The
heavy application of neonicotinoid insecticides in agricultural
production has burdened the environment. In the present study, interactions
of two neonicotinoid insecticides imidacloprid and thiamethoxam with
dissolved organic matter (DOM) were investigated by spectroscopic
techniques, molecular modeling, and density functional theory (DFT)
calculations. The static mechanism of imidacloprid and thiamethoxam
quenching the endogenous fluorescence of DOM was assessed through
time-resolved analyses. During the binding process, a protein-like
substance binds imidacloprid and thiamethoxam later than a humic-like
substance, as analyzed by two-dimensional correlation spectroscopy,
but more strongly than the humic-like substance, as suggested by molecular
modeling and DFT calculations. The conformational changes of DOM are
attributed to imidacloprid and thiamethoxam, as assessed with three-dimensional
spectra. Fourier transform infrared spectroscopy indicated that DOM
binds imidacloprid and thiamethoxam by hydroxyl, aliphatic C–H,
amide I, and carboxyl to form stable DOM–imidacloprid and DOM–thiamethoxam
complexes. Understanding the changes in the structural conformation
of humic-like and protein-like substances with imidacloprid and thiamethoxam
helps further understand the fate of the neonicotinoids in the environment.
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