure and reactivity and also in synthetic processes. Some of the most general methods are outlined here. These reactions will be encountered again when specific examples are discussed. For the most part, we will defer discussion of the reactions of the radicals until then. 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

Peresters are also sources of radicals. The acyloxy portion normally loses carbon dioxide, so peresters yield an alkyl (or aryl) and an alkoxy radical  

The thermal decompositions described above are unimolecular reactions that should exhibit first-order kinetics. Under many conditions, peroxides decompose at rates faster than expected for unimolecular thermal decomposition and with more complicated kinetics. This behavior is known as induced decomposition and occurs when part of the peroxide decomposition is the result of bimolecular reactions with radicals present in solution, as illustrated below specifically for diethyl peroxide. 

The amount of induced decomposition that occurs depends on the concentration and reactivity of the radical intermediates and the susceptibility of the substrate to radical attack. The radical X- may be formed from the peroxide, but it can also be derived from subsequent reactions with the solvent. For this reason, both the structure of the peroxide and the nature of the reaction medium are important in determining the extent of induced decomposition, relative to unimolecular homolysis. 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical, t-Butyl hydroperoxide is often used as a radical source. Detailed studies have been reported on the mechanism of the decomposition, which is a somewhat more complicated process than simple unimolecular decomposition. Dialkyl peroxides decompose to give two alkoxy radicals.  

The decomposition of peroxides, which occurs thermally in the examples cited above, can also be readily accomplished by photochemical excitation. The alkyl hydroperoxides are also sometimes used in conjunction with a transition metal ion. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as a result of one-electron reduction by the metal ion.  

Peroxides are a common source of radical intermediates. An advantage of the generation of radicals from peroxides is that reaction generally occurs at relatively 

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Peroxide-decomposing antioxidants destroy hydroperoxides, the sources of free radicals in polymers. Phosphites and thioesters such as tris(nonylphenyl) phosphite, distearyl pentaerythritol diphosphite, and dialkyl thiodipropionates are examples of peroxide-decomposing antioxidants. 

In this section we discuss the initiation step of free-radical polymerization. This discussion is centered around initiators and their decomposition behavior. The first requirement for an initiator is that it be a source of free radicals. In addition, the radicals must be produced at an acceptable rate at convenient temperatures have the required solubility behavior transfer their activity to... 

When results are compared for polymerization experiments carried out at different frequencies of blinking, it is found that the rate depends on that frequency. To see how this comes about, we must examine the variation of radical concentration under non-stationary-state conditions. This consideration dictates the choice of photoinitiated polymerization, since in the latter it is almost possible to turn on or off—with the blink of a light—the source of free radicals. The qualifying almost in the previous sentence is actually the focus of our attention, since a short but finite amount of time is required for the radical concentration to reach [M-] and a short but finite amount of time is required for it to drop back to zero after the light goes out. 

Remember from Sec. 1.3 that graft copolymers have polymeric side chains which differ in the nature of the repeat unit from the backbone. These can be prepared by introducing a prepolymerized sample of the backbone polymer into a reactive mixture—i.e., one containing a source of free radicals—of the side-chain monomer. As an example, consider introducing polybutadiene into a reactive mixture of styrene ... 

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

List two photoacceptors in addition to nitrogen dioxide that provide an initial source of free radicals. 

Another quite general source of free radicals is the decomposition of azo compounds. The products are molecular nitrogen and the radicals derived from the substituent groups ... 

The acyl derivatives of A-hydroxypyridine-2-thione are a synthetically versatile source of free radicals. These eompounds are readily prepared from reactive acylating agents, sueh as acyl chlorides, and a salt of A-hydroxypyridine-2-thione ... 

If there is an external source of free radicals (e.g. from thermal initiation in S polymerization or from an added conventional initiator) eq. 5 may again apply. The rate of polymerization becomes independent of the concentration oflX and, as long as the number of radicals generated remains small with respect to [IX] , a high fraction of living chains and low dispersilies is still possible. The validity of these equations has been confirmed for NMP and with appropriate modification has also been shown to apply in the case of ATRP.3... 

The origin of these radical species is also not known. It is often considered that they may result from recoil, either from the original molecule or from fragments of other molecules following collision Perhaps the most commonly assumed, and the most likely, source of free radicals is radiolysis of the target compound The pre-... 

These extensive alterations in cell structure and the biochemical machinery are indicative of entry into an ametabolic condition. In this condition damage from free radicals is potentially decreased, certainly the loss of chlorophyll and chloroplast structure removes a major source of free radical generation. About 50% of the extremely desiccation tolerant monocots exhibit extensive loss of chlorophyll and ultrastructural organisation when desiccated. Dicots, ferns and bryophytes retain most of their chlorophyll and exhibit small changes in structure when dry (see Gaff,... 

