Heterogeneous Nitration of Tyrosine by NO­3 and N2O5: Rates, Mechanisms and Product Yields

Abstract:

Nitration of protein-bound tyrosine has been identified as a casual connection between air pollution and human health. Tyrosine is a common amino acid, 4-hydroxyphenylalanine, HO-C₆H₄-CH₂-CH(NH₂)-C(O)OH), and is present in many atmospheric bio-aerosols. Nitration of the aromatic units of protein molecules in polluted air enhances their allergenicity. The mechanism of heterogeneous nitration process of bio-aerosols by common nitrating agents in the atmosphere, O₃/NO₂, NO₃, N₂O₅ is not well understood. This chemistry is thought to proceed via reactions with O₃ and NO₂ on particle surfaces, through mechanisms that are still uncertain. The possible role of higher nitrogen oxides also remains uncertain, partly due to a lack of measurements of fundamental chemical and physical parameters. In this work, we undertook measurements of reactive uptake of NO₃, N₂O₅, as a function of relative humidity and temperature in a tyrosine coated flow tube reactor with chemical ionization mass spectrometric (CIMS) detection. Uptake coefficients on tyrosine coated flow tube were small under low relative humidity but were enhanced by an order of magnitude in the presence of high relative humidity, particularly for N₂O₅. The measured uptake coefficients were mostly due to reaction with water adsorbed on the surface of the flow tube. Only ~10% of the reactive uptake could be attributed to reaction with tyrosine. Following uptake, the contents of the flow tube were extracted, and analyzed using electrospray ionization - mass spectrometer (ESI-MS) to identify and quantify the products of the nitration reaction. The only organic reaction product detected was 3-nitro-tyrosine (3-NT). The measured uptake coefficients, mechanism of the title reactions and the possible atmospheric implications of these findings will be discussed.