Comparison of Experimental and Calculated Hydrogen Bonding Properties of Some Urea and Triazine Inhibitors of Photosystem II

Previous studies o f structure/activity relationships o f photosystem II inhibitors, including comparisons o f their inhibitory power in herbicide-resistant and susceptible chloroplasts, have led to predict the role o f hydrogen bonding, associated to hydrophobicity, in the binding onto the Qb site. The crystallographic structures o f bacterial reaction centers now allow these bonds to be identified. In order to be able to understand the binding o f various herbicides and the effects o f resistance mutations within the QB site, a reliable estimation o f hydrogen bonding strengths is needed. We show here, by calculating interactions with model compounds, con­ trolled by physicochemical measurements, that the hydrogen bonding properties o f the C = X nucleophilic moiety present in most PS II inhibitors are different for triazines as compared to urea or amide derivatives. Semiempirical methods (AM 1) fail to reproduce the energies o f hy­ drogen bonds between a triazine ring nitrogen and a phenolic proton. An empirical method (SI BFA), designed to reproduce interaction energies, has been adapted with the aim o f calculat­ ing the binding energies o f various herbicides with models o f the QB site.