Pharmacological characterization of cis-nitromethylene neonicotinoids in relation to imidacloprid binding sites in the brown planthopper, Nilaparvata lugens

Enantioselective metabolism of novel chiral insecticide Paichongding by human cytochrome P450 3A4: A computational insight

As a novel chiral neonicotinoid insecticide, Paichongding (IPP) has been widely applied in agriculture due to its excellent insecticidal activity. However, the enantioselective metabolism of IPP stereoisomers (5R7R-IPP, 5S7S-IPP, 5R7S-IPP, and 5S7R-IPP) mediated by enzymes in non-target organisms, especially the cytochrome P450s (CYPs), remains unknown. To address this knowledge gap, we developed an integrated computational framework to elucidate the binding interactions and enantioselective metabolism of IPP stereoisomers in human CYP3A4. The results reveal that 5R7R-IPP shows much stronger binding affinity to CYP3A4 than 5S7S-IPP, while enantiomers 5R7S-IPP and 5S7R-IPP have no essential difference in their binding potential, owing to their specific interactions with key CYP3A4 residues. Although enantiomers 5R7R-IPP and 5S7S-IPP feature distinct binding modes resulting from the chiral differences, their transformation activities are slightly different, with C5 and C13 being the primary metabolic sites, respectively. In contrast, CYP3A4 preferably metabolizes 5R7S-IPP over 5S7R-IPP. The metabolism of epimers 5R7R-IPP and 5R7S-IPP share C5-hydroxylation routes due to the conserved 5R-conformaitons, but differ with the transformation routes at C11/C13 and C3 sites. The 7R-chirality of 5S7R-IPP significantly reduces the metabolic potency compared to 5S7S-IPP. CYP3A4-catalyzed hydroxylation and desaturation of IPP stereoisomers generate various chiral metabolites, with C5- and C13-hydroxyIPPs further transforming into depropylated products. Furthermore, the toxicity assessment reveals that IPP, along with the majority of its hydroxylated, desaturated, and depropylated metabolites, can potentially induce adverse effects on human health, specifically hepatotoxicity, respiratory toxicity, and carcinogenicity. This study provides valuable insights into the enantioselective fate of chiral IPP metabolism by CYP3A4, and the identified metabolites can serve as potential biomarkers for monitoring IPP exposure and associated health risk in human body.

Drosophila nicotinic acetylcholine receptor subunits and their native interactions with insecticidal peptide toxins

Drosophila nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that represent a target for insecticides. Peptide neurotoxins are known to block nAChRs by binding to their target subunits, however, a better understanding of this mechanism is needed for effective insecticide design. To facilitate the analysis of nAChRs we used a CRISPR/Cas9 strategy to generate null alleles for all ten nAChR subunit genes in a common genetic background. We studied interactions of nAChR subunits with peptide neurotoxins by larval injections and styrene maleic acid lipid particles (SMALPs) pull-down assays. For the null alleles, we determined the effects of α-Bungarotoxin (α-Btx) and ω-Hexatoxin-Hv1a (Hv1a) administration, identifying potential receptor subunits implicated in the binding of these toxins. We employed pull-down assays to confirm α-Btx interactions with the Drosophila α5 (Dα5), Dα6, Dα7 subunits. Finally, we report the localisation of fluorescent tagged endogenous Dα6 during Drosophila CNS development. Taken together, this study elucidates native Drosophila nAChR subunit interactions with insecticidal peptide toxins and provides a resource for the in vivo analysis of insect nAChRs.

Pleiotropic Effects of Loss of the Dα1 Subunit in Drosophila melanogaster: Implications for Insecticide Resistance

Nicotinic acetylcholine receptors (nAChRs) are a highly conserved gene family that form pentameric receptors involved in fast excitatory synaptic neurotransmission. The specific roles individual nAChR subunits perform in Drosophila melanogaster and other insects are relatively uncharacterized. Of the 10 D. melanogaster nAChR subunits, only three have described roles in behavioral pathways; Dα3 and Dα4 in sleep, and Dα7 in the escape response. Other subunits have been associated with resistance to several classes of insecticides. In particular, our previous work has demonstrated that an allele of the Dα1 subunit is associated with resistance to neonicotinoid insecticides. We used ends-out gene targeting to create a knockout of the Dα1 gene to facilitate phenotypic analysis in a controlled genetic background. To our knowledge, this is the first report of a native function for any nAChR subunits known to be targeted by insecticides. Loss of Dα1 function was associated with changes in courtship, sleep, longevity, and insecticide resistance. While acetylcholine signaling had previously been linked with mating behavior and reproduction in D. melanogaster, no specific nAChR subunit had been directly implicated. The role of Dα1 in a number of behavioral phenotypes highlights the importance of understanding the biological roles of nAChRs and points to the fitness cost that may be associated with neonicotinoid resistance.

