CHIRAL INVERSION IN THE ENVIRONMENT

Studies on the chiral inversion of chiral pollutants in the environment are scarce. Most studies focused on chiral inversion of pharmaceuticals in mammals with the goal of establishing the safety and efficacy of the compounds. At present, there are no studies on chiral inversion of chiral pollutants due to abiotic processes such as hydrolysis and photolysis. However, chiral inversion observed in environmental systems has been attributed to microbial action. A study on the biotransformation of 2-phenylbutyric acid, a transformation product of linear alkylbenzene sulfonates, was shown to undergo unidirectional chiral inversion mediated by Xanthobacterflavus strain PA1 (Liu et al., 2011). The X. flavus PA1 strain was isolated from sediments collected from a mangrove. This suggested that chiral inversion could occur in the environment via microbial mediation. Chiral inversion of (7?)-fipronil to (S)-fipronil was observed in Chinese pond mussels (Anodonta woodiana) (Qu et al., 2016). Acute toxicity testing using LC50 at 72-h, (S)-fipronil (0.63 mg L') was significantly more toxic than both the racemate (1.21 mg L-1) and (/?)-fipronil (3.27 mg L4) (Qu et al., 2016). These results suggest chiral inversion resulted in an increase in toxicity to A. woodiana. Hence, the need for risk assessments to consider enantiomers of chiral pollutants individually.

Chiral inversion of chiral pesticides has been observed in distinct types of soils. Incubation of (S)-(-)- and (/?)-(+)-diclofop in soil under aerobic conditions resulted in bidirectional inversion with the (S) β€”> (R) inversion proceeding with a higher rate (Diao et al., 2010). However. Diao et al. (2010) found that the degree of chiral inversion depended on the soil type. (5) β€”> (R) inversion has been observed in herbicides such as haloxyfop (Poiger et al., 2015), fluazifop-butyl (Bewick, 1986), fenoxaprop (Wink and Luley, 1988), and quizalofop-ethyl (Li et al., 2012). Incubation of (S)-fluazifop-butyl in soils collected from Beijing and Harbin yielded the (/?)-enantiomer with concentration increasing gradually with the decrease in (S)-fluazifop-butyl but not in soils from Anhui (Qi et al., 2016). The Beijing soils yielded more (/?)-fluazifop-butyl up to 2.38 mg/kg. These results suggested that the biological and physicochemical properties of the soil may influence chiral inversion of chiral pesticides. However, chiral inversion in soil is not necessarily due to microbial activity. A study on chiral stability of phenthoate in soil found that there was higher chiral inversion of (+)-phenthoate to (-)-phenthoate in sterilized soils than in unsterilized soils suggesting microbial activity inhibited chiral inversion (Li et al., 2007).

Chiral inversion in pharmaceuticals has been reported in wastewater treatment plants and soil. A recent study found that naproxen and ibuprofen underwent bidirectional chiral inversion in soil matrices (Bertin et al., 2020). Pharmaceuticals enter soil through irrigation with recycled water, landfill disposal, or soil amendment using biosolids (Fu et al., 2016). Previous studies reported the chiral inversion of (5)-(+)-naproxen to (/?)-(-)-naproxen in wastewater treatment plants (Hashim and Khan, 2011; Khan et al., 2014; Suzuki et al., 2014). (S)-naproxen is commercially marketed as a single enantiomer. Hashim et al. (2011) found that the enantiomeric fraction of (S)-naproxen in wastewater influent was approximately 1.0 but decreased significantly in the effluent suggesting the formation of (/?)-naproxen. Similar results of chiral inversion of naproxen, ketoprofen. and ibuprofen were obtained in a membrane bioreactor (Hashim et al., 2011). As observed in mammalian studies, naproxen and profens can also undergo bidirectional chiral inversion in the environment. For example, bidirectional chiral inversion of naproxen, ketoprofen, and ibuprofen was observed in a membrane bioreactor (Nguyen et al., 2017). Hence, when chiral pharmaceuticals are discharged into the environment, their enantiomeric composition can be altered by various processes such as chiral inversion which can alter the toxicity of the pollutant.

DETERMINATION OF CHIRAL INVERSION

Dynamic chromatography and electrophoresis are powerful tools for assessing the changes in enantiomeric composition as a function of time (Wolf, 2005). Enantioselective analysis of chiral pollutants is often achieved using enantioselective gas chromatography and liquid chromatography. Following a successful separation of an analyte with one chiral center or axis, two peaks are observed on the chromatogram. However, when the analyte is conformationally or configuration- ally labile, the peaks coalesce (Krupcik et al., 2003). The degree of coalescence will depend on the rate of chiral inversion and the enantioresolution. With time, the peaks will move from being distinct to a plateau. Dynamic chromatography shows the changes in elution profile with time, and this is useful for determining the effect of pH, temperature, and solvents on the chiral inversion that occur on-column, on the injector, or in the detector (Wolf, 2005). Since chiral stationary phase is widely used in dynamic chromatography and electrophoresis, initial preparation of enantiopure

TABLE 2.2

Instruments and Experimental Approaches Used for Assessing Chiral Inversion of Conformationally or Configurationally Labile Enantiomers (Krupcik et al., 2003)

Instrument

Experimental Approaches

Dynamic NMR

Integrating enantioselective separation with classical kinetic studies

Dynamic gas chromatography

Continuous flow models

Dynamic supercritical fluid chromatography

Peak form analysis which involves experimentally obtained and simulated peaks

Dynamic liquid chromatography

Stopped-flow method

Dynamic capillary electrophoresis

Stochastic methods

Dynamic micellar electrokinetic chromatography

Deconvolution methods

Dynamic capillary electrochromatography

Approximation functions method

Chiroptical methods

compounds prior to chiral inversion analysis is required (Krupcik et al., 2003). The inversion energy barrier range that can be investigated using dynamic chromatography is influenced by the run time, mobile phase, analysis temperature, and analyte stability. In addition, the inversion energy barriers obtained are also influenced by the type of chiral stationary phase used.

The choice for the instrument used in the analysis depends on the physicochemical characteristics of the chiral compound (i.e., the solubility, vapor pressure, thermal and solvent stability, and detection) (Krupcik et al., 2003). For example, an ionizable compound with high vapor pressure and high solubility in polar solvents can be analyzed using capillary electrophoresis or liquid chromatography (Sanganyado et al., 2017). A thermally stable compound with low vapor pressure can be determined using gas chromatography. Supercritical fluid chromatography has gained prominence in the analysis of chiral pollutants as it pairs well with mass spectrometers and offers fast analysis, high sensitivity, and a green approach compared to gas or liquid chromatography (Chen et al., 2019). In recent times, multidimensional approaches have been introduced to improve separation and detection. Table 2.2 shows the list of techniques and experimental approaches commonly used to determine chiral inversion. Any of the experimental approaches listed in Table 2.2 can then be used to determine the chiral inversion.

 
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