Hydroxyl radical sources

From the above it is clear that DMPO can undergo the addition-oxidation mechanism with water as the nucleophile, provided a suitable oxidant is present. With a primary alcohol competing, the O-connected alkoxy spin adduct is formed in addition to HO-DMPO". On the other hand, with a hydroxyl radical source a competing alcohol will undergo hydrogen abstraction by HO" and form an a-hydroxyalkyl radical which forms a C-connected spin adduct. This criterion clearly can distinguish between the two mechanisms at least in model systems (for recent examples, see Reszka and Chignell, 1995 Janzen et al., 1995 Thomas et al., 1996). 

Adam W, Hartung J, Okamoto FI, Saha-Moller CR, Spehar K (2000b) N-hydroxy-4-(4-chlorophenyljthiazole-2(3//)-thione as a photochemical hydroxyl-radical source photochemistry and oxidative damage of DNA (strand breaks) and 2 -deoxyguanosine (8-oxodG formation). Photochem Photobiol 72 619-624... 

Whalley LK, Edwards PM, Fumeaux KL, Goddard A, Ingham T, Evans MJ, Stone D, Hopkins JR, Jones CE, Karunaharan A, Lee JD, Lewis AC, Monks PS, Moller S, Heard DE (2011) Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest. Atmos Chem Phys 11 7223-7233... 

HaOj (a hydroxyl radical source) are similar to those formed by peroxidase but occur in different proportions (167). Therefore peroxidatic oxidations and oxidations by hydroxyl radical may involve some common step or steps, although hydroxyl radical is not formed by peroxidase (266,150,751, and 176, cited by 266). This step may be the removal of one electron from donor, forming radicals which couple at positions of most probable free electron location (compare 604) (equations 30 and 31). 

Photolysis of ferric ions (and also nitrate ions) has also been used" as a hydroxyl radical source for the degradation of 1,4-dichlorobenzene. The major product, 2,5-dichlorophenol, was formed by hydrox-ylation at one of the unsubstituted ring positions. 

Adam, W., Ballmaier, D., Epe, B., Grimm, Gn., and Saha-MoUer, C. R., N-hydroxypyridinethiones as photochemical hydroxyl radical sources for oxidative DNA-damage, Angew. Chem. Int. Ed. Engl, 34, 2156,1995. 

Adam, W., Marquardt, S., and Saha-Moller, C.R., Oxidative DNA damage in the photolysis of N-hydroxy-2-pyridone, a specific hydroxyl-radical source, Photochem. PhotobioL, 70, 287, 1999. 

Adam, W., Marquardt, S., and Saha-Moller, C.R., Oxidative DNA damage in the photolysis of N-hydroxy-2-pyridone, a specific hydroxyl-radical source, Photochem. Photobiol, 70, 287-291, 1999. Adam, W., Hartung, J., Okamoto, H., Marquardt, S., Nau, W.M., Pischel, U., Saha-MoUer, C.R., and Spehar, K., Photochemistry of N-isopropoxy-substituted 2(lEf)-pyridone and 4-p-tolylthiaz-ole-2(3H)-thione alkoxyl-radical release (spin-trapping, EPR and transient spectroscopy) and its significance in the photooxidative induction of DNA strand breaks, /. Org. Chem., 67, 6041-6049, 2002. 

A third source of initiator for emulsion polymerisation is hydroxyl radicals created by y-radiation of water. A review of radiation-induced emulsion polymerisation detailed efforts to use y-radiation to produce styrene, acrylonitrile, methyl methacrylate, and other similar polymers (60). The economics of y-radiation processes are claimed to compare favorably with conventional techniques although worldwide iadustrial appHcation of y-radiation processes has yet to occur. Use of y-radiation has been made for laboratory study because radical generation can be turned on and off quickly and at various rates (61). 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

One of the important consequences of neuronal stimulation is increased neuronal aerobic metabolism which produces reactive oxygen species (ROS). ROS can oxidize several biomoiecules (carbohydrates, DNA, lipids, and proteins). Thus, even oxygen, which is essential for aerobic life, may be potentially toxic to cells. Addition of one electron to molecular oxygen (O,) generates a free radical [O2)) the superoxide anion. This is converted through activation of an enzyme, superoxide dismurase, to hydrogen peroxide (H-iO,), which is, in turn, the source of the hydroxyl radical (OH). Usually catalase... 

