Estimating the potential toxicity of chiral diclofop-methyl: Mechanistic insight into the enantioselective behavior

Abstract

Chiral pollutants are widely distributed in the environment; however, the enantioselective toxic effects of these chemicals have still not fully been clarified. Using wet experiments and computational toxicology, this story was to explore the static and dynamic toxic reactions between chiral diclofop-methyl and target protein at the enantiomeric level, and further unveil the microscopic mechanism of enantioselective toxicity of chiral pesticide. Steady-state and time-resolved results indicated that both (R)-/(S)-enantiomers can form the stable toxic conjugates with target protein and the bioaffinities were 1.156 × 10⁴ M^-1/1.734 × 10⁴ M^-1, respectively, and significant enantioselectivity was occurred in the reaction. Results of the modes of toxic action revealed that diclofop-methyl enantiomers located in the subdomain IIA, and the strength of important noncovalent interactions between (S)-diclofop-methyl and the residues was greater than that of (R)-diclofop-methyl. The Gibbs free energies of the chiral reactions were －26.89/－29.40 kJ mol^-1 and －25.79/－30.08 kJ mol^-1, respectively, which was consistent with the outcomes of photochemistry and site-specific competitive assay. Dynamic enantioselective processes explained that the impact of intrinsic protein conformational flexibility on the toxic reaction of (R)-diclofop-methyl was lower than that of (S)-diclofop-methyl, which originates from the conformational changes and spatial displacement of the four loop regions (i.e. h6↔h7, h1↔h2, h5↔h6, and h8↔h9). The quantitative data of circular dichroism spectra confirmed such results. Energy decomposition displayed that the electrostatic energy of the target protein-(S)-diclofop-methyl system (－25.86 kJ mol^-1) was higher than that of the target protein-(R)-diclofop-methyl complex (－18.21 kJ mol^-1). Some crucial residues such as Lys-195, Lys-199, Ser-202, and Trp-214 have been shown to be of different importance for the enantioselective toxicity of chiral diclofop-methyl. Obviously this scenario will contribute mechanistic clues to assessing the potential hazards of chiral environmental pollutants to the body.

Keywords: Chiral diclofop-methyl; Dynamic reaction; Enantioselective toxicity; Environmental chemical; Risk assessment; Toxic mode.