Photosynthesis-inhibiting effects of 2-benzylsulphanylbenzimidazoles in spinach chloroplasts

Inhibition of photosynthetic electron transport (PET) in spinach chloroplasts by nineteen 2-benzylsulphanylbenzimidazoles (BZA) was studied. BZA were found to inhibit photosynthetic electron transport (PET) and for their inhibitory efficiency, electronic properties of the R substituent on the benzyl moiety are decisive. The PET inhibiting activity of the studied BZA expressed as IC50 varied in the range from 28.5 μM (R = 3,5-(CF3)2) to 394.5 μM (R = 2,4-(NO2)2). For compounds with R = H, 4-CH3, 3-CH3, 3-OCH3, 4-F, 3-F, 4-Cl, 3-Cl, 2-Cl, 4-Br, 3-Br, 3,4-F2, 3,4-Cl2, 3-CF3, 3,5-(CF3)2 linear increase of the inhibitory activity with the increasing value of the substituent’s σ constant up to 0.86 was observed. Further increase of the σ constant resulted in a sharp activity decrease of the corresponding compounds (R = 2-F-6-Cl, 2-NO2, 3,5-(NO2)2, 2,4-(NO2)2). Using EPR spectroscopy and an artificial electron donor diphenyl carbazide it was found that the site of BZA action in the photosynthetic apparatus is situated on the donor side of PS 2, prior to the Z·/D· intermediate.