Propagation of free radicals

It has been shown in many studies that protective effects of carotenoids can be observed only at small carotenoid concentrations, whereas at high concentrations carotenoids exert pro-oxidant effects via propagation of free radical damage (Chucair et al., 2007 Lowe et al., 1999 Palozza, 1998, 2001 Young and Lowe, 2001). For example, supplementation of rat retinal photoreceptors with small concentrations of lutein and zeaxanthin reduces apoptosis in photoreceptors, preserves mitochondrial potential, and prevents cytochrome c release from mitochondria subjected to oxidative stress induced by paraquat or hydrogen peroxide (Chucair et al., 2007). However, this protective effect has been observed only at low concentrations of xanthophylls, of 0.14 and 0.17 pM for lutein and zeaxanthin, respectively. Higher concentrations of carotenoids have led to deleterious effects (Chucair et al., 2007). 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

A compound added to prevent the propagation of free-radical chain reactions. In most cases, the inhibitor reacts to form a radical that is too stable to propagate the chain, (p. 161)... 

Vitamin E is the major hydrophobic chain-breaking antioxidant that prevents the propagation of free radical reactions in the lipid components of membranes, vacuoles and plasma lipoproteins. 

Initiators often trigger free radical reactions, because they easily form free radicals while inhibitors consume free radicals and thus prevent the initiation or propagation of free radical reactions. 

HALS additives function by Interfering with the propagation of free radicals via the following general reactions ... 

By this moment the chain of events—since the initiation—results in fracture of the first polymer chain, forming of two free radicals ready to break two more polymer molecules, just-about fracture of a second polymer molecule, and a further propagation of free radicals-induced polymer breakage ... 

Principia Partners, 1, 2, 44, 45 Procell, 58, 60, 110, 311, 587 Processability, 61 Processing speed, 50 Processing window, 660, 664 Product crumbling, 496, 567 Product failure, 496 Product property, 226 Progressive degradation, 502 Propagation of free radicals, 208 Propioconazole, 416 Propylene plastics, 68 PVC-based boards, 36... 

In this chapter, we review the theoretical work on thermal FP. We begin with a derivation of a base mathematical model which governs propagation of free-radical polymerization waves. We demonstrate that this model reduces in a limiting case to the famous gasless combustion (GC) model, which has been extensively studied in the context of SHS. We discuss some theoretical approaches to the simpler GC model and then apply them to the base model of free-radical FP. Some extensions of the base model are also discussed. 

The kinetic stability of a radical is largely controlled by steric factors. When the radical center is crowded, the radical becomes less reactive and persists longer under normal conditions (it has a longer life-time). Aromatic compounds that can form allylic radicals show similar benzylic stabilization. If the radical center is sterically crowded by bulky tertiary butyl substituents, the allylic radical intermediates formed by hydrogen transfer have kinetic stability that imparts important antioxidant properties (see Chapter 9). Thus, when phenolic compounds contain three bulky tertiary butyl substituents, they form persistent radicals after hydrogen donation and inhibit lipid oxidation by intermpting the propagation of free radicals (see Chapter 9). 

Strictly speaking, any model based on the time-independent thermodynamics cannot be used to adequately predict the concentration of monomer in latex particles during Smith-Ewart Interval II. This is because the free radical polymerization of monomer in the discrete latex particles is governed by the simultaneous kinetic events such as the generation of free radicals in the continuous aqueous phase, the absorption of free radicals by the particles, the propagation of free radicals with monomer molecules in the particles, the bimolecular termination of free radicals in the particles, and the desorption of free radicals out of the particles. The equilibrium (or saturation) concentration of monomer in the growing latex particles may not be achieved if the rate of consumption of monomer in the major reaction loci is much faster than that of diffusion of monomer molecules from the monomer droplets to the reaction loci. Therefore, the equilibrium concentration of monomer in the latex particles represents an upper limit that is ultimately attainable in the course of polymerization. Nevertheless, the general... 

Cellular systems have evolved a powerful and complex antioxidant defence system to limit inappropriate exposure to these stressors. a-Tocopherol is quantitatively the most important chain-breaking antioxidant in plasma and biological membranes. The antioxidant activities of chain-breaking antioxidants are determined primarily by how rapidly they scavenge peroxyl radicals, thereby preventing the propagation of free radical reactions. When the chromanol phenolic group of a-tocopherol (TOH) encounters a ROO it forms hydroperoxide (ROOH), and in the process a tocopheroxyl radical (TO ) is formed ... 

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