Quantitative Structure-Activity Relationship of Substituted Benzoquinones as Inhibitors of Photosynthetic Electron Transport

1.4-benzoquinones have been tested for their inhibitory activity on photosynthetic NADP+ reduction by chloroplasts. Benzoquinones with one or two branched alkyl side chains are inhibitors of electron flow. Inhibitory activity can be highly increased if one - and even more if two - halogen substitutents are introduced into the quinone moiety. Iodine substitution yields a better inhibitor than bromine than chlorine. A quantitative structure activity relationship according to a bilinear model, as developed by Kubinyi, with the lipophilicity as the only parameter could be established.