Multiphase reactions of proteins in the air: Oligomerization, nitration and degradation of bovine serum albumin upon ambient exposure.

Proteins in atmospheric aerosol can react with atmospheric pollutants such as ozone (O) and nitrogen dioxide (NO) in the atmosphere via the reactions of oxidation, nitration, and cross-linking etc. Currently, the reactions have been more thoroughly studied in the laboratory but rarely investigated in the ambient environment. In this study, we used bovine serum albumin (BSA) as the model protein to conduct the exposure experiment in the ambient environment in southern China, an area with increasing oxidative capacity, to investigate the reactions of proteins in the atmosphere. We observed the occurrence of oligomerization, nitration and degradation of BSA upon exposure. The mass fraction of BSA monomer decreased by 5.86 ± 1.61% after exposure and those of dimers, trimers and higher oligomers increased by 1.04 ± 0.49%, 1.37 ± 0.74% and 3.40 ± 1.06%, respectively. Simultaneously, the nitration degrees of monomers, dimers, trimers and higher oligomers increased by 0.42 ± 0.15%, 0.53 ± 0.15%, 0.55 ± 0.28% and 2.15 ± 1.01%, respectively. The results show that oligomerization was significantly affected by O and temperature and nitration was jointly affected by O, temperature and relative humidity, indicating the important role of atmospheric oxidants in the atmospheric reactions of protein. Atmospheric degradation of BSA was observed with the release of free amino acids (FAAs) such as glycine, alanine, serine and methionine. Glycine was the dominant FAA with a molar yield ranging from ∼8% to 33% for BSA. The estimated stoichiometric coefficient (α) of glycine is 10-10 for the degradation of BSA upon O. Our observation suggests the occurrence of protein reactions in the oxidative ambient environment, leading to the production of nitrated products, oligomers and low molecular weight products such as peptides and FAAs. This study may deepen the current understanding of the atmospheric reaction mechanisms and reveal the influence of environmental factors in the atmosphere.