Effect of formulation and repeated applications on the enantioselectivity of metalaxyl dissipation and leaching in soil

Enantioselective resolution of (R,S)-DMPM to prepare (R)-DMPM by an adsorbed-covalent crosslinked esterase PAE07 from Pseudochrobactrum asaccharolyticum WZZ003

(R)-N-(2,6-dimethylphenyl) aminopropionic acid methyl ester ((R)-DMPM) is an important chiral intermediate of the fungicide N-(2,6-Dimethylphenyl)-N-(methoxyacetyl)-alanine methyl ester ((R)-Metalaxyl). In this study, (1) D3520 (macroporous acrylic anion resin), selected from the ten resins, was used to immobilize the esterase from Pseudochrobactrum asaccharolyticum WZZ003 (PAE07) for resoluting the (R,S)-DMPM to obtain (R)-DMPM. (2) Up to 20 g/L PAE07 could be immobilized onto D3520 with a high enzymatic activity of 32.4 U/g. Moreover, the Km and Vmax values of 19.1 mM and 2.8 mM/min for D3520-immobilized PAE07 indicated its high activity and stereoselectivity. (3) The optimal temperature and pH for the immobilized PAE07 were 40 ℃ and 8.0, and substrate concentration was up to 0.35 M. After 15 h reaction, the conversion rate from (R,S)-DMPM to (R)-DMPM was 48.0% and the e.e.p and E values were 99.5% and 1393.0, respectively. In scale-up resolution, 200 g/L substrate and 12.5 g immobilized esterase PAE07 condition, a conversion rate from substrate to product of 48.1% and a product e.e.p of 98% were obtained within 12 h, with the activity of immobilized PAE07 retained 80.2% after 5 cycles of reactions. These results indicated that the D3520-immobilized esterase PAE07 had great potential for enzymatic resolution of (R,S)-DMPM to prepare (R)-Metalaxyl.

Fate of the neonicotinoid insecticide cycloxaprid in different soils under oxic conditions

Neonicotinoids are the most widely used pesticides worldwide due to their high toxicity to invertebrates. However, these compounds also increase the probability of environmental contamination. Cycloxaprid (CYC) is a promising neonicotinoid due to its insecticidal effectiveness and low cross resistance, but little is known about its fate in soils. Using radioisotope tracing techniques, the fate of ¹⁴C-labeled CYC enantiomers and racemic mixtures in aerobic soil was investigated in this research. After 100 d of incubation, the extractable residue (ER) of CYC decreased from 89.6% to 36.4% in red clay soil, from 46.1% to 10.1% in yellow loam soil, and from 93.2% to 12.2% in coastal saline soil. The radioactivity was substantially lower in methanol than in the other two solvents, but the distribution of CYC ER in various solvents across the three soils dramatically differed. The fraction of radioactive CYC that diffused into bound residue (BR) in the three soils increased over time to 56.8-83.0%. The variability in BR was influenced by soil properties such as organic matter concentration, pH, and residual microbial activity. Among the soils, yellow loam soil had the greatest tendency (53.0-83.0%) to form BR, while red clay soil showed the lowest capacity (7.5-61.2%). Cumulative mineralization (MI) to ¹⁴CO₂ accounted for 0.12-0.23%, 6.69-7.31% and 14.82-20.06% in acidic soil, neutral soil and alkaline soil, respectively, which suggests that the environmental fate of chiral pesticides may be influenced by soil pH. No stereoselective behavior was detected in this study. These findings provide a framework to assess the environmental impact and ecological safety of CYC application.

Preferential catabolism of the (S)-enantiomer of the herbicide napropamide mediated by the enantioselective amidohydrolase SnaH and the dioxygenase Snpd in Sphingobium sp. strain B2

The (R)- and (S)-enantiomers of the chiral herbicide napropamide (NAP) show different biological activities and ecotoxicities. These two enantiomers behave differently in the environment due to enantioselective catabolism by microorganisms. However, the molecular mechanisms underlying this enantioselective catabolism remain largely unknown. In this study, the genes (snaH and snpd) involved in the catabolism of NAP were cloned from Sphingobium sp. B2, which was capable of catabolizing both NAP enantiomers. Compared with (R)-NAP, (S)-NAP was much more rapidly transformed by the amidase SnaH, which initially cleaved the amide bonds of (S)/(R)-NAP to form (S)/(R)-2-(1-naphthalenyloxy)-propanoic acid [(S)/(R)-NP] and diethylamine. The α-ketoglutarate-dependent dioxygenase Snpd, showing strict stereoselectivity for (S)-NP, further transformed (S)-NP to 1-naphthol and pyruvate. Molecular docking and site-directed mutagenesis analyses revealed that when the (S)-enantiomers of NAP and NP occupied the active sites, the distance between the ligand molecule and the coordination atom was shorter than that when the (R)-enantiomers occupied the active sites, which facilitated formation of the transition state complex. This study enhances our understanding of the preferential catabolism of the (S)-enantiomer of NAP on the molecular level.

