Differential Metal Binding Interactions of the Imidazolinones Revealed by NMR and UV Spectroscopy

NMR and UV spectroscopy and molecular modeling methods were applied to probe the interaction of the two imidazolinones, imazethapyr (5‐ethyl‐2‐(4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl)nicotinic acid) and its structural isomer CL 303,135 (5‐ethyl‐3‐(4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl)picolinic acid), with metal ions. Both the imidazolinones inhibit the enzyme acetohydroxyacid synthase (AHAS) in vitro. However, while imazethapyr is a herbicide that is used widely in agriculture, CL303,135 does not exhibit herbicidal activity. Imazethapyr and CL303,135 exhibited considerable differences in their interactions with metals. In the metal complex of imazethapyr, the carboxyl moiety binds strongly and the pyridine nitrogen binds weakly with metals. In the case of CL303,135, both the pyridine nitrogen and the carboxyl group that are positioned ortho to each other participated strongly in the binding and were found to act together as a strong bidentate ligand to a metal ion. Both of the imidazolinones form predominantly 2:1 complexes with multivalent metal ions. However, imazethapyr binds two orders‐of‐magnitude more weakly (1·0×109 M‐2) with metal ions compared to CL303,135 (1·7×1011 M‐2). The interactions of the model compounds, nicotinic acid and picolinic acid, with metals were examined similarly. It was concluded that the strong affinity of CL303,135 for metals compared to imazethapyr may affect its absorption from soil into plants, or its translocation in plants, thereby explaining the differences in herbicidal activity of imazethapyr and CL303,135. © 1997 SCI.