Hydroperoxy! radical nitric oxide reaction

Not all oxidants formed biolc cally have the potential to promote lipid peroxidation. The free radicals superoxide and nitric oxide [or endothelium-derived relaxing aor (EDRF)] are known to be formed in ww but are not able to initiate the peroxidation of lipids (Moncada et tU., 1991). The protonated form of the superoxide radical, the hydroperoxy radical, is capable of initiating lipid peroxidation but its low pili of 4.5 effectively precludes a major contribution under most physiological conditions, although this has been suggested (Aikens and Dix, 1991). Interestingly, the reaction product between nitric oxide and superoxide forms the powerful oxidant peroxynitrite (Equation 2.6) at a rate that is essentially difiiision controlled (Beckman eta/., 1990 Huie and Padmaja, 1993). 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

There were two important innovations in the development of these oxidative cycles the use of carbon monoxide which had previously been considered a relatively inert molecule in the atmosphere to regenerate the hydroperoxy radical via Reactions 2-6 and 2-7 and the use of peroxy radicals HO, and RO, to oxidize nitric oxide to nitrogen dioxide. 

The participation of hydroxyl and hydroperoxy radicals in the oxidation of nitric oxide raises the possibility that these radicals might also attack hydrocarbons. In the case of hydroxyl these reactions are known to be fairly rapid. On the basis of e rate constants that have been measured and estimates of those which have not the rates of attack of hydroxyl and hydroperoxy radicals appear to be large enough to explain the excess consumption of propylene shown in Figure 2-5. 

Although the above reactions generate a few fi radicals, most of the oxidation of nitric oxide to nitrogen dioxide is carried out by the alkyl-peroxy, RO, and hydroperoxy radicals that are formed in later reactions involving reactive hydrocarbons, aldehydes, or even carbon monoxide. One such example is shown in Figure 2-7. There is still considerable uncertainty as to the mechanism of these secondary reactions. The modeling studies should be consulted for details. ... 

Similarly, recent experiments" have been interpreted to mean that about 10% of the reaction of hydroperoxy radical with nitric oxide gives per-nitrous add, HOONO, instead of nitrogen dioxide and hydroxyl radical. Because this reaction is of major importance, even 10% of a second channel would be important, although it has been argued that such compounds would not be sufFidently stable to accumulate in the atmosphere." Whether such peroxynitrogen compounds are stable in the gas phase and whether they can be found in the atmosphere must await further experiments. 

This reaction competes favorably with other CH3O2 reactions, such as (R16) and (R20), and offers a fast pathway to the methoxy radical (CH3O). In a similar reaction, nitric oxide converts the hydroperoxy radical (HO2) to the more reactive hydroxyl radical,... 

Nitric oxide (NO) plays a central role in atmospheric chemistry, influencing both ozone cycling and the tropospheric oxidation capacity through reactions with hydroperoxy- and organic peroxy-radicals. When the NO concentration exceeds 40 pptv (pptv = parts per trillion by volume) it catalyzes the production of ozone (O3) ... 

The major loss reactions of OH radicals involve carbon monoxide (50-70%) and methane (10-20%). These reactions lead to the formation of hydroperoxy radicals, HO2, that can regenerate OH radicals, either by reaction with nitric oxide, NO, or by reacting with O3, for example... 

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