Preventing nitrite contamination in tetramethylammonium peroxynitrite solutions

Peroxynitrite prepared from superoxide and nitric oxide in liquid ammonia does not contain detectable levels of nitrite. However, the dissolution of nitrite salts can lead to variable levels of peroxynitrite depending on the conditions used to disolve the salt. Low levels of nitrite result when frozen peroxynitrite solutions are first brought to +1 degrees C and then to room temperature. These undergo only 2-3% decomposition after 1 h, in contrast with the findings of a recent report (Lymar, S. V.; Khairutdinov, R. F.; Hurst, J. K. Inorg. Chem. 2003, 42, 5259-5266), where high levels of nitrite ( approximately 20%) result from rapid thawing of these solutions to room temperature. Warming the frozen peroxynitrite solution directly to room temperature in 30 min leads to a nitrite level of 28%.     

Hydrogenation of

The course of the hydrogenation of [5]- and [6]metacyclophane (1b and 1c) and their thermochemistry is described. Both compounds are hydrogenated rapidly (within 10 s) to furnish the bridgehead olefins 13b and 12c. The accompanying hydrogenation enthalpies are -220 and -141 kJmol(-1), respectively. Strain energies (SE) and olefinic strains (OS) of a number of bridgehead olefins have been evaluated by DFT calculations; it was concluded that 13b belongs to the class of hyperstable olefins which correlates nicely with its reluctance to undergo hydrogenation. By combining experimental hydrogenation enthalpies and DFT calculations, SE of 187 and 121 kJmol(-1) were derived for 1b and 1c.     

Homolysis of the peroxynitrite anion detected with permanganate

The reaction of peroxynitrite with violet-colored MnO4- leads to the formation of green MnO42-. The rate constant for the reaction at pH 11.7, 5.5 mM ionic strength, and 25 degrees C, 0.020 +/- 0.001 s(-1), is independent of the MnO4- concentration; homolysis of ONOO- to NO* and O2*- is the rate-determining step. Both NO* and O2*- react with MnO4- with rate constants of (3.5 +/- 0.7) x 10(6) M(-1)s(-1) and (5.7 +/- 0.9) x 10(5) M(-1)s(-1), respectively. The activation volume and activation energy for breaking the N-O bond are 12.6 +/- 0.8 cm(3)mol(-1) and 102 +/- 2 kJ mol(-1), respectively. In combination with the known standard Gibbs energies of formation of NO* and O2*-, the rate of the reaction of NO* and O2*-, and the pKa of ONOOH, we find a standard Gibbs energy of formation of ONOO- of +68 +/- 1 kJ mol(-1), and of ONOOH of +31 +/- 1 kJ mol(-1).     

Reactions of Peroxynitrite with Phenolic and Carbonyl Compounds: Flavonoids are not Scavengers of Peroxynitrite

Peroxynitrite (ONOO⁻, oxoperoxonitrate(1−)), an isomer of nitrate that oxidizes and nitrates biomolecules, is likely to be formed in vivo from the reaction of superoxide with nitrogen monoxide. To determine whether flavonoids scavenge peroxynitrite as postulated in the literature, we studied the reactions of peroxynitrite with phenol, hydroquinone, catechol, and the flavonoid monoHER. These reactions are first‐order with respect to peroxynitrous acid and zero‐order with respect to the organic compounds, and proceed as fast as the isomerization of peroxynitrous acid to nitrate. In vivo, a large fraction of all peroxynitrite is likely to react with carbon dioxide to form an unstable adduct, the 1‐carboxylato‐2‐nitrosodioxidane anion (ONOOCO). The presence of phenolic compounds did not influence the rate of disappearance of this adduct, which was ca. 4×10² s⁻¹. On the basis of these kinetic studies, it can be concluded that flavonoids are not scavengers of peroxynitrite. The products from the reaction of peroxynitrite with hydroquinone (benzene‐1,4‐diol) and catechol (benzene‐1,2‐diol) are para‐benzoquinone and ortho‐benzoquinone, respectively; no nitrated products were found. In a subsequent reaction, ortho‐quinone reacted with nitrite, a common contaminant of peroxynitrite preparations to form 1,2‐dihydroxy‐4‐nitrobenzene. We also investigated whether carbonyl compounds could redirect the reactivity of peroxynitrite toward nitration, as carbon dioxide does. The reaction with acetone is first‐order with respect to peroxynitrite and first‐order with respect to the carbonyl compound. The rate constant is 1.8 M ⁻¹s⁻¹ at neutral pH and 20°; peroxynitrite does not react with the carbonyl compounds dimethyl acetamide, L ‐alanylalanine, or methyl acetate. It is not likely that the carbonyl compounds or the mono‐, di‐, or polyphenolic compounds can scavenge peroxynitrite in vivo.