Heterocyclic ortho-quinones, a novel type of Photosystem II inhibitors

Members of the new chemical class of 7-substituted 6-bromo-benzo[4,5]imidazo[1,2alpha]pyridin-8,9-diones were found to be excellent inhibitors at the Q(B) site of the photosystem II D1 reaction center protein. The best inhibitors with pI(50)-values of >7 are: dimethyl-propyl, 7.05; i-pentyl, 7.36; t. butyl, 7.47; and i-propyl, 7.51. Displacement experiments with [14C]atrazine revealed that the 8,9-diones behave non-competitively in respect of Photosystem II herbicides and, hence, have to be considered as a new type of Photosystem II inhibitors. This notion is further corroborated by their inhibitory activity in D1 mutants of Chlamydomonas reinhardtii.