Dissipation of herbicides after repeated application in soils amended with green compost and sewage sludge

Certain agricultural practices, such as the repeated application of herbicides or organic amendments to soil, can influence herbicide dissipation. This research has studied the effects of two repeated applications of mesotrione, pethoxamid, and triasulfuron on their dissipation rates in unamended soil (S) and soil amended with green compost (S+GC) or sewage sludge (S+SS). The dissipation experiment has also included an evaluation of the adsorption of the three herbicides by soils and of changes in soil dehydrogenase activity (DHA). The adsorption of the three herbicides by amended soils (Kf range 0.83-2.98) was higher than by unamended soil (Kf range 0.20-0.64). The adsorption coefficients (Kd) of mesotrione and triasulfuron were higher for S+SS, while that of pethoxamid was higher for S+GC, but no relationship between values for the time to 50% degradation (DT₅₀) and adsorption coefficients could be determined. The repeated application of mesotrione decreased its dissipation rate in unamended soil (DT₅₀ increased from 4.75 to 8.15 days) and amended soils (DT₅₀ increased from 11.7 to 28.2 days in S+GC and from 17.7 to 37.9 in S+SS), whereas the repeated application of pethoxamid increased its dissipation rate in all the treatments, and the rate for triasulfuron increased only in amended soils. The highest DT₅₀ values for pethoxamid (12.3 days) and triasulfuron (57.1 days) were in S+GC, and the lowest in S+SS (8.35 and 24.7 days). Soil DHA was stimulated by the presence of GC in the soil and by the first application of mesotrione. The second application of mesotrione and pethoxamid positively affected soil DHA, but this did not occur for triasulfuron. The repeated applications of herbicides and soil organic amendments have different effects on herbicide dissipation, adsorption, and soil DHA, and they should be taken into account when assessing soil quality and other potential environmental implications of pesticide use.

Enantiomer-specific stable carbon isotope analysis (ESIA) to evaluate degradation of the chiral fungicide Metalaxyl in soils

Chiral pesticides are often degraded enantioselectively in soils, leading to disparity among enantiomers that may display different toxicity levels. Monitoring pesticide degradation extents and processes remains out of reach in the field using conventional bulk and enantiomer concentration analyses. Enantioselective stable carbon isotope analysis (ESIA) combines compound specific isotope analysis (CSIA) and enantioselective analysis, and bears potential to distinguish enantiomer degradation from non-destructive dissipation. We developed ESIA of the fungicide Metalaxyl, providing the ¹³C/¹²C ratios for S-Metalaxyl and R-Metalaxyl separately, and applied it to follow degradation in soil incubation experiments. Significant enantioselective degradation (k_S-MTY = 0.007-0.011 day^-1 < k_R-MTY = 0.03-0.07 day^-1) was associated with isotope fractionation (Δδ¹³C_S-MTY ranging from 2 to 6‰). While R-Metalaxyl degradation was rapid (T_1/2≈10 days), concomitant enrichment in heavy isotopes of the persistent S-Metalaxyl occurred after 200 days of incubation (ε_S-Metalaxyl ranging from -1.3 to -2.7‰). In contrast, initial racemic ratios and isotopic compositions were conserved in abiotic experiments, which indicates the predominance of microbial degradation in soils. Degradation products analysis and apparent kinetic isotope effect (AKIE) suggested hydroxylation as a major enantioselective degradation pathway in our soils. Altogether, our study underscores the potential of ESIA to evaluate the degradation extent and mechanisms of chiral micropollutants in soils.

