Quinolones and their N-oxides as inhibitors of photosystem II and the cytochrome b(6)/f-complex

4(1H)-quinolones (2-alkyl- (1), 2-alkyl-3-methyl- (2), 2-methyl-3-alkyl- (3 ), 1-hydroxy-2-methyl-3-alkyl- (4) and 1-hydroxy-2-alkyl- (5)) with n-alkyl side chains varying from C-5 to C-17 have been synthesized and tested for biological activity in photosystem II and the cytochrome b(6)/f-complex. In photosystem II, quinolones 1 and 2 showed only moderate activity, whereas 3 < 5 < 4 (increasing activity) were potent inhibitors. Displacement experi ments with [C-14]atrazine indicated that the quinolones share an identical binding site with other photosystem II commercial herbicides. In the cytoch rome b(6)/f-complex, only 3 < 4 showed enhanced activity. Maximal inhibitor y potency was achieved at a carbon chain length of 12-14 <Angstrom>(.) Furt her increase of the chain length decreased activity. In a quantitative stru cture-activity relationship inhibitory activity in photosystem II and the c ytochrome b(6)/f-complex could be correlated to the physicochemical paramet ers lipophilicity ir and/ or to STERIMOL L. (C) 2001 Elsevier Science B.V. All rights reserved.