Antifungal Activities of Fluorinated Pyrazole Aldehydes on Phytopathogenic Fungi, and Their Effect on Entomopathogenic Nematodes, and Soil-Beneficial Bacteria

Abstract

Fluoro-substituted pyrazoles have a wide range of biological activities, such as antibacterial, antiviral, and antifungal activities. The aim of this study was to evaluate the antifungal activities of fluorinated 4,5-dihydro-1H-pyrazole derivatives on four phytopathogenic fungi: Sclerotinia sclerotiorum, Macrophomina phaseolina, Fusarium oxysporum f. sp. lycopersici, and F. culmorum. Moreover, they were tested on two soil beneficial bacteria-Bacillus mycoides and Bradyrhizobium japonicum-as well as two entomopathogenic nematodes (EPNs)-Heterorhabditis bacteriophora and Steinernema feltiae. The molecular docking was performed on the three enzymes responsible for fungal growth, the three plant cell wall-degrading enzymes, and acetylcholinesterase (AChE). The most active compounds against fungi S. sclerotiorum were 2-chlorophenyl derivative (H9) (43.07% of inhibition) and 2,5-dimethoxyphenyl derivative (H7) (42.23% of inhibition), as well as H9 against F. culmorum (46.75% of inhibition). Compounds were shown to be safe for beneficial soil bacteria and nematodes, except for compound H9 on EPN H. bacteriophora (18.75% mortality), which also showed the strongest inhibition against AChE (79.50% of inhibition). The molecular docking study revealed that antifungal activity is possible through the inhibition of proteinase K, and nematicidal activity is possible through the inhibition of AChE. The fluorinated pyrazole aldehydes are promising components of future plant protection products that could be environmentally and toxicologically acceptable.

Keywords: antibacterial activity; antifungal activity; molecular docking; nematicidal activity; plant protection; pyrazole aldehydes; toxicity.

Scheme 1

Structures of analysed antifungal agents, fluorinated pyrazole aldehydes (H1–H10). Their synthesis and characterization were reported previously [15].

Figure 1

Projection of variables, antifungal activities of ten fluorinated pyrazole aldehydes against four plant pathogenic fungi (Macrophomina phaseolina, Sclerotinia sclerotiorum, Fusarium oxysporum f. sp. lycopersici, and Fusarium culmorum) (blue), and molecular docking scores energies (kcal mol⁻¹) for six target enzymes related to antifungal activity (red) (three enzymes responsible for fungal growth: demethylase (sterol 14α-demethylase (CYP51), chitinase, and transferase (N-myristoyltransferase); three plant cell wall-degrading enzymes: endoglucanase, proteinase K, and pectinase (endopolygalacturonase)) on the factor-plane.

Figure 2

(a) Three-dimensional representation of interactions into binding site of proteinase K with docked fluorinated pyrazole aldehyde H2; two-dimensional diagram of main interactions and bond lengths (Å) with amino acid residues (b) of compound H2; (c) two-dimensional diagram of coumarin as standard ligand. (green = conventional hydrogen bond; light green = van der Waals; very light green = fluorine interactions; dark purple = π–π T-shaped interactions; pink = π-alkyl). Active site of proteinase K consists of catalytic triad Asp39-His69-Ser224. Receptor–ligand interactions were visualized with BIOVIA Discovery Studio Visualizer 4.5.

Figure 3

Hydrophobicity surface representation of proteinase K binding site with docked fluorinated pyrazole aldehyde H2. Dark blue for most hydrophilic amino acids, through white, to orange and red for most hydrophobic amino acids. Compound H9 is located along active-site gorge, interacting with amino acid residues. Surface representation of enzyme binding site with docked compound was obtained using UCSF Chimera ver. 1.14.

Figure 4

(a) Three-dimensional representation of interactions between fluorinated pyrazole aldehyde H9 and amino acid residues of acetylcholinesterase (AChE); (b) two-dimensional diagram of main interactions and bond lengths (Å) with amino acid residues. (Green = conventional hydrogen bond; light green = van der Waals; dark purple = π–π T-shaped interactions; purple = π–π stacked interactions; light purple = π-alkyl interactions). Receptor-ligand interactions were visualized with BIOVIA Discovery Studio Visualizer 4.5.

Figure 5

Hydrophobicity surface representation of acetylcholinesterase (AChE) active-site gorge with docked fluorinated pyrazole aldehyde H9. Dark blue for most hydrophilic amino acids, through white, to orange and red for most hydrophobic amino acids. Compound H9 is located in peripheral active site (PAS), aromatic site (AS), acyl pocket (AP), and oxyanion hole (OH) of AChE, blocking narrow passage to catalytic site. Surface representation of enzyme binding site with docked compound was obtained using UCSF Chimera ver. 1.14.