These equations identify the dominant source and loss processes for HO and H02 when NMHC reactions are unimportant. Imprecisions inherent in the laboratory measured rate coefficients used in atmospheric mechanisms (for instance, the rate constants in Equation E6) can, themselves, add considerable uncertainty to computed concentrations of atmospheric constituents. A Monte-Carlo technique was used to propagate rate coefficient uncertainties to calculated concentrations (179,180). For hydroxyl radical, uncertainties in published rate constants propagate to modelled [HO ] uncertainties that range from 25% under low-latitude marine conditions to 72% under urban mid-latitude conditions. A large part of this uncertainty is due to the uncertainty (la=40%) in the photolysis rate of 0(3) to form O D, /j. 

This means that the observed change in M mainly reflects a change in the source flux Q or the sink function. As an example we may take the methane concentration in the atmosphere, which in recent years has been increasing by about 0.5% per year. The turnover time is estimated to be about 10 years, i.e., much less than Tobs (200 years). Consequently, the observed rate of increase in atmospheric methane is a direct consequence of a similar rate of increase of emissions into the atmosphere. (In fact, this is not quite true. A fraction of the observed increase is probably due to a decrease in sink strength caused by a decrease in the concentration of hydroxyl radicals responsible for the decomposition of methane in the atmosphere.)... 

To test the validity of this hypothesis, a 30% solution of hydrogen peroxide, a good source of hydroxyl radicals, was injected into the reactor during photockalytic methane... 

For example, a microwave discharge on H2 in an inert diluent, such as argon gas, is an efficient method for producing H atoms as reactants. Subsequent reaction of these H atoms with NO2 will yield OH and NO, and can serve as a useful source of hydroxyl radicals. These methods of reactant formation are well suited for experiments involving either static or flow reactor systems. 

Acrylonitrile is readily volatile, and significant quantities escape into air during manufacture and use. Volatilization may also occur from hazardous waste sites. In air, acrylonitrile is degraded primarily by reaction with hydroxyl radicals, with an estimated half-life of 5 to 50 hours. Acrylonitrile has been detected in air in the vicinity of various industrial sources at concentrations from 0.1 to 325 ppb, but has not been detected in typical ambient air. 

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]. 

Vione D, Falletti G, Maurino V, Minero C, Pelizzetti E, Malandrino M, Ajassa R, Olariu R-I, Arsene C (2006) Sources and sinks of hydroxyl radicals upon irradiation of natural water samples. Environ Sci Technol 40 3775-3781... 

Tropospheric chemistry is strongly dependent on the concentration of the hydroxyl radical (OH), which reacts very quickly with most trace gases in the atmosphere. Owing to its short boundary layer lifetime ( 1 s), atmospheric concentrations of OH are highly variable and respond rapidly to changes in concentrations of sources and sinks. Photolysis of ozone, followed by reaction of the resulting excited state oxygen atom with water vapour, is the primary source of the OH radical in the clean troposphere ... 

Besides these external processes, formation of ROS may also take place intrac-ellularly. Photooxidative stress, including UVB, stimulates various cellular processes leading to the production of superoxide radicals and hydrogen peroxide, as well as singlet-oxygen and hydroxyl radicals. The sources and production sites of ROS are mainly related to photosynthetic activities such as the pseudocyclic photophosphorylation and the Mehler reaction, which stimulate the accumulation of hydrogen peroxide (Asada 1994 Elstner 1990). 

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