Environment Free-radical

Subczynski, W. K., J. Widomska, and J. B. Feix. 2009. Physical properties of lipid bilayers from EPR spin labeling and their influence on chemical reactions in a membrane environment. Free Radic. Biol. Med., 46, 707-718. 

O Neill P (1989) Pulse radiolysis of DNA - effects of environment. Free Radical Res Commun 6 153-154... 

AH components of the reaction mixture, whatever their source, are subject to the same kind of radical attacks as the starting substrate(s). Any free-radical oxidation is inevitably a cooxidation of substrate(s) and products. The yields of final products are deterrnined by two factors (/) how much is produced in the reaction sequence, and (2) how much product survives the reaction environment. By kinetic correlations and radiotracer techniques, it is... 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

Most commercial processes involve copolymerization of ethylene with the acid comonomer followed by partial neutralization, using appropriate metal compounds. The copolymerization step is best carried out in a weU-stirred autoclave with continuous feeds of all ingredients and the free-radical initiator, under substantially constant environment conditions (22—24). Owing to the relatively high reactivity of the acid comonomer, it is desirable to provide rapid end-over-end mixing, and the comonomer content of the feed is much lower than that of the copolymer product. Temperatures of 150—280°C and pressures well in excess of 100 MPa (1000 atm) are maintained. Modifications on the basic process described above have been described (25,26). When specific properties such as increased stiffness are required, nonrandom copolymers may be preferred. An additional comonomer, however, may be introduced to decrease crystallinity (10,27). 

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. 

A radical polymerization involves free radical ends which of course do not associate and which interact only weakly with solvents. Consequently, the early investigators assumed that the course of propagation of radical polymerization is independent of the environment (see, for example, the recent monograph by Walling60). Actually, more recent studies, notably by Russell,36 showed that the nature of the solvent sometimes might considerably affect even the course of radical reactions. Therefore, unusual behavior of the propagation step might be expected in certain solvents. 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

Equation 6 would hold for a family of free radical initiators of similiar structure (for example, the frarw-symmetric bisalkyl diazenes) reacting at the same rate (at a half-life of one hour, for example) at different temperatures T. Slope M would measure the sensitivity for that particular family of reactants to changes in the pi-delocalization energies of the radicals being formed (transition state effect) at the particular constant rate of decomposition. Slope N would measure the sensitivity of that family to changes in the steric environment around the central carbon atom (reactant state effect) at the same constant rate of decomposition. 

The main environmental issue concerned with VOCs is their ability to form low-level ozone and smog through free radical air oxidation processes. The EPA has published a list detailing a number of adverse health effects, which are now thought to originate from the presence of VOCs in the environment, including ... 

As a species that has evolved in such an aggressive environment we have developed mechanisms that both defend against oxidative damage and can undertake damage repair. Free radical generation in vivo has even been adopted as a mechanism to protect against physical, chemical and biological injury. 

Coschigano PW, TS Wehrman, LY Yonng (1998) Identification and analysis of genes involved in anaerobic tolnene metabolism by strain Tl pntative role of a glycine free radical. Appl Environ Microbiol 64 1650-1656. 

Hunt, J. (1994). Glucose chemistry and atherosclerosis in diabetes mellitus. In Free Radicals in the Environment, Medicine and Toxicology. Current Aspects and Current Highlights (eds. H. Nohl, H. Esterbauer and C. Rice-Evans) pp. 137-162, Richelieu Press, London. 

While many biological molecules may be targets for oxidant stress and free radicals, it is clear that the cell membrane and its associated proteins may be particularly vulnerable. The ability of the cell to control its intracellular ionic environment as well as its ability to maintain a polarized membrane potential and electrical excitability depends on the activity of ion-translocating proteins such as channels, pumps and exchangers. Either direct or indirect disturbances of the activity of these ion translocators must ultimately underlie reperfiision and oxidant stress-induced arrhythmias in the heart. A number of studies have therefore investigated the effects of free radicals and oxidant stress on cellular electrophysiology and the activity of key membrane-bound ion translocating proteins. 

Free radicals are short-lived, highly-reactive transient species that have one or more unpaired electrons. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of, and governed by, many elementary processes involving free radicals. Studies of these elementary processes are pivotal to assessing reaction mechanisms in atmospheric and combustion chemistry, and to probing potential energy surfaces (PESs) and chemical reactivity. 

With free radical initiator added (e.g., H202), the reaction takes minutes to complete. Based on the surfactant vesicles characterization derived from current investigations, it is suggested that these surfactants could retain their effectiveness necessary for bitumen recovery in the reservoir environment for a number of years. 

The studies of this equilibrium as a function of pH enabled the estimation of the absolute one-electron reduction potentials of CAR + in an aqueous micellar environment (Edge et al. 2000, Burke et al. 2001b) (see Table 14.12 for typical results). As can be seen, the potentials of all the dietary carotenoid radical cations are very similar but LYC + has the lowest potential implying that it is the best carotenoid antioxidant against free radicals (of course, this is an oversimplification, see above). 

Sym-octahydrophenanthrene (HgPh) would be expected to follow the same rearrangement-dehydrogenation reactions as Tetralin, except with more isomer and product possibilities. The reactions shown in Figure 1 illustrate the many structures expected from sym-HgPh in the presence of free radical acceptors. Unlike Tetralin, hydrophenanthrenes have multiple structures which each, in turn, form various isomers. The amounts of these isomers are dependent upon the type of hydrogen-transfer reactions and the environment of the system. 

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