Nitric oxide direct reactions

The reaction in water at pH 7.4 has been much studied since the discovery of the importance of nitric oxide. The products are as for the thermal and photochemical reactions, except that the final product is nitrite ion. This is to be expected since nitric oxide in aerated water at pH 7.4 also yields quantitatively nitrite ion25, by it is believed the series of equations 7-9, which involves oxidation to nitrogen dioxide, further reaction to give dinitrogen trioxide which, in mildly alkaline solution, is hydrolysed to nitrite ion. Under anaerobic conditions it is possible to detect nitric oxide directly from the decomposition of nitrosothiols using a NO-probe electrode system26. Solutions of nitrosothiols both in... 

Four routes to form peroxynitrite from nitric oxide. The reaction of nitric oxide with superoxide is only one mechanism leading to the formation of peroxynitrite. Supetoxide could also reduce the nitrosyidioxyl radical. If nitric oxide is directly reduced to nitroxyl anion, it will react with molecular oxygen to form peroxynitrite. At acidic pH, nitrite may form nitrous acid and nitrosonium ion, which reacts with hydrogen peroxide to form peroxynitrite. 

The nitric oxide oxidation and water absorption reactions (Eqs. 11.38 and 11.40) are both much slower than the ammonia oxidation reaction (Eq. 11.35), and involve a significant volume decrease on reaction, 3 mol (volumes) to 2, and 3 mol to 1, respectively. Thus the absorbers, where the bulk of these reactions occur, must be large to provide sufficient residence time, and cooled to favor the equilibria in the desired direction. Raising the pressure in the absorbers achieves a significant improvement in performance, in accord with Le Chatelier s principle, because of the volume decrease observed for the absorber reactions. For an increase to 8 atm from 1 atm, the rate of the very slow nitric oxide reoxidation reaction (Eq. 11.45) is accelerated by a factor of the cube of this pressure increase, or 512 times [42]. 

The second direct reaction pathway, one-electron reduction of a target by nitric oxide, could occur only if the target was itself a strong oxidant, since nitric oxide does not readily give up its unpaired electron. Oxidation of nitric oxide would result in the formation of NO, which would rapidly nitrosate nucleophiles such as amines, sulfhydryls, or aromatics. In fact, the best one-electron oxidants would be radicals such as -NOi or hydroxyl radical or even ONOO itself. In such cases the net effect would be nitric oxide addition reactions (nitrosations), regardless of whether the mechanism is considered to be transfer of an electron from nitric oxide followed by attack of NO or simple radical-radical combination. Thus, under most conditions, one-electron reduction of a target by nitric oxide becomes a simple addition reaction. 

Nitric oxide addition reactions and nitrosations (often referred to as nitrosylations) are currently the subject of much debate. Evidence exists that nitrosation, particularly thiol nitrosation, serves to prolong the biological activity of nitric oxide by acting as a slow-releasing reservior (Stamler et al, 1992 Keaney et al., 1993). It has also been proposed that alteration of the redox state of critical thiols by nitric oxide may serve a signaling function (Sucher and Lipton, 1991 Lipton et al., 1993) quite distinct from the ability of nitric oxide to directly stimulate cGMP production. 

The direct use of nitric oxide (NO) reactions to identify and quantify hydroxyl (OH) and/or hydroperoxy (OOH) groups is complicated by the formation of equimolar mixtures of nitrates (with absorption bands at 1630, 1302, 1290, 1278 and 860 cm ) and nitrites (with a strong absorption band at 1645, and weak bands at 780-760 cm " ) ... 

Hydroxylamine sulfate is produced by direct hydrogen reduction of nitric oxide over platinum catalyst in the presence of sulfuric acid. Only 0.9 kg ammonium sulfate is produced per kilogram of caprolactam, but at the expense of hydrogen consumption (11). A concentrated nitric oxide stream is obtained by catalytic oxidation of ammonia with oxygen. Steam is used as a diluent in order to avoid operating within the explosive limits for the system. The oxidation is followed by condensation of the steam. The net reaction is... 

Benzyl chloride readily forms a Grignard compound by reaction with magnesium in ether with the concomitant formation of substantial coupling product, 1,2-diphenylethane [103-29-7]. Benzyl chloride is oxidized first to benzaldehyde [100-52-7] and then to benzoic acid. Nitric acid oxidizes directly to benzoic acid [65-85-0]. Reaction with ethylene oxide produces the benzyl chlorohydrin ether, CgH CH20CH2CH2Cl (18). Benzylphosphonic acid [10542-07-1] is formed from the reaction of benzyl chloride and triethyl phosphite followed by hydrolysis (19). 

The presence of PSCs also leads to the removal of nitrogen oxides (NO and NO2) from the gas phase. As long as there are significant amounts of NO2 it will react with chlorine monoxide (CIO) to produce chlorine nitrate (reaction 11). This species subsequently reacts with HQ on PSC surfaces to produce nitric acid (reaction 13), which remains in the condensed phase. Also, nitric acid directly condenses with water to form nitric acid trihydrate particles, hence it is not available to regenerate NO2 by photochemical processes, as it does when it is in the gas phase. 

Nitric oxide may also be an antioxidant by virtue of the feet that it can directly inhibit NADPH oxidase and thus prevent superoxide production (Clancy etaJ., 1992). This inhibition was reported to be independent of the reaction between nitric oxide and superoxide, which might be expected to be pro-oxidant (see Section 2.2.3). 

Mouse peritoneal macrophages that have been activated to produce nitric oxide by 7-interferon and lipopolysac-charide were shown to oxidize LDL less readily than unactivated macrophages. Inhibition of nitric oxide synthesis in the same model was shown to enhance LDL oxidation (Jessup etal., 1992 Yates a al., 1992). It has recently been demonstrated that nitric oxide is able to inhibit lipid peroxidation directly within LDL (Ho etal., 1993c). Nitric oxide probably reacts with the propagating peroxyl radicals thus terminating the chain of lipid peroxidation. The rate constant for the reaction between nitric oxide and peroxyl radicals has recently been determined to be 1-3 X10 M" s (Padmaja and Huie, 1993). This... 

Tertiary phosphine sulfides are generally stable compounds and are not easily oxidised by air, although they can be oxidised by hydrogen peroxide or dilute nitric acid. The analogous tertiary phosphine selenides and tellurides are however, more reactive to oxidation. Similar to the sulfides they can be prepared from the direct reaction of elemental chalcogen with a tertiary phosphine (Equation 1). Tertiary phosphine selenides are also accessible from tertiary phosphines using KSeCN as the selenium source instead of the element itself. 

Brovkovych et al. [38] applied the electrochemical porphyrinic sensor technique for the direct measurement of NO concentrations in the single endothelial cell. It was found that NO concentration was the highest at the cell membrane (about 1 pmoll-1) and decreased exponentially with distance from the cell, becoming undetectable at the distance of 50 pm. Now we will consider the principal reactions of nitric oxide relevant to real biological systems. 

Thus the competition between stimulatory and inhibitory effects of NO depends on the competition between two mechanisms the direct interaction of NO with free radicals formed in lipid peroxidation and the conversion of NO into peroxynitrite or other reactive NO metabolites. Based on this suggestion, Freeman and his coworkers [42-44] concluded that the prooxidant and antioxidant properties of nitric oxide depend on the relative concentrations of NO and oxygen. It was supposed that the prooxidant effect of nitric oxide originated from its reaction with dioxygen and superoxide ... 

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. 

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