Influence of Soil Factors on the Stereoselective Fate of a Novel Chiral Insecticide, Paichongding, in Flooded Paddy Soils

In this study, the fate of paichongding was investigated in three soils with contrasting soil properties. In general, low soil pH has the potential to retard the mineralization and promote the dissipation of paichongding and the formation of its primary transformation product and to accelerate the formation of bound residue. The dissipation of paichongding stereoisomers was very fast and diastereoselective. This selectivity was found only between diastereomers and not between enantiomers and was observed to be soil dependent. In the acidic soil, the enantiomers (5R,7R)- and (5S,7S)-paichongding were degraded more quickly than (5R,7S)- and (5S,7R)-paichongding, whereas a contrary trend was observed in the neutral soil, and such selectivity did not occur in the alkaline soil. The OM and clay contents also played important roles in the fate of paichongding. This effect of soil properties should be considered in risk assessment of chiral pesticides and their application in the field.

Isolation of the Novel Chiral Insecticide Paichongding (IPP) Degrading Strains and Biodegradation Pathways of RR/SS-IPP and SR/RS-IPP in an Aqueous System

Chiral insecticide paichongding (IPP) is a member of cis-nitromethylene neonicotinoid insecticides used in China. IPP was the promising replacement for imidacloprid as a result of its higher activity against imidacloprid-resistant insects. Two pairs of enantiomers, RR/SS-IPP and SR/RS-IPP, were separated by preparative high-performance liquid chromatography and employed in an aqueous system to investigate their biodegradation process. In this study, the strains G1-13/G1-14 and G2-19 with effective IPP degrading capability were isolated from agricultural soils. G1-14 was mutated from G1-13 by ultraviolet light exposure. Sequence alignment of 16S rRNA proved that these three strains belonged to the genus of Sphingobacterium. The degradation rate of RR/SS-IPP by Sphingobacterium sp. G1-13 and G1-14 reached 13 and 30% within 6 and 4 days, respectively. The degradation rate of SR/RS-IPP by Sphingobacterium sp. G2-19 could reach 35% within 5 days. Degradation intermediates (I1-I6) of enantiomers were detected, and two possible biodegradation pathways were proposed on the basis of the identification of metabolites.

Stereoselective Degradation and Transformation Products of a Novel Chiral Insecticide, Paichongding, in Flooded Paddy Soil

Paichongding is a chiral neonicotinoid insecticide currently marketed as racemate against sucking and biting insects. Under anaerobic condition, all paichongding stereoisomers underwent appreciable degradation in soil during 100 days of incubation, with estimated t_1/2 values between 0.18 and 3.15 days. Diastereoselectivity in paichongding degradation was observed, with enantiomers (5S,7R)- and (5R,7S)-paichongding being more preferentially degraded in soils than enantiomers (5R,7R)- and (5S,7S)-paichongding. The half-lives of (5R,7R)- and (5S,7S)-paichongding were 3.05 and 3.15 days, respectively, as compared to 0.18 day for (5R,7S)- and (5S,7R)-paichongding. A total of nine intermediates were identified, of which depropylated paichongding was the predominant metabolite and appeared to be stable and recalcitrant to further degradation. Paichongding is degraded via denitration, depropylation, nitrosylation, demethylation, hydroxylation, and enol-keto tautomerism, producing chiral and biologically active products. These findings could have implications for environmental risk and food safety evaluations.

Specific Synergist for Neonicotinoid Insecticides: IPPA08, a cis-Neonicotinoid Compound with a Unique Oxabridged Substructure

Insecticide synergists are key components to increase the control efficacy and reduce active ingredient use. Here, we describe a novel insecticide synergist with activity specific for insecticidal neonicotinoids. The synergist IPPA08, a cis configuration neonicotinoid compound with a unique oxabridged substructure, could increase the toxicity of most neonicotinoid insecticides belonging to the Insecticide Resistance Action Committee (IRAC) 4A subgroup against a range of insect species, although IPPA08 itself was almost inactive to insects at synergistic concentrations. Unfortunately, similar effects were observed on the honey bee (Apis mellifera) and the brown planthopper (Nilaparvata lugens), resistant to imidacloprid. IPPA08 did not show any effects on toxicity of insecticides with different targets, which made us define it as a neonicotinoid-specific synergist. Unlike most insecticide synergists, by inhibition of activities of detoxification enzymes, IPPA08 showed no effects on enzyme activities. The results revealed that IPPA08 worked as a synergist through a distinct way. Although the modulating insect nicotinic acetylcholine receptors (nAChRs, targets of neonicotinoid insecticides) were supposed as a possible mode of action for IPPA08 as a neonicotinoid-specific synergist, direct evidence is needed in further studies. In insect pest control, IPPA08 acts as a target synergist to increase neonicotinoid toxicity and reduce the amount of neonicotinoid used. Combinations of IPPA08 and insecticidal neonicotinoids may be developed into new insecticide formulations. In summary, combining an active ingredient with a "custom" synergist appears to be a very promising approach for the development of effective new insecticide products.

