Superoxide and hydroperoxyl radical

Work by Harbour, Chow and Bolton (1974) on the spin adducts of superoxide (or HOO )13 with nitrones paved the way for a number of investigations of superoxide and hydroperoxyl radical chemistry. Harbour and Bolton (1975) used DMPO to trap superoxide formed by spinach chloroplasts in the presence of 02. The signal strength was greatly enhanced when methylviologen was present, consistent with the hypothesis that this bis-pyridinium dication accepts an electron from the primary acceptor of photoprotein I, and then transfers it to molecular oxygen. 

Superoxide Radical. Reactions of the superoxide and hydroperoxyl radicals are studied primarily because of the role of the radicals in normal and abnormal biological systems. Bielski and co-workers (20) compiled a list of reaction rate constants for HO2 and O2 " and other properties of the radicals. Unhke the majority of the radicals discussed here, the self-reactions of HO2 and O2 " are slow (Figure 4). In particular, it is probable that no self-reaction occurs with the superoxide radical itself, and that observed decays result from reaction with H02 or solution wmponents or contaminants. 

The rate constants for two-electron reaction (8) (A+) and one-electron reaction (19) (k 19) are cited in Table 22.3. As seen from Table 22.3, ks values are about 100 1000 times greater than kig values therefore, the production of superoxide (or hydroperoxyl radical) by peroxidases might play an insignificant role compared to the two-electron oxidation of hydroperoxides. 

Ascorbate is a strong reductant with respect to singlet oxygen, superoxide, H2O2, hydroperoxyl radicals, hydroxyl radicals, and hypochlorous acid when occurring at... 

High values of reaction rates for the two dismutation steps confirm the ability of both nitroxides TPO and 3-CP to be SOD mimics. However, as follows from the above mechanism, hydroperoxyl radical and not superoxide must participate in the first dismutation step (Reaction (5)). (As expected, a rate constant for the reaction of nitroxides with superoxide is very low <103 1 mol 1 s-1 [27].) Therefore, superoxide had to be protonated before participating in Reaction (5), which will diminish the total catalytic process at physiological pH and increase it at lower pH values. 

Spin trapping of the superoxide radical anion, as well as that of hydroperoxyl and hydroxyl radicals and related species will be considered later in connection with biological chemistry (pp. 52-54). 

The hydroperoxyl radical formed, "00H, has a pKg of 4.75 and exists in water as a superoxide radical anion 02 Either species in water dismutates to produce O2 and hydrogen peroxide, H2O2, or, possibly, react with dissolved or adsorbed phenol. Dilute H2O2 does... 

The superoxide anion radical (O2 ) is produced when oxygen accepts one electron. This radical has a short lifetime in aqueous solutions, where it mainly undergoes spontaneous dismutation to hydrogen peroxide and oxygen (Reaction 1). The superoxide radical is in equilibrium with its conjugated acid, the hydroperoxyl radical (HO2), which is a stronger oxidant and generally more reactive than O2... 

The high energy electrons are emitted by the chromophore and then are free to imdergo reactions with molecules such as O2, thereby generating highly reactive and imstable species, such as the free radicals O2 (superoxide), H02 (hydroperoxyl), OH... 

The reaction between hydroxide ions and ozone leads to the formation of one superoxide anion radical 02° and one hydroperoxyl radical H02°. 

There is not room here to discuss the detailed mechanisms by which exposure to radiation causes adverse responses. Much of the effects of radiation result from its interaction with water to produce active species that include superoxide (Oi), hydroxyl radical (HO-), hydroperoxyl radical (HOO), and hydrogen peroxide (H202). These species oxidize cellular macromolecules. When DNA is so affected, mutagenesis and carcinogenesis may result. Ionizing radiation can also interact with organic substances to produce a carbonium ion, such as +CH3, that can alkylate nitrogenous bases on DNA. 

