Computational Simulation Of Metal Ions Complexation Effects On The Structure And Absorption Spectrum Of Sulfadiazine

Antibiotics are abused seriously, coexisting with heavy metal ions, and lead to combined pollutions to natural waters. In surface waters, photodegradation is an important transformation pathway for antibiotics in natural waters. However, in the combined polluted waters, the photochemical properties of antibiotics with hetero atoms (N, O, S, etc.) may be influenced by metal ions via complexation effects. Therefore, in order to understand the environmental photochemical behavior of antibiotics, it is necessary to study the effects of metal complexation on the antibiotic structure and light absorption properties.In this work, the metal complexation effects were investigated by quantum chemical computations, employing Mg2+ and sulfadiazine as a model complex system. The structure of 1:1 Mg2+-sulfadiazine complex was optimized and the interaction energy was computed based on the density functional theory (DFT). The light absorption properties of sulfadiazine and its complex were computed based on the time-dependent density functional theory (TD-DFT). The results indicate that Mg2+ complexes with N and O of the sulfadiazine hydrated molecule. The bond length, valence and dihedral angles of the complex differ from that of sulfadiazine significantly. The complex has different frontier molecular orbital (FMO) distribution and FMO energy levels. The simulated light absorption of the complex was red-shifted and enhanced compared with sulfadiazine. The computational absorption properties of sulfadiazine and the complex were consistent with the experimental results.To further verify the reliability of the prediction method and understand the metal complexation effects, the effects of Ca2+ and Zn2+ complexation on the structure and light absorption properties of sulfadiazine were studied. The results indicate that Mg2+, Ca2+ and Zn2+ can form bidentate complex with the sulfadiazine hydrated molecule, respectively. The stabilities of the 1:1 complexes follow Zn2+> Mg2+> Ca2+. The type of ligand and the formation of hydrogen bonding between hydrated molecule and the ligand are key factors influencing the stability of the complex. The geometrical structures of the antibiotics changed significantly due to the complexation of metal ions. The three complexes have different FMO distribution and energy levels. The simulated absorption spectra of the complexes were red-shifted. The stronger the metal ions complexation with sulfadiazine, the larger redshift of the absorption spectra, the more favourably sulfadiazine absorbs long-wavelength sunlight.