The evolution of heat shock protein sequences, cis-regulatory elements, and expression profiles in the eusocial Hymenoptera

Background

The eusocial Hymenoptera have radiated across a wide range of thermal environments, exposing them to significant physiological stressors. We reconstructed the evolutionary history of three families of Heat Shock Proteins (Hsp90, Hsp70, Hsp40), the primary molecular chaperones protecting against thermal damage, across 12 Hymenopteran species and four other insect orders. We also predicted and tested for thermal inducibility of eight Hsps from the presence of cis-regulatory heat shock elements (HSEs). We tested whether Hsp induction patterns in ants were associated with different thermal environments.

Results

We found evidence for duplications, losses, and cis-regulatory changes in two of the three gene families. One member of the Hsp90 gene family, hsp83, duplicated basally in the Hymenoptera, with shifts in HSE motifs in the novel copy. Both copies were retained in bees, but ants retained only the novel HSE copy. For Hsp70, Hymenoptera lack the primary heat-inducible orthologue from Drosophila melanogaster and instead induce the cognate form, hsc70-4, which also underwent an early duplication. Episodic diversifying selection was detected along the branch predating the duplication of hsc70-4 and continued along one of the paralogue branches after duplication. Four out of eight Hsp genes were heat-inducible and matched the predictions based on presence of conserved HSEs. For the inducible homologues, the more thermally tolerant species, Pogonomyrmex barbatus, had greater Hsp basal expression and induction in response to heat stress than did the less thermally tolerant species, Aphaenogaster picea. Furthermore, there was no trade-off between basal expression and induction.

Conclusions

Our results highlight the unique evolutionary history of Hsps in eusocial Hymenoptera, which has been shaped by gains, losses, and changes in cis-regulation. Ants, and most likely other Hymenoptera, utilize lineage-specific heat inducible Hsps, whose expression patterns are associated with adaptive variation in thermal tolerance between two ant species. Collectively, our analyses suggest that Hsp sequence and expression patterns may reflect the forces of selection acting on thermal tolerance in ants and other social Hymenoptera.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0573-0) contains supplementary material, which is available to authorized users.

Stereoselective fate kinetics of chiral neonicotinoid insecticide paichongding in aerobic soils

Man-made chemicals such as pesticides, when released into the soil environment, are transformed into extractable residue (ER), bound residue (BR), or mineralized. These processes all play a pivotal role in the risk assessment for the use of man-made chemicals. In this study, BR, ER, and mineralization of a novel chiral pesticide, paichongding (IPP), 1-((6-chloropyridin-3-yl)methyl)-7-methyl-8-nitro-5-propoxy-1,2,3,5,6,7-hexahydro-imidazo[1,2-a]pyridine, were investigated in different soils under aerobic conditions. Significant specificity was observed for diastereoisomers of IPP in the formation of BR or mineralization in neutral and alkaline soils. In contrast, no significant difference was found between enantiomers. The overall mineralization was less than 8% of the applied radioactivity and was related to soil pH. Our findings suggest that the environmental fate of chiral pesticides may be influenced by many factors such as soil properties (e.g. pH). More comprehensive and individualized risk assessments should be carried out for individual stereoisomers of a chiral product.

Tetrahydroindeno[1',2':4,5]pyrrolo[1,2-a]imidazol-5(1H)-ones as novel neonicotinoid insecticides: reaction selectivity and substituent effects on the activity level

Tetraheterocyclic tetrahydroindeno[1',2':4,5]pyrrolo[1,2-a]imidazol-5(1H)-one derivatives as novel neonicotinoid candidates were designed and prepared by selective etherification, chlorination and esterification of ninhydrin-heterocyclic ketene aminals adducts. Some of the designed compounds showed excellent insecticidal activity against cowpea aphids (Aphis craccivora), brown planthopper (Nilaparvata lugens), and armyworm (Mythimna separata). In particular, the activity against armyworm (Mythimna separata) improved a lot in contrast with that of imidacloprid and cycloxaprid. The research here provides a novel neonicotinoid chemotype for further development.

