Hydroxyl radicals, and oxidation

Some microorganisms in culture show methionine-dependent ethylene formation. In studies with Escherichia coli, 2-oxo-4-methylthiobutyrate (KMB) produced from methionine by transamination was suggested as the precursor of ethylene [19], and subsequently a cell-free system which produced ethylene from KMB in the presence of NAD(P)H, EDTA-Fe and oxygen was established [20]. An enzyme which catalysed a similar ethylene-forming activity was purified from Cryptococcus albidus [15]. The purified enzyme of molecular mass 62 kDa turned out to be NADH EDTA-Fe oxidoreductase. The proposed mechanism involves reduction of EDTA-Fe to EDTA-Fe by the enzyme, reduction of oxygen to superoxide by EDTA-Fe, of hydrogen peroxide to hydroxyl radical, and oxidation of KMB by hydroxyl radical to ethylene. However, an extensive physiological evaluation of this enzyme must be done before it can... 

A significant increase in 8-hydroxy-2 -deoxyguano-sine in the blood of 17 workers exposed to styrene compared to 67 non-exposed healthy volunteers provided a good indication that styrene exposure can result in generation of hydroxyl radicals and oxidative DNA damage (Marczynski et al. 1997). 

At elevated temperatures, methylene carbons cleave from aromatic rings to form radicals (Fig. 7.44). Further fragmentation decomposes xylenol to cresols and methane (Fig. 7.44a). Alternatively, auto-oxidation occurs (Fig. 1.44b ). Aldehydes and ketones are intermediates before decarboxylation or decarbonylation takes place to generate cresols and carbon dioxide. These oxidative reactions are possible even in inert atmospheres due to the presence of hydroxyl radicals and water.5... 

In broad terms, the following types of reactions are mediated by the homolytic fission products of water (formally, hydrogen, and hydroxyl radicals), and by molecular oxygen including its excited states—hydrolysis, elimination, oxidation, reduction, and cyclization. 

These have already been noted in the context of hydroxyl radical-initiated oxidations, and reference should be made to an extensive review by Worobey (1989) that covers a wider range of abiotic oxidations. Some have attracted interest in the context of the destruction of xenobiotics, and reference has already been made to photochemically induced oxidations. 

Thus, it is quite natural to consider the properties of other acceptor particles, for example, atoms of nitrogen, aminoradicals, hydroxyl radicals, and many others, adsorbed on oxide semiconductors. However, the properties of these particles are not studied yet. As to adsorbed donor particles, it was found in our experiments that liquid media with different values of the dielectric constant do not have any influence on the properties of adsorbed atoms of hydrogen. 

Results of a more recent series of investigations suggest that lead may cause hypertension in rats by increasing reactive oxygen species, which act as vasoconstrictors, and decreasing nitric oxide, a vasodilator released by the endothelium. The reactive oxygen species may be the hydroxyl radical, and did not appear to be the superoxide anion (Ding et al. 1998). 

It is also possible that COS might be a precursor for CS2 in soil and vegetation chemical analogies have been proposed.12 The reverse reaction, CS2—>COS, occurs in our oxidative atmosphere via hydroxyl radical and other gas phase oxidants. 

W. P., Katsuki, S., Kimura, H., Guanylate cyclase activation by azide, nitro compounds, nitric oxide, and hydroxyl radical and inhibition by... 

Figure 18.16 Hypothetical model for the metallobiology of AP in Alzheimer s disease. (From Bush, 2003. Copyright 2003, with permission from Elsevier.) The proposed sequence of events (1) concentration of iron and copper increase in the cortex with aging. There is an overproduction of APP and AP in an attempt to suppress cellular metal-ion levels. (2) Hyper-metallation of AP occurs which may facilitate H202 production. (3) Hyper-metallated AP reacts with H202 to generate oxidized and cross-linked forms, which are liberated from the membrane. (4) Soluble AP is released from the membrane and is precipitated by zinc which is released from the synaptic vesicles. Oxidized AP is the major component of the plaque deposits. (5) Oxidized AP initiates microglia activation. (6) H202 crosses cellular membranes to react with Cu and Fe, and generate hydroxyl radicals which oxidize a variety of proteins and lipids.

The anion of di-isopropyl phosphorothiolothionic acid (26) reduces hydroxyl radicals, and the radical (27) so produced is detectable by e.s.r.33 Attempts to observe these radicals by photolysis of the free acid were unsuccessful. However, the use of a spin trap (e.g. TV-methylene-t-butylamine iV-oxide) enabled radicals in this system and other closely related systems [e.g. with (28)] to be observed by e.s.r. spectroscopy. 

Just as the fate of H radicals is crucial in determining the rate of the H2—02 reaction sequence in any hydrogen-containing combustion system, the concentration of hydroxyl radicals is also important in the rate of CO oxidation. Again, as in the H2—02 reaction, the rate data reveal that reaction (3.44) is slower than the reaction between hydroxyl radicals and typical hydrocarbon species thus one can conclude—correctly—that hydrocarbons inhibit the oxidation of CO (see Table 3.1). 

Of course, all the appropriate higher-temperature reaction paths for H2 and CO discussed in the previous sections must be included. Again, note that when X is an H atom or OH radical, molecular hydrogen (H2) or water forms from reaction (3.84). As previously stated, the system is not complete because sufficient ethane forms so that its oxidation path must be a consideration. For example, in atmospheric-pressure methane-air flames, Wamatz [24, 25] has estimated that for lean stoichiometric systems about 30% of methyl radicals recombine to form ethane, and for fuel-rich systems the percentage can rise as high as 80%. Essentially, then, there are two parallel oxidation paths in the methane system one via the oxidation of methyl radicals and the other via the oxidation of ethane. Again, it is worthy of note that reaction (3.84) with hydroxyl is faster than reaction (3.44), so that early in the methane system CO accumulates later, when the CO concentration rises, it effectively competes with methane for hydroxyl radicals and the fuel consumption rate is slowed. 

Besides ozone, the main indicator of photochemical pollution, other important concomitant products are peroxyacetylnitrate (PAN), hydrogen peroxide, nitrogen dioxide, hydroxyl radicals and various aldehydes that are both products and primary pollutants, particles, sulfates, nitrates, ammonium, chloride, water, and various types of oxygenated organic compounds. The most important precursors of photochemical pollution are nitric oxide and hydrocarbons. The measurement procedures for the hydrocarbons are not as highly developed as those for ozone and the nitrogen oxides. 

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