solvents are used in these tests. Regarding photolytical isomerization, approximately
20–30% of single enantiomers of permethrin (1R-trans), cypermethrin and cyfluthrin
(1R-trans-αS) were epimerized to other enantiomers after 7 days of sunlight irradiation [27]. However, epimerization occurred only in C-1 and C-3 carbons, while
alpha-carbons of cypermethrin and cyfluthrin remained in the S-configuration. On
the cyclopropane ring, the recombination of biradicals on carbon bonds occurs after
internal rotation, resulting in chiral carbons C-1 and C-3 epimerization [81]. Photoinduced isomerization at diastereomeric or enantiomeric levels was also observed for
deltamethrin (including on alpha-carbon), tralomethrin and tralocythrin [81].
Epimerization induced during analysis procedures is only expected in the alphacyano carbon present in Type II pyrethroids. This chiral carbon is unstable under
high temperature and protic solvents, such as primary alcohols [3, 82]. For example,
methanol, ethanol, n-propanol, 2-methyl-1-propanol and n-butanol induced alphacarbon epimerization of cypermethrin enantiomers 1R-cis-αR to 1R-cis-αS and
1R-trans-αR to 1R-trans-αS [83]. On the other hand, no epimerization was observed
on C-1 and C-3 during stability tests with sterile water and aprotic solvents (acetone,
n-hexane, ethyl acetate and dichloromethane) [81, 82].
In addition, the heated injector in gas chromatography analysis (GC) induced
some epimerization on the alpha-carbon of cypermethrin and cyfluthrin [80]. In
acidic solution (0.1% acetic acid) with n-hexane, chiral centres of pyrethroids
remained stable during GC analysis, and an almost twofold increase of peak intensity
was observed compared to non-acidified solvent [84].
In light of the above findings, it should be considered that stereoisomeric profiles
found in environmental samples are the result of several transformations, both biotic
and abiotic, on the commercial formulations used. In addition, it is crucial to avoid
analytical procedures that induce changes in chiral carbon configurations in studies
addressing pyrethroid stereoisomerism.
4 Stereoisomeric Profile and Environmental Dynamics
of Chiral Pollutants
Initially, the stereoselective behaviour of chiral pesticides in the environment,
such as organochlorines (e.g. cis- and trans-chlordane and alphahexachlorocyclohexane – α-HCH), allowed the use of their degradation pattern as
a tracer of sources of contamination [85]. This approach is employed because
enantiomers present the same physicochemical characteristics (e.g. solubility in
water, vapour pressure, octanol-water partition coefficient). However, upon entering
the environment, the chiral compounds undergo selective enantiomeric degradation
in biological systems that may alter their initial isomeric pattern [86]. In this context,
the differentiation of a racemic profile of atmospheric contamination (primary
emissions from applied products) compared to a nonracemic contamination profile,
for example, from the revolatilization (secondary emission) of pesticides from
164
C. E. T. Parente et al.
20–30% of single enantiomers of permethrin (1R-trans), cypermethrin and cyfluthrin
(1R-trans-αS) were epimerized to other enantiomers after 7 days of sunlight irradiation [27]. However, epimerization occurred only in C-1 and C-3 carbons, while
alpha-carbons of cypermethrin and cyfluthrin remained in the S-configuration. On
the cyclopropane ring, the recombination of biradicals on carbon bonds occurs after
internal rotation, resulting in chiral carbons C-1 and C-3 epimerization [81]. Photoinduced isomerization at diastereomeric or enantiomeric levels was also observed for
deltamethrin (including on alpha-carbon), tralomethrin and tralocythrin [81].
Epimerization induced during analysis procedures is only expected in the alphacyano carbon present in Type II pyrethroids. This chiral carbon is unstable under
high temperature and protic solvents, such as primary alcohols [3, 82]. For example,
methanol, ethanol, n-propanol, 2-methyl-1-propanol and n-butanol induced alphacarbon epimerization of cypermethrin enantiomers 1R-cis-αR to 1R-cis-αS and
1R-trans-αR to 1R-trans-αS [83]. On the other hand, no epimerization was observed
on C-1 and C-3 during stability tests with sterile water and aprotic solvents (acetone,
n-hexane, ethyl acetate and dichloromethane) [81, 82].
In addition, the heated injector in gas chromatography analysis (GC) induced
some epimerization on the alpha-carbon of cypermethrin and cyfluthrin [80]. In
acidic solution (0.1% acetic acid) with n-hexane, chiral centres of pyrethroids
remained stable during GC analysis, and an almost twofold increase of peak intensity
was observed compared to non-acidified solvent [84].
In light of the above findings, it should be considered that stereoisomeric profiles
found in environmental samples are the result of several transformations, both biotic
and abiotic, on the commercial formulations used. In addition, it is crucial to avoid
analytical procedures that induce changes in chiral carbon configurations in studies
addressing pyrethroid stereoisomerism.
4 Stereoisomeric Profile and Environmental Dynamics
of Chiral Pollutants
Initially, the stereoselective behaviour of chiral pesticides in the environment,
such as organochlorines (e.g. cis- and trans-chlordane and alphahexachlorocyclohexane – α-HCH), allowed the use of their degradation pattern as
a tracer of sources of contamination [85]. This approach is employed because
enantiomers present the same physicochemical characteristics (e.g. solubility in
water, vapour pressure, octanol-water partition coefficient). However, upon entering
the environment, the chiral compounds undergo selective enantiomeric degradation
in biological systems that may alter their initial isomeric pattern [86]. In this context,
the differentiation of a racemic profile of atmospheric contamination (primary
emissions from applied products) compared to a nonracemic contamination profile,
for example, from the revolatilization (secondary emission) of pesticides from
164
C. E. T. Parente et al.