A STEREOCHEMICAL MODEL FOR THE ACTIVE SITE OF PHOTOSYSTEM II HERBICIDES

Abstract Photosystem II herbicides act by blocking electron transport at the secondary electron acceptor ‘Q_B’, thought to be a non‐covalently bound plastoquinone. Recent evidence suggests that these compounds work by displacing the plastoquinone from its site in the thylakoid. Since the herbicides cannot act as electron carriers, electron transport is then blocked. In this report a model is presented for the site of action of Photosystem II herbicides that encompasses biochemical, biophysical, and structure‐activity considerations. The essence of the model is that Photosystem II herbicides are non‐reducible analogues of plastoquinone or its semiquinone anion. As examples of the ways in which known herbicidal classes fit the model, the possible interactions of diuron, atrazine, the putative urea‐triazine hybrid MBAT (the a‐methylbenzyl analogue of atrazine), and dinoseb with the active site are discussed. This model provides a stereochemical basis for herbicidal activity and offers a qualitative approach for the design of novel Photosystem II herbicides.