Stereoselective degradation of metalaxyl and metalaxyl‐M in soil and sunflower plants

Abstract

A high proportion of agrochemicals are chiral compounds. Since stereoisomers often show different biological and physiological properties, the biological and metabolic responses to these compounds and their fate in the environment are expected to be different. In this work we investigate a possible stereo and/or enantioselective degradation in soil and plants (sunflower) of the fungicide Metalaxyl (rac-Metalaxyl) and the new compound Metalaxyl-M ((-)-(R)-Metalaxyl) and propose procedures for extraction, cleanup, chromatographic separation of enantiomers, and determination of the R : S ratio by using an HPLC chiral column. The degradation of the two stereoisomers of Metalaxyl proved to be enantioselective and dependent on the media: the (+)-(S)-enantiomer showed a faster degradation in plants, while the (-)-(R)-enantiomer showed a faster degradation in soil. In this study there was no evidence that racemization of Metalaxyl-M took place either in soil or in sunflowers.

Enantioselective sorption of the chiral fungicide metalaxyl on soil from non-racemic aqueous solutions: Environmental implications

“…Nevertheless, it should be noted that previous works have indicated that metalaxyl enantiomers are configurationally stable in soil, with no interconversion of R to S-enantiomers, and viceversa [31,41]. Furthermore, enantiomerization would not explain the greater total (R+S) sorption of metalaxyl from initial solutions enriched with R enantiomer (Fig.…”

Isomerization of chiral pesticides in the environment

“…The (R)<(S) enantioselectivity in metabolism of aryloxyalkanoic acid herbicides was observed in some plant species including green algae, and similarly in soil metabolism, but with no enantiomerization. 82,[112][113][114][115] Similar enantioselectivity was observed for metalaxyl (10) 89,116) and benalaxyl (11), 91) with no enantiomerization of the former herbicide reported. The simple comparison of soil and plant metabolism may suggest less contribution of enzyme-mediated enantiomerization of these pesticides in plants.…”

“…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).…”

Application of bioassays and soil column test for toxicity evaluation of selected pesticides

“…Metalaxyl-M [the (-)-(R)-enantiomer of methyl-N-(2-methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate] is the most widely known member of the phenylamide class of fungicides. It destroys the fungi by interacting with the fungal RNA polymerase I-template complex, thus inhibiting the incorporation of ribonucleotides in nascent RNA [1]. On the other hand, fosthiazate is an efficient nematicide and a member of the organophosphate class of pesticides (OPs).…”