Ring-substituted 8-hydroxyquinoline-2-carboxanilides as photosystem II inhibitors

Abstract Ring-substituted 8-hydroxyquinoline-2-carboxanilides inhibited photosynthetic electron transport (PET) through photosystem (PS) II. Their inhibitory efficiency depended on the compound lipophilicity, the electronic properties of the substituent R and the position of the substituent R on the benzene ring. The most effective inhibitors showing IC 50 values in the range 2.3–3.6 μM were substituted in C′ (3) by F, CH 3 , Cl and Br. The dependence of the PET-inhibiting activity on the lipophilicity of the compounds was quasi-parabolic for 3-substituted derivatives, while for C′ (2) ones a slight increase and for C′ (4) derivatives a sharp decrease of the activity were observed with increasing lipophilicity. In addition, the dependence of PET-inhibiting activity on electronic Hammett’s σ parameter of the substituent R was observed with optimum σ value 0.06 for C′ (4) and 0.34 for C′ (3) substituted derivatives, while the value of σ parameter did not significantly influence the PET-inhibiting activity of C′ (2) substituted compounds. Interactions of the studied compounds with chlorophyll a and aromatic amino acids present in the pigment–protein complexes mainly in PS II were documented by fluorescence spectroscopy. The section between P680 and plastoquinone Q B occurring on the acceptor side of PS II can be suggested as the site of action of the compounds.