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Bacitracin is a mixture of nonribosomal peptides (NRPs) that is extensively used as an antibiotic in both human and veterinary medicine. Despite its widespread use over the past six decades, very few studies have addressed the environmental fate of bacitracin and zinc-bacitracin complexes. In this study, the photochemical transformation of bacitracin components (i.e., cyclic dodecapeptides) in the aquatic environment was investigated. A high resolution mass spectrometry (HRMS)-based approach enabled monitoring of the photochemical degradation kinetics of individual bacitracin components, investigation of the relative contribution of reactive oxygen species (e.g., singlet oxygen, ¹O2) in dissolved organic matter-sensitized photoreactions, and identification of oxidative modifications in bacitracin photoproducts. The results of this study support the hypothesis that indirect photochemical oxidation of the histidine (His) residue by ¹O2 is a major degradation pathway for bacitracin A, the most potent congener of the mixture. Furthermore, the photooxidation rate of bacitracin A with ¹O2 decreased upon bacitracin A coordination with Zn2+, demonstrating that the photochemistry of metal-bound His is different from that of metal-free His. Overall, these results provide insight into the fate of bacitracin components in the aquatic environment and highlight the potential of utilizing this HRMS-based methodology to study transformations of other environmentally relevant NRPs. [ABSTRACT FROM AUTHOR]