Enantioselective Degradation of Metalaxyl in Soils:  Chiral Preference Changes with Soil pH

Abstract

Chiral pesticides are often degraded enantio-/stereoselectively in soils. Degradation is typically studied with one or a small number of soils so that it is not possible to extrapolate the findings on chiral preference to other soils. For this study, the fungicide metalaxyl was chosen as a "chiral probe" to investigate its enantioselective degradation in 20 different soils, selected primarily to cover a wide range of soil properties (e.g., acidic/alkaline, aerobic/ anaerobic) rather than to consider soils of agricultural importance. Racemic metalaxyl was incubated in these soils under laboratory conditions, and the degradation of the enantiomers as well as the enantioselective formation/ degradation of the primary major metabolite, metalaxyl acid, was followed over time, using enantioselective GC-MS after ethylation with diazoethane. In aerobic soils with pH > 5, the fungicidally active R-enantiomer was degraded faster than the S-enantiomer (k(R) > k(S)), leading to residues with a composition [S] > [R]. However, in aerobic soils with pH 4-5, both enantiomers were degraded at similar rates (k(R) approximately k(S)), and in aerobic soils with pH < 4 and in most anaerobic soils, the enantioselectivity was reversed (k(R) < k(S)). These considerable soil-to-soil variations were observed with soils from locations close to each other, in one case even within a single soil profile. Liming and acidification of a "nonenantioselective" soil prior to incubation resulted in enantioselective degradation with k(R)> k(S) and k(R) < k(S), respectively. While the enantioselectivity (expressed as ES = (k(R) - k(S))/(k(R) + k(S))) of metalaxyl degradation in aerobic soils apparently correlated with soil pH, no such correlation was found for metalaxyl acid. Reevaluation of published kinetic data for the herbicides dichlorprop and mecoprop indicated similar correlations between soil pH and ES as for metalaxyl.

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

“…It can exist as two enantiomers, R-(-) and S-(+) (Fig. S1), but the fungicidal activity mostly originates from the R-(-) enantiomer [29,30]. Currently, metalaxyl is marketed and used both as a racemic formulation and as R-(-)-single enantiomer-rich product, called Metalaxyl-M.…”

“…Currently, metalaxyl is marketed and used both as a racemic formulation and as R-(-)-single enantiomer-rich product, called Metalaxyl-M. Previous studies have indicated that the degradation of metalaxyl in soil is an enantioselective, microbiologically-mediated process, with S-metalaxyl being degraded more rapidly than R-metalaxyl in anaerobic soils and in aerobic soils with pH < 4, and R-metalaxyl being degraded more rapidly than S-metalaxyl in aerobic soils with pH > 5 [9,21,[29][30][31].…”

Enantioselectivity in environmental safety of current chiral insecticides

“…However, if the adverse effects are caused only by or mostly by the excluded enantiomer, using enantiomer-enriched products may offer great environmental benefits. In addition, studies show that enantioselectivity may closely depend on environmental conditions and may vary significantly even for the same compound (4,14,24,25). Further studies are needed to characterize interactions of environmental factors, such as soil͞sediment properties, vegetation types, redox conditions, and microbial structures, with enantioselectivity in the behavior of chiral pesticides.…”

“…In the brain tissue of seals around Iceland, often only (ϩ) ␣-HCH was found; (Ϫ) ␣-HCH was absent, giving indefinitely large ER values (5,13). Enantioselectivity also has been observed under laboratory conditions in the soil degradation of some chiral herbicides and fungicides, including metolachlor, metalaxyl, and dichlorprop (4,8,14).…”

Chiral pesticides

“…They concluded that the soil and the presence of bacteria that breakdown chemicals are important factors in determining which isomer might become more prominent over time. 89) Based on the comparison of measured and calculated vibrational circular dichroism, the absolute configuration of (Ϫ)-enantiomer of heptachlor (43) (Fig. 10) can be assigned unambiguously as 1R-heptachlor.…”