disruption, have been reported for enantiomers with low or no insecticidal action. In
addition, the different metabolic pathways of pyrethroid enantiomers have consequences for their persistence and bioaccumulation profiles in biological systems.
Therefore, a stereochemical approach is required to better understand the undesired
impacts of pyrethroids on the environment and on human health, since the studies
point to patterns of toxicity and persistence at enantiomeric levels. The occurrence of
degradation/persistence patterns in environmental samples may be useful for understanding enantiomeric fate, contributing to more accurate risk assessments aimed
at preventing or mitigating the impacts of continuous pyrethroid release into the
environment.
Keywords Chirality, Cypermethrin, Enantiomers, Environment, Isomerism,
Permethrin
1 Introduction
Chiral compounds are characterized by the presence of at least one asymmetric
molecular centre. Currently, approximately 30% of commercialized pesticides present chirality [1, 2]. The need to increase the efficiency and economic viability of
new pesticides, as well as the evolution of knowledge about molecular interactions
in biological systems, favoured the development of more specific chiral active
ingredients [2]. In this context, the development of chiral molecules is aligned
with the strategy of achieving efficient and environmentally sustainable pesticides
[3]. Among the various molecular structures of pesticides, asymmetric centres can
occur on carbon, sulphur, nitrogen and phosphorus atoms [4]. In pyrethroids,
chirality is due to the presence of one to three stereogenic tetrahedral carbons. The
occurrence of a chiral centre (e.g. fenpropathrin) gives this structure optical isomerism with two possible spatial configurations, which are non-superimposable mirrored forms of the same compound (R- and S-enantiomers). On typical Type I
pyrethroids (e.g. permethrin), the presence of two chiral carbons generates four
diastereomers, resulting in one pair of cis- and trans-enantiomers. On Type II
pyrethroids (e.g. cypermethrin), the inclusion of a third chiral carbon (alphacyano) generates eight diastereomers, resulting in a second pair of each cis- and
trans-enantiomer.
Due to chirality at enzymatic sites, pyrethroid enantiomers may be related to
different toxicities and preferential metabolic pathways in biological systems
[5]. Furthermore, variation in the biochemical transformation patterns of these
compounds directly influences the persistence and preferential bioaccumulation
of stereoisomers [6]. Enantioselectivity is a determining factor for the occurrence
of isomeric patterns in the environment, including the different rates of
bioaccumulation observed in species living in the same ecosystem [7].
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C. E. T. Parente et al.
addition, the different metabolic pathways of pyrethroid enantiomers have consequences for their persistence and bioaccumulation profiles in biological systems.
Therefore, a stereochemical approach is required to better understand the undesired
impacts of pyrethroids on the environment and on human health, since the studies
point to patterns of toxicity and persistence at enantiomeric levels. The occurrence of
degradation/persistence patterns in environmental samples may be useful for understanding enantiomeric fate, contributing to more accurate risk assessments aimed
at preventing or mitigating the impacts of continuous pyrethroid release into the
environment.
Keywords Chirality, Cypermethrin, Enantiomers, Environment, Isomerism,
Permethrin
1 Introduction
Chiral compounds are characterized by the presence of at least one asymmetric
molecular centre. Currently, approximately 30% of commercialized pesticides present chirality [1, 2]. The need to increase the efficiency and economic viability of
new pesticides, as well as the evolution of knowledge about molecular interactions
in biological systems, favoured the development of more specific chiral active
ingredients [2]. In this context, the development of chiral molecules is aligned
with the strategy of achieving efficient and environmentally sustainable pesticides
[3]. Among the various molecular structures of pesticides, asymmetric centres can
occur on carbon, sulphur, nitrogen and phosphorus atoms [4]. In pyrethroids,
chirality is due to the presence of one to three stereogenic tetrahedral carbons. The
occurrence of a chiral centre (e.g. fenpropathrin) gives this structure optical isomerism with two possible spatial configurations, which are non-superimposable mirrored forms of the same compound (R- and S-enantiomers). On typical Type I
pyrethroids (e.g. permethrin), the presence of two chiral carbons generates four
diastereomers, resulting in one pair of cis- and trans-enantiomers. On Type II
pyrethroids (e.g. cypermethrin), the inclusion of a third chiral carbon (alphacyano) generates eight diastereomers, resulting in a second pair of each cis- and
trans-enantiomer.
Due to chirality at enzymatic sites, pyrethroid enantiomers may be related to
different toxicities and preferential metabolic pathways in biological systems
[5]. Furthermore, variation in the biochemical transformation patterns of these
compounds directly influences the persistence and preferential bioaccumulation
of stereoisomers [6]. Enantioselectivity is a determining factor for the occurrence
of isomeric patterns in the environment, including the different rates of
bioaccumulation observed in species living in the same ecosystem [7].
150
C. E. T. Parente et al.