Transformation of Tetracyclines Mediated by Mn(II) and Cu(II) Ions in the Presence of Oxygen

Complexation of tetracyclines (TCs) with dissolved MnII and CuII ions were found to significantly enhance the transformation of these antibiotics in the presence of oxygen at pH 8−9.5 and pH 4−6, respectively. In the TC−MnII—O2 system, oxidation of the TC-complexed MnII to MnIII by oxygen occurs, followed by oxidation of TC by MnIII to regenerate MnII. In the TC−CuII—O2 system, CuII oxidizes TC within the complex and the yielded CuI is reoxidized by the present oxygen. Opposite reactivity trends were observed with the two metals: OTC (oxytetracycline) > TTC (tetracycline) ≫ iso-CTC (iso-chlorotetracycline) for the MnII-mediated reaction, whereas CTC > TTC > OTC > epimers for the CuII-mediated reaction. The reactivity results and examination of TC-metal ion complexation and transformation products suggest that the BCD-ring and A-ring of TC are crucial to interact with MnII and CuII, respectively. This study highlights that the fate of TCs in aquatic environments may differ significantly by their strong interactions with different metal species present in the systems.