X-RAY STRUCTURE-ANALYSIS OF A CYANOACRYLATE INHIBITOR OF PHOTOSYSTEM-II ELECTRON-TRANSPORT

X-ray crystallographic data for the highly potent cyanoacrylate photosynthetic electron transport inhibitor, (Z)-ethoxyethyl 3-(4-chlorobenzylamino)-2-cyano-4-methylpent-2-enoate, are presented. This compound has a particularly high affinity for the photosystem II (PS II) herbicide receptor with a pI50 value of 9.5 (in the Hill reaction under uncoupled conditions with a chlorophyll concentration of 0.1-mu-g/ml). Data regarding the structure of small ligands, such as this potent cyanoacrylate, which bind to the site with high affinity may be used to provide the basis for modelling studies of PS II/herbicide complexes. The X-ray data presented confirm the Z-stereochemistry of active cyanoacrylates and demonstrate the presence of a planar core stabilized by an intramolecular hydrogen bond between the ester carbonyl oxygen and a benzylamino hydrogen atom. In order to assess the importance of the benzylamino - NH- group in this type of cyanoacrylate, analogues containing a methylene group in its place were synthesized and found to be 100- and 1000-fold less active as Hill inhibitors.