Keywords

7.1 Introduction

Chirality is a property of some organic molecules to behave like an object that is not superposable on its mirror image. The term comes from Greek language, where χειρ (kheir) means hand, as hands are nonsuperposable objects. The two molecules (Fig. 7.1) are referred as chiral isomers.

Fig. 7.1
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Hands and chiral isomers of a generic amino acid

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Chiral isomers (optic isomers or enantiomers) are organic compounds with identical structures, (same atoms, identically bonded) except for their molecular conformation (+/−, D/L, or R/S) due to an asymmetry element (centre, plane) which provides them identical physical-chemical properties in achiral environments. Enantiomers placed in a chiral environment show different behaviours, both in the presence of a physical chiral media (polarised light plan) or of a chemical chiral one (solvent, reactant or catalyser). A racemate (racemic mixture) consists in an echimolar mixture of both enantiomers.

The biotic environment offers an excellent chiral media, due to the enzymes with protein structure, or to other chiral constituents. The presence of microorganisms in water and soil, or of proteins in plants, determines different behaviour of the chiral compounds, like some pesticides are. They will exhibit enantioselective phenomenon such as adsorption, degradation (biotic and abiotic) and leaching processes.

About 30 % of currently used pesticides are chiral being mostly used as racemates, due to economic reasons; therefore it is needed to define their enantioselectivity, to provide information for improved risk assessment [11]. Studies relieved that pesticide enantiomers have enantioselective bioactivity, toxicity, metabolism, bioaccumulation, and biodegradation behavior, thus altering the racemate, enantiomers ratio [22]. However, the research on enantioselective environmental fate and effects of chiral pesticides is still limited, particularly in the evaluation of enantioselectivity on their environmental impact, as well as their ecotoxicological and health risks [29].

This study will further present examples of studied chiral pesticides and their behavior in chiral media, as well as available methods and techniques for their determination in environmental, food and biological samples.

7.2 Chiral Pesticides and Their Enantioselectivity

Studies related to the environmental fate and biochemical transformations of the chiral organic pollutants, like chiral pesticides, chiral metabolites of polychlorinated biphenyls (PCB), synthetic chiral polycyclic musks, chiral hexabromocyclododecane (HBCDD), and chiral pharmaceuticals are, were reviewed [39]. The author underlined the need of understanding the role of stereochemistry in ecotoxicity, as well as the elucidation of factors controlling environmental fate of pollutants enantiomers.

Figure 7.2 gives the structure of the chiral herbicide dichlorprop, with the tow (R)- and (S)-enantiomers, where the asymmetric carbon atom is marked, while Table 7.1 presents a selection of chiral pesticides, that will be further subject of discussions.

Fig. 7.2
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Object and image in mirror of (R)- and (S)-enantiomers of dichlorprop

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Table 7.1 Chiral pesticides from different classes and their use
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Similar to chiral drugs, only one of the two enantiomers of the chiral pesticide is active against the target pest, the other one is either inactive, or has a different active role, or even has adverse effects on some non-target species. However, few single-enantiomer pesticides are synthesized or produced [43]. Examples of different pest activity of the two enantiomers of chiral pesticides are given in Table 7.2.

Table 7.2 Chiral pesticides and their active enatiomeric forms
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For mecoprop, dichlorprop imazapyr, imazethapyr, imazaquin the herbicidal activities are exclusively related to the R- forms, as well as for metalaxyl the fungicidal use. For indoxacarb the S-form exhibit insecticidal activity. The optical activity is even more enantioselective in the case of metolachlor, S-metolachlor having herbicidal activity, while R-metolachlor is used as fungicide.

In their review on enantioselective chromatography techniques available for the study of transformation processes of chiral environmental pollutants [14] the authors presented a short history of the studies reported in this domain: in 1989, was published the enantiomer separation of α-HCH on a γ-cyclodextrin derivative as stationary phase, which may further be used to study the enantioselective transformation of α-HCH in the environment; in 1991, the enantioselective microbial transformation of α-HCH in the marine ecosystem have been proved for the first time; in 1991 as well, the enantioselective metabolisation of the same compound in Eider ducks was demonstrated; one year later, the photochemical transformation of α-HCH was studied. To conclude, the enantioselectivity of environmental chiral pesticides is based on their potential to discriminate by microbial, enzymatic, or photochemical transformation processes [14].

As enantioselective uptake indicators enantiomeric fractions (EF) or enantiomeric ratio (ER) (Eq. 7.1 and 7.2) are used.

$$ E{F_R}=\frac{{{C_R}}}{{{C_R}+{C_S}}} $$
(7.1)
$$ E{R_R}=\frac{{{C_R}}}{{{C_S}}} $$
(7.2)

where: CR and CS are the concentrations of the (R)- and (S)-enantiomers, respectively.

Enantiomeric enrichment of chiral pesticides in the environment was also subject of a reviewing article. The authors used EFs and ERs of chiral compounds to explain the mechanisms of enantiomer enrichment in air, soil, water and biota measured, over the past 10 years [13]. These differences of pesticides enantiomers behaviour in biotic environment, lead to further development of research studies of the pesticides uptake from different media (air, water, soil) by living organisms (plants, animals), transfers that can be or not enantioselective. Table 7.3 gives some examples of enantioselective behaviour of chiral pesticides in environment.

