Organic acids enhanced the phototransformation of 1-nitronaphthalene in skin sweat: The critical role of carbon-centered radicals.

Skin contact is an important route for human exposure to atmospheric pollutants. Many atmospheric pollutants possess photochemical activity, and skin sweat provides a natural environment for the photogeneration of reactive species (RSs) from pollutants. However, the photochemical transformation of atmospheric pollutants in skin sweat and their toxicological effects on the skin remain poorly understood. Herein, 1-nitronaphthalene (1-NN), a common atmospheric pollutant, was used as a model compound to investigate the influence of key components (i.e., Organic acids, Fe3 +) of skin sweat on the degradation and transformation mechanisms of 1-NN, as well as the skin toxicity of its transformation products. Our results show that lactic acid (LA) facilitated the photodegradation of 1-NN. The excited triplet state 1-NN (31-NN*) can oxidize LA to generate carbon-centered radicals (·CH(CH3)OH) that was confirmed by EPR spin trapping and high-resolution mass spectrometry. Histidine (His) further accelerates this process by consuming dissolved oxygen (3O2), thereby inhibiting the quenching of 31-NN* as well as carbon-centered radicals by 3O2. Additionally, the presence of Fe3+ can trigger the generation of carboxyl radicals (CO2•-) via carboxylic acid ligand-to-metal charge transfer, further enhancing the photodegradation of 1-NN. Toxicity assays indicated that the photochemical transformation process of 1-NN can lead to higher cytotoxicity. These findings provide a new strategy for understanding the transformation processes and toxicity evolution of nitrated polycyclic aromatic hydrocarbons on the skin surface.