NMR structural model of the interaction of herbicides with photosynthetic reaction center from Rhodobacter spheroides

The interaction of the herbicides acifluorfen and paraquat with the photosynthetic reaction center from Rhodobacter sphaeroides has been studied by NMR relaxation measurements. Interaction in aqueous solution has been demonstrated by evaluating motional features of the bound form through cross-relaxation terms of protons at fixed distances on the herbicides. Contributions to longitudinal nonselective relaxation rates different from the proton-proton dipolar relaxation were inferred, most probably due to paramagnetic effects originating from the high-spin nonheme Fe-II ion in the reaction center. Paramagnetic contributions to proton relaxation rates were converted into distance constraints in order to build a model for the interaction. The models place paraquat in the Q(B) site, where most herbicides interact, in agreement with docking calculations, whereas acifluorfen was placed between the metal and the Q(B) site, as also demonstrated by the induced paramagnetic shifts. Acifluorfen could therefore act to break the electron-transfer pathway between the Q(A) and Q(B) sites.