Table 7.3 Chiral pesticides and their enantioselective behaviour in environment
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7.3 Ecotoxicological Effects of Chiral Pesticides

Pesticides enantiomers do not only exhibit enantioselectivity in the environment, but they also have different toxicological effects, subject that has become one of most challenging theme for researchers (Table 7.4).

Table 7.4 Chiral pesticides and their ecotoxicological effects
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It has been shown that the use of pesticides has increased the risk of intoxication of non-target species, such as birds and humans [25]. For example, synthetic pyrethroids (SPs) are among the most commonly used pesticides for agricultural and indoor pests control, especially in households to eradicate pests and insects, therefore also contributing to the exposure of humans to SPs [25].

More information about the enantioselectivity in environmental risk assessment of chiral pesticides is given in the reviewing article [43]. The co-authors are giving a survey on ecological fate and toxicology of chiral pesticides (synthetic pyrethroids, organophosphates, acylanilides, imidazolinones, phenoxypropanoic acids, organochlorines), also expressing their concern about pesticides that are not formulated as single- or enriched-enantiomer products.

7.4 Chiral Separation of Optic Active Pesticides

Pesticides qualitative and quantitative analysis are mostly based on separation methods, different chromatographic and capillary electrophoresis techniques being developed, optimized and available. Gas chromatography technique (GC) is mostly used for volatile pesticides, with different detection modes: electron capture detection (ECD) especially for organochlorine pesticides, inductively coupled plasma detector (ICP), or coupled with mass spectrometry detection (MS). High performance liquid chromatography (HPLC), in its reversed phase mode, may also be used, especially for water soluble pesticides, using UV detectors, circular dichroism detection for the pure enantiomer discrimination (CD), of in tandem with MS detection. Capillary electrophoresis (CE) is restricted to charged compounds analysis, but its micellar electrokinetic chromatography technique (MEKC) is able to also separate the neutral pesticides. Usually CE utilizes the same detection type like HPLC, UV and MS being the mostly used. The separation techniques available for the enantioselective studies are subject of reviewing studies, for chromatographic techniques [14, 21, 30, 31, 42] or for capillary electrophoresis [2, 6, 12].

Chiral chromatographic separation techniques are based on chiral recognition mechanisms and were developed to also contribute to the studies of enantioselective behavior of the chiral pesticides in environment. Enantiomers can be separated using different approaches:

  • indirect separation – is based on the racemate reaction with a chiral derivatization reagent (CDR) which is pure enantiomer, forming diastereomeric derivatives, separable in achiral system;

  • direct separation based on the use of chiral selector (CS) – that will enantioselectively (preferentially) form a complex with one of the chiral isomers;

  • direct separation based on tailor made chiral stationary phases (TMCSP).

Figure 7.3 presents chiral separation modes mostly developed for chromatographic and electrophoretic techniques (selection after Draghici C. [9]).

Fig. 7.3
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Chiral separation modes

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7.4.1 Chiral Selectors Used for Enantio Separations

Using direct enantioseparation by chromatographic techniques, the chiral selector may be introduces in either the stationary phase, forming a chiral stationary phase (CSP) or as chiral additive in the mobile phase (CAMP), in which case an achiral stationary phase is used. Sometimes, chiral selectors added in the mobile phase form a temporary dynamic chiral stationary phase (DCSP), like chiral micelles form. For capillary electrophoresis techniques the CS is more commonly added in the background electrolyte, but CSP packed columns may also be used.

Very divers chiral stationary phases were developed, some of them are also commercially available. Different criteria were used for their classification in many publications, and one based on the selective interactions between the CSP and analyte, leading to chiral recognition is presented:

  • ligand exchange mechanism;

  • multiple attractive interaction (π-π, dipole-dipole, hydrogen bonding);

  • inclusion into chiral cavities of low molecular mass

  • hydrophobic and polar interactions favored by natural, hybrid or synthetic polymers (high molecular mass).

Figure 7.4 presents the mostly used chiral selectors utilised in chromatographic and electrophoretic techniques (adapted after Draghici C. [9]).

Fig. 7.4
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Chiral stationary phases

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7.4.2 Applications of Chiral Separations of Optic Active Pesticides

Table 7.5 gives a selection of examples of chiral pesticides separation, with the indication of the chiral stationary phase (CSP) or the available commercial chiral columns that were used for enatioselective discrimination of the racemates.

Table 7.5 Enantioseparation of chiral pesticides
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The interest for enantioselectivity and separation of chiral pesticides is also reflected in studies related to the commercially available chiral pesticide standards, as agricultural reference materials [44]. The study relieved that certification of chiral organic reference materials should be performed with respect to both chemical and enantiomeric purity.

7.5 Conclusion

This study gives a presentation of the pesticides chirality and their enantioselective behavior in environment, air, water, soil, sediments and biota. Starting with the fact that chiral pesticides exhibit enantioselective uptake, from air-water-soil to plants and animals, and continuing with presentation of studies on their enantioselective toxicity, the study finally presents the separation techniques, as tools to investigate the chiral pesticides fate and behavior in the contaminated environment. More studies are required in order to detailed elucidate the different uptake, transfer or biodegradation mechanisms, results that will provide the manufacturers valuable information on the chiral (agro)chemicals production, as single or enriched-enantiomer, that might provide the expected benefit with minimum environmental impact.