In vitro evaluation of the conjugations of neonicotinoids with transport protein: photochemistry, ligand docking and molecular dynamics studies

Abstract

The main objective of this study was to assess the biological effects of neonicotinoids, together with their structure–activity relationships, by employing plasma albumin as a non-target model. Fluorescence indicated clearly that static-type quenching is the effective mechanism for the reduction of Trp-214 residue emission when c(neonicotinoid) ≤ 10 μM, yet both static and dynamic properties occurred in the system if the concentration was higher than 10 μM. The stoichiometric proportion of protein to neonicotinoid is obviously 1 : 1, and subdomain IIA was discovered to possess high affinity for these chemicals. This corroborates molecular docking, site-directed mutagenesis, molecular dynamics simulations and free-energy calculations, which show that neonicotinoids are present at the warfarin–azapropazone site and yield hydrogen bonds, π–π stacking and hydrophobic interactions with several pivotal amino acid residues, i.e. Phe-211, Trp-214 and Arg-222. These noncovalent bonds caused partial conformational changes in the protein, that is, α-helix content decreased from 55.9% to 48.5% along with an increase in the contents of β-sheet, turn and random coil, as derived from synchronous fluorescence and circular dichroism. This phenomenon agrees well with the outcomes of the assignment of protein secondary structure. According to analyses of structure–activity relationships, it can be observed that neonicotinoids with the ring-closed structure (part B), e.g., imidacloprid and thiacloprid, have relatively low affinity for proteins compared with some ring-open agents such as nitenpyram and acetamiprid. These disparities may be related to the fact that ring-open neonicotinoids have great flexibility and thus take part more easily in noncovalent interactions with the amino acid residues in the active cavity. In addition, the toxicological relevance of the biorecognition of neonicotinoids by a biopolymer is also investigated here. Perhaps this investigation could use a non-target biological model for the evaluation of neonicotinoid toxicity and might also provide helpful clues for the synthesis of novel neonicotinoid agents.