Environmental Behavior of the Chiral Aryloxyphenoxypropionate Herbicide Diclofop-Methyl and Diclofop: Enantiomerization and Enantioselective Degradation in Soil

Abstract

In this study, the degradation of diclofop-methyl (DM) and its main metabolite, diclofop (DC), in two soils under aerobic and anaerobic conditions were investigated using enantioselective HPLC. Under aerobic or anaerobic conditions, rapid hydrolysis to the corresponding acid diclofop (DC)was observed. The results from this study revealed that the degradation of DM in the two soils is not enantioselective, and the calculated half-lives (t(1/2)) for the two soils were both less than 1 day. However, the degradation of DC in the two soils is enantioselective both under aerobic and anaerobic conditions, and the S-(-)-DC was preferentially degraded, resulting in relative enrichment of the R-(+)-form. The calculated t(1/2) values of the enantiomers of DC ranged between 8.7 and 43.3 days for aerobic incubation experiments and between 14.7 and 77.0 days for anaerobic incubation experiments, respectively. The enantiopure S-(-)- and R-(+)-DC were incubated under aerobic conditions, and it revealed significant enantiomerization with inversion of the S-(-)-enantiomer into R-(+)-enantiomer, and vice versa, and the S-(-)-DC showed a significantly higher inversion tendency than the R-(+)-DC.

Enantioseparation and Enantioselective Analysis of Chiral Herbicides

“…And it was proved that the S-diclofop-methyl dissipated faster than R-diclofop-methyl while the generation and degradation rates of S-diclofop were higher than R-enantiomer in the plant by Zhou et al (Gu et al, 2010). However, in a former report of Zhou et al (Diao et al, 2010a), it was found that the degradation of diclofop-methyl in two soils was not enantioselective while the degradation of diclofop was enantioselective under both aerobic and anaerobic conditions, and the S-(-)-diclofop was preferentially degraded, resulting in relative enrichment of the R-(+)-form. To haloxyfop ethoxyethyl ester, the S-form was degraded faster than R-form (the enantiomeric fraction of R-form was about 72%) (Desiderio et al, 1997a).…”

Isomerization of chiral pesticides in the environment

“…3 (4)]. 86) The opposite enantioselectivity of degradation [(R)>(S)] was reported for acetoanilide herbicide metalaxyl (10) [88][89][90]93) and benalaxyl (11), 91,92) while the same enantioselectivity as aryloxyalkanoic acids was observed for the acid metabolite of metalaxyl (10).…”

“…The (S)-enantiomers of aryloxyalkanoic acid herbicides and their esters are favorably degraded in aerobic soils, [78][79][80][81][82][83][84][85][86][87] but with no enantiomerization reported for the esters. 79,81,[85][86][87] In most cases, the degradation of each enantiomer is faster than the chiral conversion in either direction, and the (S) (R) conversion rate (k SR ) is larger than the opposite one (k RS ); such as k SR /k RS =1. 3 (4)].…”

Evaluating the enantioselective degradation and novel metabolites following a single oral dose of metalaxyl in mice

“…Chiral pesticides usually contain one or more pairs of enantiomers, which have the same physical properties. However, enantiomers generally differ in their biological properties including pharmacodynamic and pharmacokinetic properties as a consequence of their enantioselective interaction with enzymes or other naturally occurring chiral molecules [6][7][8]. For instance, (+)-malathion is more acutely toxic to anthropods and rats than the (−)-enantiomer [9].…”