Structure-Activity Relationships of N-benzylsalicylamides for Inhibition of Photosynthetic Electron Transport
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.
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