Structure Activity Correlation of Herbicides Affecting Plastoquinone Reduction by Photosystem II: Electron Density Distribution in Inhibitors and Plastoquinone Species

Abstract

Molecular orbital calculations of the net charge and the π charge distribution in several inhibitors and herbicides of the functionally related group of the diuron and dinoseb type are reported. They confirm the model that urea, aminotriazinone and triazine herbicides all have in common a positive π-charge at a particular atom considered to be essential for binding. Phenol type inhibitors have different charge distribution and a model for their essential features is presented. The calculations support the finding that two different subunits with different binding characteristics are involved in inhibitor and plastoquinone function on the acceptor side of photosystem II. Force-field model building and MO calculations of the charge distribution of a plastoquinone analogue with a butenyl side chain, of two of its semiquinone forms and of the hydroquinone, are reported, as well as their conformation with the lowest energy content and their likely anionic forms.