Influence of environmental changes on degradation of chiral pollutants in soils

Abstract

Numerous anthropogenic chemicals of environmental concern--including some phenoxy acid herbicides, organophosphorus insecticides, polychlorinated biphenyls, phthalates, freon substitutes and some DDT derivatives--are chiral. Their potential biological effects, such as toxicity, mutagenicity, carcinogenicity, and endocrine disrupter activity, are generally enantiomer-selective, and different enantiomers are preferentially degraded (transformed) by micro-organisms in various environments. Here we use field and laboratory experiments to demonstrate that environmental changes in soils can alter these preferences, and to suggest that the preferences shift owing to different groups of related microbial genotypes being activated by different environmental changes. In Brazilian soils, almost all pasture samples preferentially transformed the non-herbicidal enantiomer of dichlorprop ((RS)-2-(2,4-dichlorophenoxy)propionic acid), while most forest samples either transformed the herbicidal enantiomer more readily or as rapidly as the non-herbicidal enantiomer. Organic nutrient enrichments shifted enantioselectivity for methyl dichlorprop ((RS)-methyl 2-(2,4-dichlorophenoxy)propionic acid) strongly towards preferentially removing the non-herbicidal enantiomer in soils from Brazil and North America, potentially increasing phytotoxicity of its residues relative to that of the racemate. Assessments of the risks chemical pollutants pose to public health and the environment need to take into account the chiral selectivity of microbial transformation processes and their alteration by environmental changes, especially for pesticides as up to 25 per cent are chiral.

Probing the Enantioselectivity of Chiral Pesticides

“…The goal was to probe the differences in activities of microbial populations in control soil samples and samples that were purposely disturbed or treated from field plots in Brazil, North America, and Norway (28). Water slurries of soils from these plots were spiked and analyzed at different times for these pesticide residues.…”

“…(c-e) The microbial transformation of methyl dichlorprop enantiomers in soil was followed by gas chromatography with a Chirasil-Dex column used for enantiomer separation. (c) Initial separation, (d) 24 h later, and (e) after 48 h (28 ).…”

Chiral pesticides

“…46) Among other methods, asymmetric catalysis is often the most efficient because a small amount of a chiral catalyst can be used to produce a large quantity of a chiral target molecule. Technical synthesis of the herbicides (S) enantiomer of metolachlor (3), (R) enantiomer of metalaxyl (1) and clozylacon (26) (Fig. 6) involved enantioselective hydrogenation of an imine intermediate using a novel iridium ferrocenyldiphosphine catalyst.…”

“…[96][97] Asymmetric synthesis is commonly used to prepare chiral compounds. The importance of chiral "synthons" in the preparation of new, structurally diverse, optically pure pesticides 98) as well as the application of enantioselective catalytic methods for the technical preparation of chiral pesticides [(S)-enantiomer of metolachlor (3), (R)-enantiomer of metalaxyl (1) and clozylacon (26)] has been reported. 99) The key step in the technical synthesis of the herbicide (S)-metolachlor (11) involved the enantioselective hydrogenation of an imine intermediate using a novel iridium ferrocenyl-diphosphine catalyst.…”

“…Studies on chiral pesticides started to appear in the early 1990s. [19][20][21][22][23][24][25][26][27][28] Companies have been deeply interested in selling synthetic chiral pesticides as single enantiomers in the past decade. The key reasons why single isomers are less common than they could be are probably limited access to chiral raw materials and economic synthetic routes.…”