ROS include oxygen-based free radicals such as superoxide, hydroxyl, alkoxyl, peroxyl, and hydroperoxyl (Table 1). Other ROS, such as hydrogen peroxide and lipid peroxides, can be converted into free radicals by transition... 

Fig. 20 Oxidative radical hydroxycarboxylation reactions catalyzed by (phthalocyanine)iron (The depiction of the hydroperoxyl radical is formal to account for the correct proton and electron balance. Another formal hydroperoxyl radical (not shown for clarity) results from coupling of the initially generated superoxide and the proton resulting from formation of the azo radical)...

Another active oxygen species is the superoxide radical, 02, and its conjugate acid form, the hydroperoxyl radical, IIO2, and these are also produced in many AOTs, but they are far less active than HO. ... 

Superoxide ion is not highly reactive towards hydrocarbons (i.e., lipids) and is readily removed from biological systems by superoxide dismutase, forming hydrogen peroxide and oxygen. However, the protonated superoxide ion or hydroperoxyl radical, HO, reacts with NADH and alkanes. Although such a reactive... 

The carboxyperoxyl radical anion thus produced should be similar in reactivity to the hydroperoxyl radical, HO. The nucleophilic activity of the superoxide ion towards carbonyl groups in acid chlorides, esters and ketones is well documented The reaction between superoxide ion and the Py-Py" cation radical, which leads to destruction of the latter, would seem more likely to mitigate the long-term effects of the Py-Py rather than promote damage to components of the cell d . The occurence of Rh(bipy) -mediated photoreduction of alkenes with NADH models and... 

The hydroperoxyl radical (HOO-) is the conjugate acid of superoxide ion (02 ) (equation 93) and constitutes about 1% of the 02 that is formed in aqueous systems at pH 7. Although the 0-0 bond of HOO- traditionally is viewed to be the same as the single a bond of HO-OH (A//dbe, 51 kcalmoH ), its bond energy (AHdbe) is about 85kcalmoH, which is more consistent with the 1.5 bond order of 02. However, HOO- is unstable in protic media (such as water and alcohols) and rapidly decomposes via het-erolytic and homolytic disproportionation (equations 94 and 95). ... 

Hydroperoxyl radical HO 10" Stronger oxidizer and more hydrophobic then superoxide anion radical. Can initiate lipid peroxidation in membrane lipids... 

The superoxide radical and its conjugate acid, the hydroperoxyl radical, rapidly equilibrate with PK45 = 4.9, The hydroxyl radical center is transferred to a superoxide radical in a two-step process HO abstracts a hydrogen atom from the formate ion, producing the carbon dioxide radical anion, which then transfers an electron to oxygen (reactions 46 and 47). 

These results support the hypothesis that superoxide or other oxygen radicals derived from it such as singlet oxygen [444] and hydroperoxyl or hydroxyl radicals may mediate 8-cell damage and the decrease in insulin, resulting in insulin-dependent diabetes [435]. 

The radioprotectant activity of Cu(II)(3,5-DIPS)2 may not be solely due to its disproportionation of superoxide, k = (1-2) X 10 M s [517], but may also be related to its reactivity with hydroxyl radical, hydroperoxyl radical, hydrated electrons, and hydrogen atoms according to the following reactions at biological pH values [324, 462, 518-520] ... 

Nitric oxide has recently been shown to react very rapidly with organic hydroperoxyl radicals (Padmaja and Huie, 1993). This rapid radical-radical addition could account for the ability of nitric oxide to inhibit lipid peroxidation (Rubbo et al., 1995). Given the relatively high lipid solubility of nitric oxide, it could readily partition into membranes, where it would be sequestered from reactive species such as superoxide and remain for longer periods to act as a chain terminator of radical-mediated lipid peroxidation. Virtually any radical species formed in or near the lipid bilayer could react with nitric oxide (e.g., tocopheryl or ascorbyl radicals) to give nitrosated intermediates or products which could in turn act as nitric oxide reservoirs. 

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