Free radical local concentration

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

The UV radiation adsorbed by chromophoric DOM stimulates production of free radical (singlet) O2. This leads to high concentrations of the free radicals within and around the CDOM, creating local conditions of high reactivity. Thus, chromophoric DOM, which tends to be HMW, can be thought of as a photochemical micro reactor in which free radical oxidations are promoted. Given its macromolecular nature, CDOM... 

ZDDP decomposes by a number of routes involving free radical and redox processes. Film composition varies from the iron-rich bonding layer, through the zinc phosphate layer to the outer surface, which contains organic material incompletely converted to precursor species. The polyphosphate chain length may vary as a function of depth into the film and the conditions under which the film is formed. Formation of polyphosphate tribofilms from simple ZDDP solutions is promoted by self-association of ZDDP molecules, which increases the local concentration of ZDDP. 

Because of their different hydrophilicities, the two free radicals formed at the same time can separate from each other quickly which can eliminate the cage effect. In a micelle, the local BA concentration may be quite high. Once a micelle is initiated, a number of BA molecules may be added quickly. As a result, some short BA blocks would be incorporated into a poly(MAETAC) chain to form something like multi-block copoly(MAETAC-BA), as shown in Fig. 19 [170]. Surfactant should stabilize the BA blocks so that the block copolymer remains in the aqueous phase. 

Radicals.—The measurement of emission intensities from electronically excited small free radicals has become an important means of determining radical concentrations in hostile environments such as flames. When combined with laser excitation, the technique is very powerful, offering temporal, spectral, and spatial resolution. Just has reviewed laser techniques for the measurement of both radical concentrations and local temperatures in flames, and has demonstrated the use of laser-induced saturated fluorescence to measure the concentrations of CH and OH radicals in low-pressure acetylene-oxygen flames. Vanderhoff ei al. used a novel Kr " and Ar laser intracavity technique to... 

As we noted in Section 4.01.1, the ability of the troposphere to chemically transform and remove trace gases depends on complex chemistry driven by the relatively small flux of energetic solar UV radiation that penetrates through the stratospheric O3 layer (Levy, 1971 Chameides and Walker, 1973 Crutzen, 1979 Ehhalt et al., 1991 Logan et al, 1981 Ehhalt, 1999 Crutzen and Zimmerman, 1991). This chemistry is also driven by emissions of NO, CO, and hydrocarbons and leads to the production of O3, which is one of the important indicators of the oxidizing power of the atmosphere. But the most important oxidizer is the hydroxyl free radical (OH), and a key measure of the capacity of the atmosphere to oxidize trace gases injected into it is the local concentration of hydroxyl radicals. 

At this time the evidence that hyperoxic-induced lung injury was due to excessive oxygen free radical production was still largely indirect. The next major event which specifically addressed this question was the important experiments of Crapo and colleagues in the early 1980 s [10]. These studies conducted with lung tissue were similar to those performed earlier in liver [11] and heart [12]. Crapo and co-workers found that superoxide production, measured as CN-insensitive respiration, was increased in lung slices when exposed to 100% O2 rather than air [10]. This work was extended to the measurement of O2 production in submitochondrial particles [13]. Clearly then exposure of tissues to elevated concentrations of O2 leads to ROS production, the extent of which, in association with local antioxidant defences, will determine the redox balance of the tissue. 

Copper-containing intrauterine contraceptive devices (10-12) became popular because local inflammatory reactions in the endometrium are more marked and the contraceptive effect is thus more pronounced (SEDA-21, 234) (13). In addition, copper ions released from intrauterine contraceptive devices reach concentrations in the luminal fluids of the genital tract that are toxic to spermatozoa and embryos. The ability of copper to induce the generation of free radicals and the formation of malonaldehyde may be involved in its contraceptive effect. 

Reaction of [ HCo(CN )5 with activated alkenes, such as ,/i-unsaturated acids, produces free-radical intermediates (see also Section 6.2.2.3), and therefore hydrogenation of these substrates is often characterized by low yields and numerous side-products. In the presence of j3-CD the yield of acrylic acid hydrogenation increased to 81% [73]. Both neutral (Brij 35) and ionic (SDS, CTAB) micellar agents were shown to increase the rate of hydrogenation of cinnamic acid as a result of an increased local concentration of the substrate within the micelles [74] (cf. Section 4.5). 

In mammalian systems, nitroaromatic compounds are further reduced to amines and/or hydroxylamines, which may subsequently form DNA and protein adducts. These stable metabolites may be formed by the reduced oxygen tension and high local single-electron transferring enzyme concentration, for example, P-450R in microsomes may favor the free radical dismutation over their reoxidation by oxygen (Equation 9.8). Subsequently, the nitroso compounds formed (Equations 9.7 and 9.8) will be reduced to hydroxylamines (ArNHOH) and/or amines (ArNHj). The stable metabolites may also be formed by the two-electron reduction of nitroaromatics by certain flavoenzymes. In fact, the enzymatic two-electron reduction may be considered as the four-electron reduction, since after the first two-electron (hydride) transfer, the reduction of an intermediate nitroso compound to hydroxylamine (ArNHOH) proceeds faster [55] ... 

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