Disproportionation Of NO Research Articles

Reactions of nitrogen dioxide in aqueous model systems: Oxidation of tyrosine units in peptides and proteins

By application of pulse radiolysis it was demonstrated that nitrogen dioxide (NO 2 ̇ ) oxidizes Gly-Tyr in aqueous solution with a strongly pH-dependent rate constant ( k 6 = 3.2 × 10 5M −1 S −1 at pH 7.5 and k 6 = 2.0 × 10 7M −1 S −1 at pH 11.3), primarily generating phenoxyl radicals. The phenoxyl can react further with NO 2 ̇ ( k 7 ~ 3 × 10 9M −1 S −1) to form nitrotyrosine, which is the predominant final product in neutral solution and at low tyrosyl concentrations under gamma-radiolysis conditions. Tyrosine nitration is less efficient in acidic solution, due to the natural disproportionation of NO 2 ̇ , and in alkaline solutions and at high tyrosyl concentrations due to enhanced tyrosyl dimerization. Selective tyrosine nitration by interaction of NO 2 ̇ with proteins (at pH 7 to 9) was demonstrated in the case of histone, lysozyme, ribonuclease A, and subtilisin Carlsberg. Nitrotyrosine developed slowly also under incubation of Gly-Tyr with nitrite at pH 4 to 5, where NO 2 ̇ is formed by acid decomposition of HONO. It is recalled in this context that NO 2 ̇ -induced oxidations, by regenerating NO 2 ̇ , can propagate NO 2 ̇ NO 2 − redox cycling under acidic conditions. Even faster than with tyrosine is the NO 2 ̇ -induced oxidation of cysteine-thiolate ( k 9 = 2.4 × 10 8M −1 S −1 at pH 9.2), involving the transient formation of cystinyl radical anions. The interaction of NO 2 ̇ with Gly-Trp was comparably slow ( k ~ 10 6M −1 S −1), and no reaction was detectable by pulse radiolysis with Met-Gly and (Cys-Gly) 2, or with DNA. Slow reactions of NO 2 ̇ were observed with arachidonic acid ( k ~ 10 6M −1 S −1 at pH 9.0) and with linoleate ( k ~ 2 × 10 5M −1 S −1 at pH 9.4), indicating that NO 2 ̇ is capable of initiating lipid peroxidation even in an aqueous environment. NO 2 ̇ -Induced tyrosine nitration, using 50 μ m Gly-Tyr at pH 8.2, was hardly inhibited, however, in the presence of 1 mM linoleate, and was not affected at all in the presence of 5 mM dimethylamine (a nitrosamine precursor). It is concluded that protein modifications, and particularly phenol and thiol oxidation, may be an important mechanism, as well as initiation of lipid peroxidation, of action of NO 2 ̇ in biological systems.

Catalytic activity of HY-type zeolite for the disproportionation of nitrogen monoxide

The disproportionation of NO on HY-type zeolite was carried out at 15°C by a pulsed microreactor. The catalytic activity began to appear at the pretreatment temperature 500°C and showed its maximum in the region of 600 to 700°C. The Lewis acid sites of HY-type zeolite seem to act as the active sites. TPD profiles after the reaction were composed of three peaks. The amount of peak β also showed its maximum at the pretreatment temperature ca. 600°C.

Catalytic activity of various Y-type zeolites on disproportionation of nitrogen monoxide

Disproportionation of NO has been investigated on various Y-type zeolites by the pulse-reaction technique. Alkaline-earth Y-type zeolites showed high activity. The order of catalytic activity presently obtained is as follows: CaY > CdY > MgY > ZnY > LaY > BeY > NaY > LiY > KY > SrY > BaY, AgY, HgY, FeY, AlY = 0. A volcano-type relationship was found between the catalytic activity and the radius of the exchanged cation in the case of alkaline-earth, zinc family and alkali-metal Y-type zeolites. CaY, CdY and NaY-type zeolites, each with a radius of exchanged cation ca. 1Å in size, showed maximun activity of their families. TPD profiles had three peaks corresponding to NO, NO and N 2O desorption with increasing temperature. The volcano-type relationship was also found between the peak area of each TPD profile and the radius of exchanged cation. The catalytic activity increases with increase of pretreatment temperature, reaches a broad maximum at 700°C and decreases with the collapse of zeolite structure in further increase of the pretreatment temperature.

Mechanism of the Reduction of Nitric Oxide with Ammonia over Cu(II) Ion-Exchanged Zeolites

Abstract The catalytic NO reduction with NH3 over Cu(II) ion-exchanged zeolite was studied with special attention to the temperature dependence of catalytic activity which reaches maximum at ca. 120 °C. ESR study showed that in the catalytic system all the exchanged copper ions exist as Cu(II) up to ca. 120 °C, above which Cu(II) ions decrease with a rise in temperature. The reaction rates measured at 110 and 140 °C were found to show similar dependence on both NO and NH3 partial pressures, indicating that the reaction mechanism does not differ much. By lowering temperature the Cu(I) ions formed at higher temperatures were reoxidized to Cu(II), accompanied by N2O formation due to the disproportionation of NO. From the results and the IR spectroscopic data a reaction mechanism was proposed. It includes a step in which NO reacts with NH3 adsorbed to Cu(II) ions to give rise to the evolution of N2 and the reduction of Cu(II) ions, and steps in which Cu(II) ions are regenerated accompanying the evolution of N2+N2O.

ESR and IR studies on the adsorption and disproportionation of nitric oxide on copper exchanged Y zeolites, and of reduction by carbon monoxide

Nitric oxide adsorption on copper exchanged Y zeolites is investigated by means of ESR and IR techniques. It is shown that NO disproportionation is copper concentration dependent as well as NO adsorption which occurs on CO-reduced samples on Cu2+ ions for low exchanged Y zeolites and on Cu+ ions for high exchanged Y zeolites.