At the conclusion of the induction period due to oxygen, polymerization sets in at a rate exceeding that for pure monomer under the same conditions. The polymeric peroxides apparently furnish a source of free radicals. Oxygen therefore combines the roles of inhibitor, comonomer, and (indirectly) of initiator. 

At the present time it is difficult to single out any one factor that could be held ultimately responsible for cell death after cerebral ischaemia. Recent studies, however, have provided us with sufficient evidence to conclude that free radical damage is at least one component in a chain of events that leads to cell death in ischaemia/reperfiision injury. As noted earlier in this review, much of the evidence for free radicals in the brain and the sources of free radicals come from studies in animals subjected to cerebral ischaemia. Perhaps the best evidence for a role for free radicals or reactive oxygen species in cerebral ischaemia is derived from studies that demonstrate protective effects of antioxidants. Antioxidants and inhibitors of lipid peroxidation have been shown to have profound protective effects in models of cerebral ischaemia. Details of some of these studies will be mentioned later. Several reviews have been written on the role of oxygen radicals in cerebral ischaemia (Braughler and HaU, 1989 Hall and Btaughler, 1989 Kontos, 1989 Floyd, 1990 Nelson ef /., 1992 Panetta and Clemens, 1993). 

Pang, C.Y. (1990). Ischaemia-induced reperfusion injury in muscle flaps pathogenesis and major source of free radicals. J. Reconstr. Microsuig. 6, 77-83. 

Essential hypertension, whose prevalence is increased nearly two-fold in the diabetic population, may be another source of free-radical activity. The vascular lesions of hypertension can be produced by free-radical reactions (Selwign, 1983). In the recent Kuopio Ischaemic Heart Risk Factor Study in Finnish men, a marked elevation of blood pressure was associated with low levels of both plasma ascorbate and serum selenium (Salonen etal., 1988). A few studies report a hypotensive effect of supplementary ascorbate in patients with hypertension, but the actual changes in both systolic and diastolic pressure after ascorbate were not statistically significant in comparison with placebo (Trout, 1991). 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Peroxidic groups in oxidized polyolefins have frequently been employed as sources of free radicals to allow grafting of vinyl monomers to polyolefins (2f[). Some of the products from the gas reactions also have interesting potential as reactive sites. For example, chloroformate groups are well known to react with alcohols, and amines 2J[). Thus chloroformate groups could be useful for example in coupling highly oriented polyolefin fibres to resins such as epoxy based systems. 

In summation, as a result of interaction with free radicals, carotenoids can themselves become a source of free radicals and may induce further damaging reactions. 

The basic reactions of Tetralin and derivatives have been extended to the use of 1-13C labels and 1,2-dihydronaphthalene, with and without a source of free radicals. The studies with Tetralin were monitored equally well with C-NMR and GLC techniques. The rate constant for the conversion of Tatralin to methyl indan in the presence of dibenzyl at 450°C was 6.4 x 10 min i which is consistent with that previously reported [2]. 

The source of free radicals is multiplied under these circumstances, arachidonic acid metabolism, activation of xanthine oxidase, perturbation of electron flow within the respiratory chain, and NOS activation. Structurally, excitotoxicity is generally described as a necrotic process involving initial swelling of the cell and of the endoplasmic reticulum, clumping of chromatin, followed by swelling of the... 

It has been shown [90] that the homogeneous dissociation of methane is the only primary source of free radicals and it controls the rate of the overall process. This reaction is followed by a series of consecutive and parallel reactions with much lower activation energies. After the formation of acetylene (C2H2), a sequence of very fast reactions occurs, leading to the production of higher unsaturated and aromatic hydrocarbons and finally carbon ... 

This means that the main source of free radicals are reactions of dioxygen with impurities and residues of the technological catalyst. 

Xanthine oxidase, a widely used source of superoxide, has been frequently applied for the study of the effects of superoxide on DNA oxidation. Rozenberg-Arska et al. [30] have shown that xanthine oxidase plus excess iron induced chromosomal and plasmid DNA injury, which was supposedly mediated by hydroxyl radicals. Ito et al. [31] compared the inactivation of Bacillus subtilis transforming DNA by potassium superoxide and the xanthine xanthine oxidase system. It was found that xanthine oxidase but not K02 was a source of free radical mediated DNA inactivation apparently due to the conversion of superoxide to hydroxyl radicals in the presence of iron ions. Deno and Fridovich [32] also supposed that the single strand scission formation after exposure of DNA plasmid to xanthine oxidase was mediated by hydroxyl radical formation. Oxygen radicals produced by xanthine oxidase induced DNA strand breakage in promotable and nonpromotable JB6 mouse epidermal cells [33]. 

There is a major difference between the role of free radicals in cancer and other pathologies such as cardiovascular diseases, hypertension, diabetes mellitus, etc. In contrast to the latter diseases where the sources of free radical overproduction are well established (vascular cells and macrophages in cardiovascular diseases and leukocytes in inflammation), the origin and the levels of free radical production in tumor cells are still uncertain. 

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