Prediction of Hydrolysis Products of Organic Chemicals under Environmental pH Conditions.

Cheminformatics-based software tools can predict the molecular structure of transformation products using a library of transformation reaction schemes. This paper presents the development of such a library for abiotic hydrolysis of organic chemicals under environmentally relevant conditions. The hydrolysis reaction schemes in the library encode the process science gathered from peer-reviewed literature and regulatory reports. Each scheme has been ranked on a scale of one to six based on the median half-life in a data set compiled from literature-reported hydrolysis rates. These ranks are used to predict the most likely transformation route when more than one structural fragment susceptible to hydrolysis is present in a molecule of interest. Separate rank assignments are established for pH 5, 7, and 9 to represent standard conditions in hydrolysis studies required for registration of pesticides in Organisation for Economic Co-operation and Development (OECD) member countries. The library is applied to predict the likely hydrolytic transformation products for two lists of chemicals, one representative of chemicals used in commerce and the other specific to pesticides, to evaluate which hydrolysis reaction pathways are most likely to be relevant for organic chemicals found in the natural environment.

Microbial transformation of triadimefon to triadimenol in soils: selective production rates of triadimenol stereoisomers affect exposure and risk.

The microbial transformation of triadimefon, an agricultural fungicide of the 1,2,4-triazole class, was followed at a nominal concentration of 50 µg/mL over 4 months under aerobic conditions in three different soil types. Rates and products of transformation were measured, as well as enantiomer fractions of parent and products. The transformation was biotic and enantioselective, and in each soil the S-(+)-enantiomer reacted faster than the R-(-) one. Rates of the first-order reactions were 0.047, 0.057, and 0.107 d(-1) for the three soils. The transformation involves reduction of the prochiral ketone moiety of triadimefon to an alcohol, resulting in triadimenol, which has two chiral centers and four stereoisomers. The abundances of the four product stereoisomers were different from each other, but abundance ratios were similar for all three soil types. Triadimenol is also a fungicide; the commercial product is composed of two diastereomers of unequal amounts (ratio of about 4.3:1), each having two enantiomers of equal amounts. However, the triadimenol formed by soil transformation of triadimefon exhibited no such stereoisomer profile. Instead, different production rates were observed for each of the four triadimenol stereoisomers, resulting in all stereoisomer concentrations being different from each other and very different from concentration/abundance patterns of the commercial standard. This result is important in risk assessment if the toxicity of the environmental transformation product were to be compared to that of the commercial triadimenol. Because triadimenol stereoisomers differ in their toxicities, at least to fungi and rats, the biological activity of the triadimenol formed by microbes or other biota in soils depends on the relative abundances of its four stereoisomers. This is an exposure and risk assessment issue that, in principle, applies to any chiral pesticide and its metabolites.

Application of capillary electrophoresis to study the enantioselective transformation of five chiral pesticides in aerobic soil slurries.

The enantiomers of five chiral pesticides of environmental interest, metalaxyl, imazaquin, fonofos (dyfonate), ruelene (cruformate), and dichlorprop, were separated analytically using capillary electrophoresis (CE) with cyclodextrin chiral selectors. For metalaxyl, imazaquin, and fonofos, aqueous slurries of soil samples from two sites in Georgia and one in Ohio were spiked with the racemate of each pesticide at 50-60 mg/L of aqueous phase of the slurry, and CE analyses were performed at various time intervals to determine enantiomer fractions (EF). Metalaxyl underwent enantioselective transformation; in one soil, the half-life of the target active R-(+)-enantiomer was 17 days while that for the S-(-)-enantiomer was 69 days. Transformation occurred more slowly in the other two soils but was still selective for the R-(+)-enantiomer. Imazaquin and fonofos exhibited nonselective enantiomer loss over their 3 months of incubation time; this could have been due to abiotic or nonselective microbial reactions. Ruelene and dichlorprop were transformed selectively in a variety of soils in a previously reported study (7) that showed the influence of environmental changes on the transformation of chiral pollutants in soils; analytical methods used in that study are reported here to further illustrate the application of CE. CE is shown to be a simple, efficient, and inexpensive way to follow the transformation of chiral pesticides in laboratory microcosms where concentrations can be made high enough (25-50 mg/L initial racemate concentration) for detection of residual parent enantiomers during most of the process.

Estimation of microbial reductive transformation rates for chlorinated benzenes and phenols using a quantitative structure-activity relationship approach.

A set of literature data was used to derive several quantitative structure-activity relationships (QSARs) to predict the rate constants for the microbial reductive dehalogenation of chlorinated aromatics. Dechlorination rate constants for 25 chloroaromatics were corrected for the effects of hydrophobic partitioning and adjusted for the observed distribution of product species. A number of physicochemical properties and molecular parameters were considered for inclusion in the QSARs. Multivariate statistical analyses were used to select the optimal set of descriptors to minimize multicollinearity between the descriptors, as well as to minimize the p-value of the regression coefficients. The final QSAR included four descriptors: The logarithm of the octanol-water partition coefficient (Kow), the summation of the Hammett sigma constants, and the sigma induction constants in the ortho and meta positions relative to the transformation reaction center. The predictive ability of this QSAR was evaluated using 24 site-specific rate constants that were measured in five separate studies and were not used to derive the expression. The peer-reviewed literature was screened carefully to ensure that all rate constant data were representative of environmentally relevant conditions.

