Insecticidal and Bactericidal Activities of Cassia nigricans Vahl and Molecular Docking Analysis of Insect Acetylcholinesterase.

Abstract

Objectives: This study focused on the phytochemical, insecticidal, and bactericidal activities of Cassia nigricans Vahl, as well as molecular docking analysis of an acetylcholinesterase (AChE) inhibitor as a promising natural insecticide.

Materials and methods: The leaves of C. nigricans were successively extracted with n-hexane, acetone, and methanol. Silica gel column chromatography of the methanol extract yielded compound 1. The insecticidal properties of the extracts and compound 1 were evaluated in terms of contact toxicity against Sitophilus zeamais. Bactericidal activity was achieved by photodynamic inactivation of fecal coliforms (FCs) and enterococci in water using extracts and compound one as natural photosensitizers. Compound 1 was analyzed for physicochemical and pharmacokinetic parameters and molecular docking against the AChE protein (6XYU).

Results: Compound 1 was characterized as emodin (1,3,8-trihydroxy-6-methylanthracene-9,10-dione) using 1D-2D-¹H-¹³C nuclear magnetic resonance and mass spectrometry. Insecticidal properties showed that emodin exhibited the highest toxicity with an lethal concentration 50 (LC₅₀) = 5.00 mg/mL compared with all extracts. The n-hexane extract showed the highest insecticidal activity (LC₅₀ = 177.48 mg/mL) compared with the methanol (LC₅₀ = 195.08 mg/mL) and acetone (LC₅₀ = 374.14 mg/mL) extracts. Complete inhibition of fecal enterococci by photosensitization was observed after 60 min of light exposure to emodin-treated water at all concentrations (1-5 mg/mL) and 120 minutes for FCs under the same conditions. Based on the docking score, the binding energy of emodin (-6.38 kcal/mol) was close to that of the marketed insecticide pirimiphos-methyl (-6.25 kcal/mol).

Conclusion: In addition, emodin was subjected to insecticide probability prediction and absorption, distribution, metabolism, excretion, and toxicity analysis and was found to be satisfactory as a natural insecticide. Emodin is a promising candidate for insecticidal pest control.