Experimental and theoretical studies on the photochemical transformation of levofloxacin: Environmental factor effects, chiral state, and low-energy barrier pathways

The widespread occurrence and persistence of fluoroquinolone antibiotics in aquatic environments necessitate a deeper understanding of their photo-transformation. This study, the photodegradation mechanism of levofloxacin was systematically investigated by integrating HPLC-MS, transient absorption spectroscopy, and quantum chemical calculations. The results revealed that the photodegradation of levofloxacin proceeded more rapidly under neutral conditions. Cl - and SO 4 2- promoted photodegradation via reactive species, while the divalent metal ions (e.g., Cu 2+ ) inhibited it by forming photostable complexes. Quantum calculations predicted five pathways, with the piperazine ring cleavage pathway having the lowest energy barrier (5.3 kcal/mol). Experimental analysis further confirmed that the major photoproducts arose from demethylation, decarboxylation, piperazine ring oxidation, and hydroxylation–defluorination reactions, among which the decarboxylation and piperazine ring oxidation processes were consistent with theoretical predictions. Notably, a time-dependent change in optical rotation was observed and a low-energy pathway (9.1 kcal/mol) was identified, indicating the ring containing the chiral atom was disrupted during photolysis. This integrated experimental and theoretical study provided novel mechanistic insights and solutions for targeting the removal of fluoroquinolone antibiotic residues from the environment.