Electrochemical properties of the herbicide cacalol and its derivatives in protic and aprotic solvents by using cyclic voltammetry. Correlation with Hill's reaction activities

The redox behavior of cacalol, a natural product, and its semisynthetic derivatives, which were found acting as electron transport inhibitors in photosystem II, was examined by cyclic voltammetry. The studied compounds were cacalol (1), 2-acetylcacalol (2), cacalol acetate (3), 2-acetylcacalol acetate (4), and cacalol methyl ether (5). The voltammograms of 1 and 2 in a protic solvent were indicative of an electrochemically irreversible oxidation of the phenolic system. The electrooxidation of compounds 1-5 in an aprotic solvent was irreversible and more complex than in the protic solvent but presented similar oxidation patterns; however, compounds containing an unsubstituted phenolic system presented E(pa), which is near the potential for the water-splitting enzyme. This suggests that these compounds suffer oxidation and act as inhibitors to the oxygen evolution complex enzyme. Compounds 3-5 with higher oxidation potentials could not form a redox pair with the enzymes of photosystem II. Therefore, it is believed that these derivatives acted as inhibitors without any oxidation or reduction reaction.