Mode of action and mo calculation of two classes of herbicides interacting with the reducing side of photosystem II

Abstract Two chemically different classes of herbicides - represented by diuron and by dinoseb - are inhibitors of photosynthetic electron flow at the same functional site between the primary acceptor of photosystem II and plastoquinone. Recent functional and binding studies with radioactive labelled compounds with intact membranes from chloroplasts from susceptible and herbicide resistant plants or algae gave important new data on the biochemistry involved in the mode of action of these herbicides. In particular, photoaffinity labels identified the peptide substructure of the herbicide target: the B-protein on the thylakoid membrane. This membrane protein catalyses electron flow from photosystem II to plastoquinone and is composed of two subunits: a 32 kDa peptide carrying the binding site for diuron, triazine, amino-triazinone and related herbicides and a 42 kDa peptide carrying the binding site for phenol herbicides, like dinoseb and ioxynil. Already QSAR studies had let to quite diverse types of correlations for the two classes of herbicides. Molecular orbital calculations presented here provide further evidence that the two classes of photosystem II inhibitors differ significantly in the charge distribution at the atoms essential for binding to the membrane. π charge density maps of the basic structure reveal an asymmetric distribution in an amino-triazinone with a large positive charge at the nitrogen in the 4-position. The phenolic compounds in contrast are rather symmetric. The net charge distribution of compounds binding to the 42 kDa peptide is also quite different from those binding to the 32 kDa peptide.