Abstract

3-Nitrotyrosine has been reported as an important biomarker of oxidative stress that may play a role in a variety of diseases. In this work, transient UV-visible absorption spectra and kinetics observed during the reaction of the hydrated electron, eaq, with 3-nitrotyrosine and derivatives thereof were investigated. The absorption spectra show characteristics of aromatic nitro anion radicals. The absorptivity of radical anion product at 300 nm is estimated to be (1.0 ± 0.2) × 104 M−1 cm−1 at pH 7.3. The rate constants determined for the reaction of eaq with 3-nitrotyrosine, N-acetyl-3-nitrotyrosine ethyl ester and glycylnitrotyrosylglycine at neutral pH (3.0 ± 0.3) × 1010 M−1 s−1, (2.9 ± 0.2) × 1010 M−1 s−1 and (1.9 ± 0.2) × 1010 M−1 s−1, respectively, approach the diffusion-control limit and are almost two orders of magnitude higher than those for the reactions with tyrosine and tyrosine-containing peptides. The magnitude of the rate constants supports reaction of eaq at the nitro group, and the product absorbance at 300 nm is consistent with formation of the nitro anion radical. The pH dependence of the second-order rate constant for eaq decay (720 nm) in the presence of 3-nitrotyrosine shows a decrease with increasing pH, consistent with unfavorable electrostatic interactions. The pH dependence of the second-order rate constant for formation of radical anion (300 nm) product suggests that deprotonation of the amino group slows the rate, which indicates that deamination to form the 1-carboxy-2-(4-hydroxy-3-nitrophenyl)ethyl radical occurs. We conclude that the presence of the nitro group activates tyrosine and derivatives toward reaction with eaq and can affect the redox chemistry of biomolecules exposed to oxidative stress.

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