Abstract

Inhibition of photosynthetic electron transport (PET) in spinach chloroplasts by sixty-one ring-substituted N-benzylsalicylamides was investigated. The inhibitory potency of the compounds expressed by IC₅₀ value varied from 2.0 to 425.3 μmol/L. Several evaluated compounds can be considered as effective PET inhibitors; these include N-(3,4- dichlorobenzyl)-2-hydroxy-5-nitrobenzamide (IC₅₀ = 2.0 μmol/L), 3,5-dibromo-N-(3,4-dichlorobenzyl)-2-hydroxybenzamide (IC₅₀ = 2.3 μmol/L) and 3,5-dibromo-N-(4-chlorobenzyl)-2-hydroxybenzamide (IC₅₀ = 2.6 μmol/L) with activity comparable with that of the standard Diuron (IC₅₀ = 1.9 μmol/L). The PET inhibiting activity increased approximately linearly with increasing lipophilicity of the compounds as well as with the increasing sum of Hammett σ constants of the substituents on the acyl fragment (R¹ = H, 5-OCH₃, 5-CH₃, 5-Cl, 5-Br, 5-NO₂, 4-OCH₃, 4-Cl, 3,5-Cl and 3,5-Br) and the benzylamide fragment (R² = H, 4-OCH₃, 4-CH₃, 4-F, 4-Cl and 3,4-Cl). Based on the evaluated structure-PET inhibiting activity relationships (QSAR) it was confirmed that the inhibitory activity of the compounds depends on lipophilicity (log P or distributive parameters π ¹ and π²of individual substituents) and electronic properties of the substituents on the acyl (σ¹) and the benzylamide fragments (σ²), the contribution of σ¹ being more significant than that of σ².

Keywords: Hammett constants, lipophilicity, photosynthetic electron transport inhibition, ring-substituted N-benzylsalicylamides, spinach chloroplasts, structure-activity relationships.

Graphical Abstract