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Nitration Activates Tyrosine toward Reaction with the Hydrated Electron
Wei-Qun Shi, Hai-Ying Fu, Patricia L. Bounds, Yusa Muroya, Ming-Zhang Lin, Yosuke Katsumura, Yu-Liang Zhao and Zhi-Fang Chai
Vol. 176, No. 1 (July 2011), pp. 128-133
Published by: Radiation Research Society
Stable URL: http://www.jstor.org/stable/25835573
Page Count: 6
You can always find the topics here!Topics: Electrons, Free radicals, Absorption spectra, Radiolysis, Kinetics, Adducts, Nitration, Anions, Amino acids, Aqueous solutions
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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, e aq ⁻, 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 e aq ⁻ 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 e aq ⁻ 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 e aq ⁻ 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 e aq ⁻ and can affect the redox chemistry of biomolecules exposed to oxidative stress.
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