Hydroperoxyl Radical 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... 

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

In aqueous solutions, superoxide may act as a weak base by accepting protons to form a hydroperoxyl radical (HO ) that can dissociate to regenerate protons, by the equilibrium G). However, not much superoxide is present at physiological pH. 

Measurements of tropospheric HO and H02 concentrations have been accomplished by both direct and indirect means. Direct techniques are based on the measurement of the hydroxyl or hydroperoxyl radical using some physical property of the radicals themselves e.g. optical absorption. Indirect techniques refer to methods based on the measurement of compounds that are uniquely and/or quantitatively formed from or destroyed by HO or HO2. Examples of these techniques for both [HO ] and [HO2 ] will be given. 

The CBS-QB3 potential energy surface accounts for the various experimentally observed products, including hydroperoxyl radical, propene, HO, propanal, and oxirane (c-CsHgO). The activation barrier for simultaneous 1,4-H transfer and HO2 expulsion, obtained via calculations, compares well to the experimentally observed barrier (26.0kcal/mol) of DeSain et al. This work provides some ramifications for larger alkylperoxy radicals multiple conformers of long alkylperoxy radicals are likely to play a role in the overall oxidation chemistry and dictate consideration for correct treatment of thermochemistry at lower temperatures T< 500 K), unimolecular reactions dictate peroxy radical chemistry. 

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. 

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. ... 

The Fenton process is very effective for HO generation, but an excess of Fe2+, H202, hydroperoxyl radicals or halogens (if present) can act as HO scavengers. 

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). ... 

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). 

As we have already noted, the hydrogen atom combines immediately with O, to yield HO,. The formyl radical, HCO, also reacts very rapidly with O, to yield the hydroperoxyl radical and CO,... 

The alkylperoxyl radical can be expected to react with sulfur amino acids by mechanisms similar to those for HOO- and HO-. Reactions of the hydroxyl and hydroperoxyl radicals have been more extensively studied because of their importance in radiation biochemistry. Thus, for cysteine, glutathione, and sulfhydryl proteins,... 

Hydroxyl radical (HO ), which reacts with some hydrocarbons at rates that are almost diffusion-controlled (meaning that if a molecule collides with a hydroxyl radical they react), is the predominant reactant in early stages of smog formation. Significant contributions are made by hydroperoxyl radical (HOO ) and O3 after smog formation is well under way. 

Major destruction routes of OH radicals are the addition to olefins, the 11 atom abstraction from olefins and aldehydes, and the reaction with C( > Another radical, hydroperoxyl (H02), has been considered as a major oxidizing agent for NO and to a lesser extent for hydrocarbons. The HO, radicals are probably formed by the photolysis of formaldehyde [see Section VI1-4, p. 277]... 

The gas phase reactions which may be undergone by HO 2 depend on the conditions. It is these elementary steps which are involved in the slow reaction and at the third limit. Salt coated vessels may be assumed to be moderately or highly efficient for the destruction of hydroperoxyl at the surface. In such vessels the radical concentration is likely to be low and reactions between radicals are unlikely to be important. The reactions proposed for the HO2 under these conditions were (xi) [1, 23], or (xia) [7], the first of which leads to the formation of hydrogen peroxide and has since been shown to be the faster of the two [45]. Assuming that the hydrogen peroxide is decomposed without formation of further chain centres, reactions (iv) and (xi) then form a chain propagating cycle which continues until either a H or HO2 is destroyed at the vessel surface. 

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