Enantioselectivity of protoporphyrinogen oxidase-inhibiting herbicides

Protoporphyrinogen oxidase (Protox) was inhibited stereoselectively by three pairs of enantiomers belonging to diphenyl ether (DPE) and pyrazole phenyl ether (PPE) herbicide classes. The (R) enantiomers were 10- to 44-fold more active than the (S) enantiomers as inhibitors of Protox from barley etioplasts. Similarly, the (R) enantiomers caused green barley tissue to accumulate greater amounts of porphyrins and caused greater tissue damage than the (S) enantiomers. The (R) enantiomers competed more successfully with [ 14 C]acifluorfen than the (S) enantiomers for the binding sites on Protox. In the PPE class, the in-vitro and in-vivo activity differences were wider in the isopropyl pairs than in the n-propyl pairs. The DPE enantiomers were tested on ten dicotyledonous and six monocotyledonous weed species and ten crops for weed control and crop safety. In general, neither enantiomer had pre-emergence activity on monocotyledons, but the (R) enantiomer provided some monocotyledonous weed control when applied post-emergence. On dicotyledons, the (R) enantiomer provided excellent pre-emergence control, whereas the (S) enantiomer was inactive. The (R) enantiomer caused no injury to corn, cotton, peanuts, rice, sorghum, or soybeans applied pre-emergence, but it severely injured crops when applied post-emergence. There was a positive correlation between the activities of the compounds at the molecular, cellular and whole plant levels. The only molecular property differences found to account for differences in activity between members of chiral pairs were steric parameters.