Position-specific isotope modeling of organic micropollutants transformation through different reaction pathways

Kinetic isotope effects of C and N indicate different transformation mechanisms between atzA- and trzN-harboring strains in dechlorination of atrazine

Compound-specific stable isotope analysis provides an alternative method to insight into the biotransformation mechanisms of diffuse organic pollutants in the environment, e.g., the endocrine disruptor herbicide atrazine. Biotic hydrolysis process catalyzed by chlorohydrolase AtzA and TrzN plays an important role in the detoxification of atrazine, while the catalytic mechanism of AtzA is still speculative. To investigate the catalytic mechanism of AtzA and answer whether both enzymes catalyze hydrolytic dechlorination of atrazine by the same mechanism, in this study, apparent kinetic isotope effects (AKIE) for carbon and nitrogen were observed by three atzA-harboring bacterial isolates and their membrane-free extracts. The AKIEs obtained from atzA-harboring bacterial isolates (AKIEC = 1.021 ± 0.010, AKIEN = 0.992 ± 0.003) were statistically different from that of trzN-harboring strains (AKIEC = 1.040 ± 0.006, AKIEN = 0.983 ± 0.006), confirming the different activation mechanisms of atrazine preceding to nucleophilic aromatic substitution of Cl atom in actual enzymatic reaction catalyzed by AtzA and TrzN, despite the limitation of variable dual-element isotope plots. The lower degree of normal carbon and inverse nitrogen isotope fractionation observed from atzA-harboring strains, suggesting AtzA catalyzing hydrolytic dechlorination of atrazine by coordination of Cl and one aromatic N to the Fe2+ drawing electron density from carbon-chlorine bond that facilitating the nucleophilic attack, rather than in TrzN case that protonation of aromatic N increasing nucleophilic substitution of Cl atom. This study suggests considering the potential influences of phylogenetic diversity of bacterial isolates and evolution of enzymes on the applications of CSIA method in future study.

Position-specific isotope effects during alkaline hydrolysis of 2,4-dinitroanisole resolved by compound-specific isotope analysis, 13C NMR, and density-functional theory

Compound-specific isotope analysis (CSIA), position-specific isotope analysis (PSIA), and computational modeling (e.g., quantum mechanical models; reactive-transport models) are increasingly being used to monitor and predict biotic and abiotic transformations of organic contaminants in the field. However, identifying the isotope effect(s) associated with a specific transformation remains challenging in many cases. Here, we describe and interpret the position-specific isotope effects of C and N associated with a S_N2Ar reaction mechanism by a combination of CSIA and PSIA using quantitative ¹³C nuclear magnetic resonance spectrometry, and density-functional theory, using 2,4-dinitroanisole (DNAN) as a model compound. The position-specific ¹³C enrichment factor of O-C₁ bond at the methoxy group attachment site (ε_C1) was found to be approximately -41‰, a diagnostic value for transformation of DNAN to its reaction products 2,4-dinitrophenol and methanol. Theoretical kinetic isotope effects calculated for DNAN isotopologues agreed well with the position-specific isotope effects measured by CSIA and PSIA. This combination of measurements and theoretical predictions demonstrates a useful tool for evaluating degradation efficiencies and/or mechanisms of organic contaminants and may promote new and improved applications of isotope analysis in laboratory and field investigations.

Photodegradation of pesticides using compound-specific isotope analysis (CSIA): a review

Pesticides are commonly applied in agriculture to protect crops from pests, weeds, and harmful pathogens. However, chronic, low-level exposure to pesticides can be toxic to humans. Photochemical degradation of pesticides in water, soil, and other environmental media can alter their environmental fate and toxicity. Compound-specific isotope analysis (CSIA) is an advanced diagnostic tool to quantify the degradation of organic pollutants and provide insight into reaction mechanisms without the need to identify transformation products. CSIA allows for the direct quantification of organic degradation, including pesticides. This review summarizes the recent developments observed in photodegradation studies on different categories of pesticides using CSIA technology. Only seven pesticides have been studied using photodegradation, and these studies have mostly occurred in the last five years. Knowledge gaps in the current literature, as well as potential approaches for CSIA technology for pesticide monitoring, are discussed in this review. Furthermore, the CSIA analytical method is challenged by chemical element types, the accuracy of instrument analysis, reaction conditions, and the stability of degradation products. Finally, future research applications and the operability of this method are also discussed.

