Nitration and Hydroxylation of Phenolic Compounds by Peroxynitrite

Abstract

The kinetics and products of the reaction of peroxynitrite with the phenolic compounds phenol, tyrosine, and salicylate were studied as a function of pH. All reactions are first-order in peroxynitrite and zero-order in the phenolic compound. Relative to the hydroxyl group, electrophilic substitution in the 2- and 4-positions (if available) leads to hydroxylated and nitrated products. The total yield of the products is proportional to the concentration of peroxynitrite. The sum of the rates of hydroxylation and nitration of phenol, determined by the stopped-flow technique, is approximately equal to the rate constant for the isomerization of peroxynitrite to nitrate. The rate vs pH profiles of the nitration and hydroxylation reactions parallel the yield vs pH profile with nitration maxima at pH 1.8 and 6.8, while hydroxylation is dominant between these two pH values. The activation energies for both hydroxylation and nitration are 18.8 ± 0.3 kcal mol^-1, identical to that of the isomerization of peroxynitrite to nitrate. Ethanol decreases the yield of hydroxylation, but has less effect on the nitration. The rate of reaction in the presence of metal complexes is first-order in metal complex and peroxynitrite and zero-order in the phenolic compound. The enhancement of the nitration of phenol by Fe(III)−edta and −nta is pH-dependent, with a maximum near pH 7, while Fe(III)−citrate, Cu(II)−edta, and CuSO₄ affect the nitration much less. The second-order rate constants for Fe(III)−edta at pH 4.8 and 7.2 are 1.4 × 10³ and 5.5 × 10³ M^-1 s^-1, respectively, at 25 °C. The activation energies for the nitration reaction in the presence of Fe(III)−edta are 11.5 and 12.2 kcal mol^-1 at pH 4.8 and 7.2, respectively. The nitration of tyrosine and salicylate by peroxynitrite is maximally enhanced by Fe(III)−edta.