title: Environmental behavior of the chiral acetamide pesticide metalaxyl: enantioselective degradation and chiral stability in soil.
abstract: Racemic metalaxyl is currently being replaced in many countries by metalaxyl-M, the fungicide enantiomerically enriched with the biologically active R-enantiomer. This "chiral switch" is expected to reduce the amount of pesticide released into the environment as well as potential side-effects on nontarget organisms. Detailed knowledge of the environmental behavior of such chiral compounds should include information on the chiral stability (interconversion of enantiomers). In the present study, the degradation/ dissipation of metalaxyl and its primary carboxylic acid metabolite (MX-acid) in soil was investigated under laboratory conditions using enantioselective gas chromatography mass spectrometry (GC-MS). Racemic and the enantiopure R- and S-compounds were incubated in separate experiments. The degradation of metalaxyl was shown to be enantioselective with the fungicidally active R-enantiomer being faster degraded than the inactive S-enantiomer, resulting in residues enriched with S-metalaxyl when the racemic compound was incubated. The relatively high enantioselectivity suggests that degradation/dissipation was largely biological. The data indicated a conversion of 40-50% of metalaxyl to MX-acid, and the remaining metalaxyl being degraded via other pathways. The degradation of MX-acid was also enantioselective. Metalaxyl and MX-acid were both configurationally stable in soil, showing no interconversion of R- to S-enantiomers, and vice-versa. Furthermore, the conversion of metalaxyl to MX-acid proceeded with retention of configuration. Degradation followed approximate first-order kinetics but showed significant lag phases.
authors.affiliation: Swiss Federal Research Station, Wädenswil. hans-rudolf.buser@faw.admin.ch
authors.forename: Hans-Rudolf
authors.initials: HR
authors.lastname: Buser
authors.forename: Markus D
authors.initials: MD
authors.lastname: Müller
authors.forename: Thomas
authors.initials: T
authors.lastname: Poiger
authors.forename: Marianne E
authors.initials: ME
authors.lastname: Balmer
mesh_terms: D000409:Alanine / Q000031:analogs & derivatives* / Q000032:analysis* / Q000378:metabolism*; D001673:Biodegradation, Environmental; D005659:Fungicides, Industrial / Q000032:analysis* / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D007700:Kinetics; D008968:Molecular Conformation; D012989:Soil Pollutants / Q000032:analysis* / Q000378:metabolism*
chemical_list: D005659:Fungicides, Industrial; D012989:Soil Pollutants; C028175:metalaxyl; D000409:Alanine