Tracking photodegradation products and bond-cleavage reaction pathways of triclosan using ultra-high resolution mass spectrometry and stable carbon isotope analysis

Triclosan (TCS) is an antimicrobial compound ubiquitously found in surface waters throughout the world. Although several studies have focused on the photochemical degradation of TCS, there is still limited knowledge about its environmental fate. In this study, we got molecular-level insights into the photochemical degradation of TCS. Significant stable carbon isotope fractionation was observed during photodegradation; different bond-cleavage reaction pathways under different photolytic conditions were characterized, using compound specific isotope analysis (CSIA). Photochemical modeling of TCS photodegradation showed that direct photolysis would be the main transformation pathway if pH > 7, even in presence of dissolved organic matter. Moreover, by use of ultrahigh resolution mass spectrometry, FT-ICR-MS, a broad and complex spectrum of organic by-products (some of which potentially toxic, as assessed by a quantitative structure-activity relationship approach) were identified. A detailed reaction mechanism was developed on the basis of the detected compounds. A possible sequence of steps leading to some of the detected product compounds in aqueous solution is suggested.

Simulation of dual carbon-bromine stable isotope fractionation during 1,2-dibromoethane degradation

We performed a model-based investigation to simultaneously predict the evolution of concentration, as well as stable carbon and bromine isotope fractionation during 1,2-dibromoethane (EDB, ethylene dibromide) transformation in a closed system. The modelling approach considers bond-cleavage mechanisms during different reactions and allows evaluating dual carbon-bromine isotopic signals for chemical and biotic reactions, including aerobic and anaerobic biological transformation, dibromoelimination by Zn(0) and alkaline hydrolysis. The proposed model allowed us to accurately simulate the evolution of concentrations and isotope data observed in a previous laboratory study and to successfully identify different reaction pathways. Furthermore, we illustrated the model capabilities in degradation scenarios involving complex reaction systems. Specifically, we examined (i) the case of sequential multistep transformation of EDB and the isotopic evolution of the parent compound, the intermediate and the reaction product and (ii) the case of parallel competing abiotic pathways of EDB transformation in alkaline solution.

Experimental Determination of Isotope Enrichment Factors - Bias from Mass Removal by Repetitive Sampling

Application of compound-specific stable isotope approaches often involves comparisons of isotope enrichment factors (ε). Experimental determination of ε-values is based on the Rayleigh equation, which relates the change in measured isotope ratios to the decreasing substrate fractions and is valid for closed systems. Even in well-controlled batch experiments, however, this requirement is not necessarily fulfilled, since repetitive sampling can remove a significant fraction of the analyte. For volatile compounds the need for appropriate corrections is most evident, and various methods have been proposed to account for mass removal and for volatilization into the headspace. In this study we use both synthetic and experimental data to demonstrate that the determination of ε-values according to current correction methods is prone to considerable systematic errors even in well-designed experimental setups. Application of inappropriate methods may lead to incorrect and inconsistent ε-values entailing misinterpretations regarding the processes underlying isotope fractionation. In fact, our results suggest that artifacts arising from inappropriate data evaluation might contribute to the variability of published ε-values. In response, we present novel, adequate methods to eliminate systematic errors in data evaluation. A model-based sensitivity analysis serves to reveal the most crucial experimental parameters and can be used for future experimental design to obtain correct ε-values allowing mechanistic interpretations.

Joint interpretation of enantiomer and stable isotope fractionation for chiral pesticides degradation

Chiral pesticides are important contaminants affecting the health and functioning of aquatic systems. The combination of stable isotope and enantiomer analysis techniques has been recently proposed to better characterize the fate of these contaminants in natural and engineered settings. We introduce a modeling approach with the aim of unifying and integrating the interpretation of isotopic and enantiomeric fractionation. The model is based on the definition of enantiomer-specific isotopologues and jointly predicts the evolution of concentration, enantiomer fractionation, as well as changes in stable isotope ratios of different elements. The method allows evaluating different transformation pathways and was applied to investigate enzymatic degradation of dichlorprop (DCPP), enzymatic degradation of mecoprop methyl ester (MCPPM), and microbial degradation of α-hexachlorocyclohexane (α-HCH) by different bacterial strains and under different redox conditions. The model accurately reproduces the isotopic and enantiomeric data observed in previous experimental studies and precisely captures the dual-dimensional trends characterizing different reaction pathways. Furthermore, the model allows testing possible combinations of enantiomer analysis (EA), compound specific isotope analysis (CSIA), and enantiomer specific isotope analysis (ESIA) to identify and assess isotope and enantiomer selective reaction mechanisms.

Compound-specific isotope analysis (CSIA) of micropollutants in the environment - current developments and future challenges

Over the last decade, the occurrence of micropollutants in the environment has become a worldwide issue of increasing concern. Compound-specific stable-isotope analysis (CSIA) of natural isotopic abundance may greatly enhance the evaluation of sources and transformation processes of micropollutants, such as pesticides, personal care products or pharmaceuticals. We summarize recent advances from laboratory studies, review current limitations and analytical challenges associated with low concentrations and high polarity of micropollutants, and delineate the potential of micropolluant CSIA for field applications. We highlight future challenges and prospects regarding source apportionment, identification of biotic and abiotic transformation reactions on a mechanistic level, as well as integrative evaluation of degradation hot spots on the catchment scale. Such advances may feed into a framework for risk assessment of micropollutants that includes CSIA.