Reaction of peroxynitrite with L-tryptophan

The reaction between peroxynitrous acid (hydrogen oxoperoxonitrate) and L-tryptophan is 130 M(-1)s(-1) at 25°C. The pH dependence of the second-order rate constant shows a maximum at pH 5.1. The enthalpy and entropy of activation at pH 7.1 are 10.6 ± 0.4 kcal.mol(-1) and -16 ± 2 cal.mol(-1)K(-1) respectively. High-performance liquid chromatography analysis revealed a number of reaction products, two of which were identified as 5- and 6- nitrotryptophan. Hydroxytryptophans were not observed, even at low peroxynitrite concentrations where most of the peroxynitrite decays to nitrate via a first-order process. These results support the hypothesis that isomerization of protonated peroxynitrite to nitrate does not involve formation of the hydroxyl radical.