title: The effect of Cu(2+) chelation on the direct photolysis of oxytetracycline: A study assisted by spectroscopy analysis and DFT calculation.
abstract: The extensive usage of OTC and Cu(2+) in livestock and poultry industry caused high residues in natural environment. Co-contamination of OTC and Cu(2+) was a considerable environmental problem in surface waters. In this study, Cu(2+) mediated direct photolysis of OTC was studied. Cu(2+) chelating with OTC was found to greatly inhibit OTC photodegradation. To reveal the chelation mechanism of OTC-Cu complexes, multiple methods including UV-Vis absorption spectra, Infrared (IR) spectra, mass spectroscopy, and density functional theoretical (DFT) modeling were performed. Four OTC-Cu complexes were proposed. Cu(2+) preferably bond to O11O12 site with the binding constants logK = 8.19 and 7.86 for CuHL+ and CuL±, respectively. The second chelating site was suggested to be O2O3 with the binding constants of logK = 4.41 and 4.62 for Cu2HL3+ and Cu2L2+, respectively. The suppressed quantum yield of OTC by Cu2+ chelation was accused for their intra-/inter-molecular electron transfer, by which the energy in activated states was distributed. The occurrence of electron transfer between BCD ring and A ring also from BCD ring to Cu was evidenced by the TD-DFT result only for the OTC-Cu complexes. Besides, the cyclic voltammetry measurement also suggested one OTC-Cu(II)/OTC-Cu(I) redox couple. These results suggested that the persistence of OTC in environmental surface waters will probably be underestimated for neglecting the chelating effect of Cu2+. The photolysis quantum yield of OTC-Cu complexes, as well as the specific molar absorption constants, the equilibrium binding constants of Cu2+ with OTC could contribute to more accurate kinetic models of OTC.
authors.affiliation: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China; State Key Laboratory of Pollutant Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210023, China; University of Chinese Academy of Sciences, Beijing, 100049, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Jin
authors.affiliation: National Quality Supervision and Inspection Center for Industrial Explosive Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China.
authors.forename: Shanshan
authors.initials: S
authors.lastname: Qiu
authors.affiliation: Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY, 12180, USA.
authors.forename: Ke
authors.initials: K
authors.lastname: Wu
authors.affiliation: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China.
authors.forename: Mingyun
authors.initials: M
authors.lastname: Jia
authors.affiliation: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China.
authors.forename: Fang
authors.initials: F
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China.
authors.forename: Chenggang
authors.initials: C
authors.lastname: Gu
authors.affiliation: State Key Laboratory of Pollutant Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210023, China; Research Center for Eco-Environmental Science, Chinese Academy of Sciences, Beijing, 100085, China.
authors.forename: Aiqian
authors.initials: A
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China; University of Chinese Academy of Sciences, Beijing, 100049, China. Electronic address: jiangxin@issas.ac.cn.
authors.forename: Xin
authors.initials: X
authors.lastname: Jiang
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry*; D002614:Chelating Agents / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D008962:Models, Theoretical; D010118:Oxytetracycline / Q000737:chemistry*; D010782:Photolysis; D013057:Spectrum Analysis; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D000900:Anti-Bacterial Agents; D002614:Chelating Agents; D014874:Water Pollutants, Chemical; D003300:Copper; D010118:Oxytetracycline
keywords: Chelation; Cu(2+); Direct photolysis; Oxytetracycline; TD-DFT