Bio-preparation of (R)-DMPM using whole cells of Pseudochrobactrum asaccharolyticum WZZ003 and its application on kilogram-scale synthesis of fungicide (R)-metalaxyl

Methyl (R)-N-(2,6-dimethylphenyl)alaninate ((R)-DMPM) is a key chiral intermediate for the production of (R)-metalaxyl, which is one of the best-selling fungicides. A new strain, Pseudochrobactrum asaccharolyticum WZZ003, was identified as a biocatalyst for the enantioselective hydrolysis of (R,S)-DMPM. The key parameters including pH, temperature, rotation speed and substrate concentrations were optimized in the enantioselective hydrolysis of (R,S)-DMPM. After the 48 h hydrolysis of 256 mM (R,S)-DMPM under the optimized reaction conditions, the enantiomeric excess of product (e.e._p ) was up to 99% and the conversion was nearly 50%. Subsequently, the unhydrolyzed (S)-DMPM was converted to (R,S)-DMPM through the n-butanal-catalyzed racemization. Furthermore, stereoselective hydrolysis of (R,S)-DMPM catalyzed by whole cells of P. asaccharolyticum WZZ003 was scaled up to kilogram-scale, offering (R)-MAP-acid with 98.6% e.e._p and 48.0% yield. Moreover, (R)-metalaxyl was prepared at kilogram scale after subsequent esterification and coupling reactions. Therefore, a practical production process of (R)-DMPM and (R)-metalaxyl with the prospect of industrialization was developed in this study. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:921-928, 2018.

Impact of TiO2 on the chemical and biological transformation of formulated chiral-metalaxyl in agricultural soils

The impacts of TiO₂ on the chemical and biological transformation of racemic metalaxyl wettable powder (rac-metalaxyl WP) in agricultural soils, and soil microorganisms were investigated. Under simulated solar irradiation, TiO₂ highly promoted the transformation of rac-metalaxyl WP without changing the enantiomer fraction, with the promotion amplitude (60-1280%) being dependent on TiO₂ characteristics. TiO₂ characteristics showed different influence on the transformation of rac-metalaxyl WP in soils and aqueous solutions because their characteristics changed differently in soils. The impact of the mancozeb and other co-constituents on the transformation of rac-metalaxyl WP was smaller in soil media than in aqueous solution. Autoclave sterilization changed soil properties and subsequently weakened the promotion effects of TiO₂ on the chemical transformations of rac-metalaxyl WP to 0-233%. Microorganism biomass and bacterial community were not statistically significant changed by TiO₂ exposure regardless of rac-metalaxyl WP, suggesting that the promotional effects occurred mainly through chemical processes. The results also showed TiO₂-soil interactions may be strengthened with TiO₂ (Degussa P25) aging time in soils, which decreased its promotion amplitude from 1060% (without aging) to 880% (aging for 20 days). Intermediate formed in soil biological transformation process was different from that in TiO₂ photocatalysis process.

Metalaxyl Degradation by Mucorales Strains Gongronella sp. and Rhizopus oryzae

In this study, the degradation of metalaxyl was investigated in the presence of two Mucorales strains, previously isolated from soil subjected to repeated treatments with this fungicide and selected after enrichment technique. Fungal strains were characterised by a polyphasic approach using phylogenetic analysis of the Internal Transcribed Spacer (ITS) gene region, phenotypic characterisation by Matrix-Assisted Laser Desorption Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF MS) spectral analysis, and growth kinetics experiments. The strains were identified as Gongronella sp. and Rhizopus oryzae. The fungal growth kinetics in liquid cultures containing metalaxyl fits with Haldane model. Under laboratory conditions, the ability of Gongronella sp. and R. oryzae cultures to degrade metalaxyl was evaluated in liquid cultures and soil experiments. Both species were able to: (a) use metalaxyl as the main carbon and energy source; and (b) degrade metalaxyl in polluted soils, with rates around 1.0 mg kg^-¹ d^-¹. This suggests these strains could degrade metalaxyl in soils contaminated with this fungicide.

Assessing the Effect of Organoclays and Biochar on the Fate of Abscisic Acid in Soil

The potential use of allelopathic and signaling compounds as environmentally friendly agrochemicals is a subject of increasing interest, but the fate of these compounds once they reach the soil environment is poorly understood. This work studied how the sorption, persistence, and leaching of the two enantiomers of the phytohormone abscisic acid (ABA) in agricultural soil was affected by the amendments of two organoclays (SA-HDTMA and Cloi10) and a biochar derived from apple wood (BC). In conventional 24-h batch sorption experiments, higher affinity toward ABA enantiomers was displayed by SA-HDTMA followed by Cloi10 and then BC. Desorption could be ascertained only in BC, where ABA enantiomers presented difficulties to be desorbed. Dissipation of ABA in the soil was enantioselective with S-ABA being degraded more quickly than R-ABA, and followed the order unamended > Cloi10-amended > BC-amended > SA-HDTMA-amended soil for both enantiomers. Sorption determined during the incubation experiment indicated some loss of sorption capacity with time in organoclay-amended soil and increasing sorption in BC-amended soil, suggesting surface sorption mechanisms for organoclays and slow (potentially pore filling) kinetics in BC-amended soil. The leaching of ABA enantiomers was delayed after amendment of soil to an extent that depended on the amendment sorption capacity, and it was almost completely suppressed by addition of BC due to its irreversible sorption. Organoclays and BC affected differently the final behavior and enantioselectivity of ABA in soil as a consequence of dissimilar sorption capacities and alterations in sorption with time, which will affect the plant and microbial availability of endogenous and exogenous ABA in the rhizosphere.

