Transformation of tetracyclines mediated by Mn(II) and Cu(II) ions in the presence of oxygen

Complexation of tetracyclines (TCs) with dissolved Mn ^II and Cu ^II 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-Mn ^II-O ₂ system, oxidation of the TC-complexed Mn ^II to Mn ^III by oxygen occurs, followed by oxidation of TC by Mn ^III to regenerate Mn ^II. In the TC-Cu ^II-O ₂ system, Cu ^II oxidizes TC within the complex and the yielded Cu ^I is reoxidized by the present oxygen. Opposite reactivity trends were observed with the two metals: OTC (oxytetracycline) > TTC (tetracycline) ≫ iso-CTC (isochlorotetracycline) for the Mn ^II-mediated reaction, whereas CTC > TTC > OTC > epimers for the Cu ^II-mediated reaction. The reactivity results and examination of TC-metalion complexation and transformation products suggest that the BCD-ring and A-ring of TC are crucial to interact with Mn ^II and Cu ^II, respectively. This study highlights thatthe fate of TCs in aquatic environments may differ significantly by their strong interactions with different metal species present in the systems.