Hydroxyl radical oxidation, formation

Small alkyl radicals, such as ethyl, are oxidized predominantly to the corresponding alkene because the isomerization of an a-hydroperoxy-alkyl radical to the corresponding alkylperoxy radical competes successfully with its decomposition to an oxiran and a hydroxyl radical. The formation of alkenes via a-hydroperoxyalkyl radicals (and not vice-versa) cannot be excluded, however. 

Hydroxyl radical oxidation of thiocyanate ion in acid and neutral solution leads to the formation of a transient species which absorbs strongly at a wavelength of 475nm. Figure 2 shows the growth in absorption of this species at intervals from 20ns to 800ns. 

Reaction of Neurospora PPO with uniformly labeled phenol, under conditions similar to those used by Wood and Ingraham (1965), led to covalent incorporation of < 0.03 mol labeled phenol/mol PPO (Pfiffner et al., 1981). The enzyme lost its activity during the reaction there was also a loss of one mol of histidine and one mol of copper per mol of Neurospora PPO. Brooks and Dawson (1966) had shown earlier that k inactivation of mushroom PPO was accompanied by loss of copper from the enzyme. Lerch (1978) proposed that inactivation of Neurospora PPO may have occurred as a result of the oxidation of histidine due to formation of singlet oxygen or hydroxyl radicals during the reaction. However, Kahn et al. (1982) could not detect superoxide ion (Oj) or hydroxyl radical (0H ) formation during the reaction of mushroom PPO with substrates. 

In this model, for every methane molecule which reacts, the sequence leads to 4 ozone and 2 hydroxyl radicals, extra. Formation of ozone in the lower troposphere is therefore catalysed by photochemical oxidation of organic molecules, but it does require comparatively high levels of NO (mixing ratio > 5 — 10 x 10 ) to be present. If it goes to completion, OH can react further with CO to make CO2 thus completing the oxidation of methane (Scheme 5.2). At low NO levels, the net reaction is the destruction of ozone via the reaction with CO [Scheme 5.2b]. 

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

Hydroxyl radicals, generated from hydrogen peroxide and titanium trichloride, add to the sulfur atom of 2-methylthiirane 1-oxide leading to the formation of propene and the radical anion of sulfur dioxide (Scheme 102) (75JCS(P2)308). 

The reverse reaction (that is, the oxidation of a vinyl radical by Fe to the corresponding vinyl cation) may be involved in the reaction of the dimethyl ester of acetylenedicarboxyUc acid 261 with Fenton s reagent [Fe —H2O2, (217)] (216). When 261 was treated with Fe —H2O2 and the reaction mixture was extracted with ether, a small amount of furan 262 was isolated. A possible mechanism (216) for its formation may be addition of hydroxyl radical to the triple bond of 261, followed by addition of the intermediate vinyl radical to a second molecule of 261 and oxidation of the resulting radical with Fe to the corresponding vinyl cation, followed by cyclization to 262, as shown in Scheme XX. 

These relatively facile oxidations may involve hydroxyl transfer, in preference to radical-cation formation, viz. 

The transformation of isoquinoline has been studied both under photochemical conditions with hydrogen peroxide, and in the dark with hydroxyl radicals (Beitz et al. 1998). The former resulted in fission of the pyridine ring with the formation of phthalic dialdehyde and phthalimide, whereas the major product from the latter reaction involved oxidation of the benzene ring with formation of the isoquinoline-5,8-quinone and a hydroxylated quinone. 

Combined treatment of atrazine with ozone and H2O2 resulted in retention of the triazine ring, and oxidative dealkylation with or without replacement of the 2-chloro group by hydroxyl (Nelieu et al. 2000). Reaction with ozone and hydroxyl radicals formed the analogous products with the additional formation of the acetamido group from one of the N-alkylated groups (Acero et al. 2000). 

Ascorbate is known to act as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, reduced ascorbate can react non-enzymatically with molecular oxygen to produce dehydroascorbate and hydrogen peroxide. Also, ascorbate in the presence of light, hydrogen peroxide and riboflavin, or transition metals (e.g. Fe, Cu " ), can give rise to hydroxyl radicals (Delaye and Tardieu, 1983 Ueno et al., 1987). These phenomena may also be important in oxidative damage to the lens and subsequent cataract formation. 

Patients in which oxidative damage may be an important aetiological factor cataract formation include those with Down s syndrome, since there is now evidence that they have increased indices of free-radical activity and lipid peroxidation. It has been su ested that this is due to the increased levels of Cu/Zn-SOD (carried on chromosome 21) generating increased concentrations of hydrogen peroxide (Bras etal., 1989). In the presence of superoxide radicals these produce highly reactive hydroxyl radicals. 

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