Enantioselective microbial transformation of the phenylpyrazole insecticide fipronil in anoxic sediments.

Fipronil, a chiral insecticide, was biotransformed initially to fipronil sulfide in anoxic sediment slurries following a short lag period. Sulfidogenic or methanogenic sediments transformed fipronil with half-lives of approximately 35 and 40 days, respectively. In all microbially active sediment slurries tested, the transformation of fipronil to fipronil sulfide was enantioselective. In the sulfidogenic sediment slurry, the enantiomeric fraction (EF) of fipronil decreased from an initial racemic EF value of 0.46 to a value of 0.22 during the incubation period of active fipronil transformation, indicating preferential transformation of the S-(+)-enantiomer. A previously unidentified product, 5-amino1-[2,6-dichloro-4-(trifluoromethyl)-phenyl]-4-(trifluoromethylthio)-1-H-pyrazole-3-carboxyamide, or fipronil sulfide-amide, was detected in the sulfidogenic slurries and coincided with the loss of fipronil sulfide. Biota from methanogenic freshwater sediment slurries also transformed fipronil enantioselectively but with a preference for the R-(-)-enantiomer. In all microbially inhibited (autoclaved) sediment slurries tested, no changes in the enantiomeric fractions of fipronil were observed and only low levels (< 5% of the added fipronil) of the fipronil sulfide metabolite were detected. In defined (model) chemical experiments, solutions of pyrite (FeS2) and iron sulfide (FeS) non-enantioselectively transformed fipronil primarily to either 2,6-dichloro-4-(trifluoromethyl)-aniline or to fipronil sulfide and fipronil amide, respectively. This report provides the first experimental evidence of enantioselective microbial transformation of fipronil in a natural environment (soil, water, and sediment) as well as identification of a novel fipronil biotransformation product.

H2 consumption during the microbial reductive dehalogenation of chlorinated phenols and tetrachloroethene.

Competition for molecular hydrogen exists among hydrogen-utilizing microorganisms in anoxic environments, and evidence suggests that lower hydrogen concentrations are observed with more energetically favorable electron-accepting processes. The transfer of electrons to organochlorines via reductive dehalogenation reactions plays an important role in hydrogen dynamics in impacted systems. We studied the flux of aqueous hydrogen concentrations in methanogenic sediment microcosms prior to and during reductive dehalogenation of a variety of substituted chlorophenols (CP) and tetrachloroethene (perchloroethylene, PCE). Mean hydrogen concentrations during reductive dehalogenation of 2,4-CP, 2,3,4-CP, and PCP were 3.6 nM, 4.1 nM, and 0.34 nM, respectively. Sediment microcosms that were not dosed with chlorophenols yet were actively methanogenic maintained a significantly higher mean hydrogen concentration of 9.8 nM. During active PCE dehalogenation, sediment microcosms maintained a mean hydrogen concentration of 0.82 nM. These data indicate that during limiting hydrogen production, the threshold ecosystem hydrogen concentration is controlled by microbial populations that couple hydrogen oxidation to thermodynamically favorable electron accepting reactions, including reductive dehalogenation of chloroaromatic and chloroaliphatic compounds. We also present revised estimates for the Gibbs free energy available from the reductive dehalogenation of a variety of substituted chlorophenols based on recently published values of vapor pressure, solubility, and pKa for these compounds.

Changes in enantiomeric fractions during microbial reductive dechlorination of PCB132, PCB149, and araclor 1254 in Lake Hartwell sediment microcosms.

The enantioselectivity of microbial reductive dechlorination of chiral PCBs in sediments from Lake Hartwell, SC, was determined by microcosm studies and enantiomer-specific GC analysis. Sediments from two locations in the vicinity of the highest levels of PCB contamination were used as inocula. Dechlorination activity was monitored by concentration decreases in the spiked chiral PCBs and formation of dechlorination products using both achiral and chiral chromatography. Live microcosms spiked with PCB132 (234-236) exhibited dechlorination of PCB132 to PCB91 (236-24) and PCB51 (24-26). Meta dechlorination was the dominant mechanism. Microcosms spiked with PCB149 (245-236) exhibited preferential para dechlorination of PCB149 to PCB95 (236-25), followed by meta dechlorination to PCB53 (25-26) and subsequently PCB19 (26-2). Dechlorination of chiral PCB132 and PCB149 was not enantioselective. In Aroclor 1254-spiked microcosms, reductive dechlorination of PCB149 also was nonenantioselective. These results suggest that dechlorinating enzymes responsible for the dehalogenation of the chiral PCB132 and PCB149 congeners bind the two enantiomers equally. Reductive dechlorination of PCB91 and PCB95, however, occurred in an enantioselective manner, indicating that the dechlorinating enzymes for these PCBs are enantiomer-specific. The chlorine substitution pattern on the biphenyl ring appears to influence whether reductive dechlorination of chiral PCB congeners is enantioselective. Enantioselective PCB dechlorination by the microbial population of Lake Hartwell sediments occurs for select chiral PCBs; thus, certain chiral PCBs might be useful as markers for in situ reductive dechlorination.