Evaluation of an organo-layered double hydroxide and two organic residues as amendments to immobilize metalaxyl enantiomers in soils: A comparative study

Many pollutants released into the environment as a result of human activities are chiral. Pollution control strategies generally consider chiral compounds as if they were achiral and rarely consider enantiomers separately. We compared the performance of three different materials, an organically-modified anionic clay (HT-ELA) and two organic agro-food residues (ALP and ALPc), as amendments to immobilize the chiral fungicide metalaxyl in two soils with different textures, addressing the effects of the amendments on the sorption, persistence, and leaching of each of the two enantiomers of metalaxyl (R-metalaxyl and S-metalaxyl) separately. The effects of the amendments were both soil- and amendment-dependent, as well as enantiomer-selective. The organo-clay (HT-ELA) was much more efficient in increasing the sorption capacity of the soils for the two enantiomers of metalaxyl than the agro-food residues (ALP and ALPc), even when applied at a reduced application rate. The enhanced sorption in HT-ELA-amended soils reduced the bioavailability of metalaxyl enantiomers and their leaching in the soils, mitigating the particularly high leaching potential of the more persistent S enantiomer. The immobilizing capacity of the agro-food residues was more variable, mainly because their addition did not greatly ameliorate the sorption capacity of the soils and had variable effects on the enantiomers degradation rates. HT-ELA showed potential to reduce the bioavailability and mobility of metalaxyl enantiomers in soil and to mitigate the contamination problems particularly associated with the higher leaching potential of the more persistent enantiomer.

Environmental fate of the fungicide metalaxyl in soil amended with composted olive-mill waste and its biochar: An enantioselective study

A large number of pesticides are chiral and reach the environment as mixtures of optical isomers or enantiomers. Agricultural practices can affect differently the environmental fate of the individual enantiomers. We investigated how amending an agricultural soil with composted olive-mill waste (OMWc) or its biochar (BC) at 2% (w:w) affected the sorption, degradation, and leaching of each of the two enantiomers of the chiral fungicide metalaxyl. Sorption of metalaxyl enantiomers was higher on BC (Kd ≈ 145 L kg(-1)) than on OMWc (Kd ≈ 22 L kg(-1)) and was not enantioselective in either case, and followed the order BC-amended>OMWc-amended>unamended soil. Both enantiomers showed greater resistance to desorption from BC-amended soil compared to unamended and OMWc-amended soil. Dissipation studies revealed that the degradation of metalaxyl was more enantioselective (R>S) in unamended and OMWc-amended soil than in BC-amended soil. The leaching of both S- and R-metalaxyl from soil columns was almost completely suppressed after amending the soil with BC and metalaxyl residues remaining in the soil columns were more racemic than those in soil column leachates. Our findings show that addition of BC affected the final enantioselective behavior of metalaxyl in soil indirectly by reducing its bioavailability through sorption, and to a greater extent than OMWc. BC showed high sorption capacity to remove metalaxyl enantiomers from water, immobilize metalaxyl enantiomers in soil, and mitigate the groundwater contamination problems particularly associated with the high leaching potential of the more persistent enantiomer.

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

Mechanisms governing the enantioselectivity of the processes that determine the behavior of chiral pollutants in the environment need to be better understood. Understanding these mechanisms should help improve predictions of the hazards and risks chiral compounds can pose to people and the environment. We report the results of batch sorption experiments indicating that the sorption of the chiral fungicide metalaxyl on soil from non-racemic initial solutions was enantioselective. While from a racemic initial solution the two enantiomers of metalaxyl were sorbed on the soil to the same extent, increasing the fraction of R-enantiomer in the initial solution led to enhanced sorption of this enantiomer and to reduced sorption of the S-enantiomer. Considering the shape of the sorption isotherms (S-type) and the sorption behavior of model sorbents, we attributed this effect to molecular interactions between metalaxyl enantiomer species at the sorbed state, where R-R metalaxyl interactions appeared to be more favorable than R-S metalaxyl interactions. We discuss important environmental implications of the proposed mechanism, such as those related to the fact that the biological degradation of metalaxyl is known to be an enantioselective process that can yield non-racemic residues in soils shortly after application of the fungicide as a racemic mixture.