Atmospheric protein chemistry influenced by anthropogenic air pollutants: Nitration and oligomerization upon exposure to ozone and nitrogen dioxide

The allergenic potential of airborne proteins may be enhanced via post-translational modification induced by air pollutants like ozone (O₃) and nitrogen dioxide (NO₂). The molecular mechanisms and kinetics of the chemical modifications that enhance the allergenicity of proteins, however, are still not fully understood. Here, protein tyrosine nitration and oligomerization upon simultaneous exposure of O₃ and NO₂ were studied in coated-wall flow-tube and bulk solution experiments under varying atmospherically relevant conditions (5-200 ppb O₃, 5-200 ppb NO₂, 45-96% RH), using bovine serum albumin as a model protein. Generally, more tyrosine residues were found to react via the nitration pathway than via the oligomerization pathway. Depending on reaction conditions, oligomer mass fractions and nitration degrees were in the ranges of 2.5-25% and 0.5-7%, respectively. The experimental results were well reproduced by the kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB). The extent of nitration and oligomerization strongly depends on relative humidity (RH) due to moisture-induced phase transition of proteins, highlighting the importance of cloud processing conditions for accelerated protein chemistry. Dimeric and nitrated species were major products in the liquid phase, while protein oligomerization was observed to a greater extent for the solid and semi-solid phase states of proteins. Our results show that the rate of both processes was sensitive towards ambient ozone concentration, but rather insensitive towards different NO₂ levels. An increase of tropospheric ozone concentrations in the Anthropocene may thus promote pro-allergic protein modifications and contribute to the observed increase of allergies over the past decades.