Influence of short-time imidacloprid and acetamiprid application on soil microbial metabolic activity and enzymatic activity

The influence of two neonicotinoids, i.e., imidacloprid (IMI) and acetamiprid (ACE), on soil microbial activities was investigated in a short period of time using a combination of the microcalorimetric approach and enzyme tests. Thermodynamic parameters such as Q T (J g(-1) soil), ∆H met (kJ mol(-1)), J Q/S (J g(-1) h(-1)), k (h(-1)), and soil enzymatic activities, dehydrogenase, phosphomonoesterase, arginine deaminase, and urease, were used to evaluate whole metabolic activity changes and acute toxicity following IMI and ACE treatment. Various profiles of thermogenic curves reflect different soil microbial activities. The microbial growth rate constant k, total heat evolution Q T (expect for IMI), and inhibitory ratio I show linear relationship with the doses of IMI and ACE. Q T for IMI increases at 0.0-20 μg g(-1) and then decreases at 20-80 μg g(-1), possibly attributing to the presence of tolerant microorganisms. The 50 % inhibitory ratios (IC50) of IMI and ACE are 95.7 and 77.2 μg g(-1), respectively. ACE displays slightly higher toxicity than IMI. Plots of k and Q T against microbial biomass-C indicate that the k and Q T are growth yield-dependent. IMI and ACE show 29.6; 40.4 and 23.0; and 23.3, 21.7, and 30.5 % inhibition of dehydrogenase, phosphomonoesterase, and urease activity, respectively. By contrast, the arginine deaminase activity is enhanced by 15.2 and 13.2 % with IMI and ACE, respectively. The parametric indices selected give a quantitative dose-response relationship of both insecticides and indicate that ACE is more toxic than IMI due to their difference in molecular structures.

Target-site resistance to neonicotinoids

Neonicotinoid insecticides selectively target the invertebrate nicotinic acetylcholine receptor and disrupt excitatory cholinergic neurotransmission. First launched over 20 years ago, their broad pest spectrum, variety of application methods and relatively low risk to nontarget organisms have resulted in this class dominating the insecticide market with global annual sales in excess of $3.5 bn. This remarkable commercial success brings with it conditions in the field that favour selection of resistant phenotypes. A number of important pest species have been identified with mutations at the nicotinic acetylcholine receptor associated with insensitivity to neonicotinoids. The detailed characterization of these mutations has facilitated a greater understanding of the invertebrate nicotinic acetylcholine receptor.

Stereoisomeric isolation and stereoselective fate of insecticide paichongding in flooded paddy soils

Chiral insecticide paichongding (IPP) is one of the prospective substitutes for imidacloprid used in China due to its higher activity against imidacloprid-resistant insects. However, little is known about the fate of IPP in soils, including especially the different behaviors among its stereoisomers. In this study, four stereoisomers of IPP were separated and applied in flooded soils. Kinetics of mineralization, extractable residues, and bound residues showed diastereoselectivity in IPP degradation, with enantiomers (5S,7R)-IPP (IPP-SR) and (5R,7S)-IPP (IPP-RS) being more readily mineralized and preferentially bound to soils than enantiomers (5R,7R)-IPP (IPP-RR) and (5S,7S)-IPP (IPP-SS). The overall mineralization was rather limited and did not exceed 4% of the spiked rate. Concurrent to the decreases of extractable residues, the fraction of bound residues increased with time and reached about 34% of the applied radioactivity for (14)C-IPP-SR and (14)C-IPP-RS as compared to about 23% for (14)C-IPP-RR or (14)C-IPP-SS. Soil properties such as organic matter content and pH likely contributed to the variability. Relatively rapid formation of bound residue suggests that IPP may be quickly detoxified in flooded paddy soil, decreasing the potential for off-site transport such as leaching or runoff, especially for enantiomers IPP-SR and IPP-RS.

Stereoselective uptake and distribution of the chiral neonicotinoid insecticide, Paichongding, in Chinese pak choi (Brassica campestris ssp. chinenesis)

Neonicotinoid chiral insecticidal Paichongding is a promising substitute for the widely used imidacloprid. Four stereoisomers of Paichongding, 5R,7R, 5S,7S, 5S,7R and 5R,7S, were employed in both foliage and roots of Chinese pak choi to investigate their stereoselective uptake and distribution in pak choi. Results showed that after foliar application, no stereoselective absorption into pak-choi plants was observed among the enantiomers. Total absorptions were 35.40% of the applied amount for 5R,7R, 36.66% for 5S,7S, 36.80% for 5S,7R and 38.20% for 5R,7S at 96 HAT. The translocation of the four absorbed stereoisomers within pak choi occurred both acropetally and basipetally and the transport of (14)C from enantiomers 5R,7R and 5S,7S were significantly higher than for 5R,7S and 5S,7R. Significant stereoselective translocation inside plants was observed between Paichongding epimers. Total root uptake reached 16.49-19.85% for 5R,7R and 5S,7S, and 24.57-28.82% for 5S,7R and 5R,7S at 144 HAT. Both enantioselective and diastereoselective root uptake into pak-choi occurred between the four stereoisomers. The 5R,7S and 5S,7R enantiomers were more readily uptaken by the roots than 5R,7R and 5S,7S and accumulated in the edible leaves. These results will help to develop an understanding of Paichongding using only the target-active enantiomer of pesticides.

Caste load and the evolution of reproductive skew

Reproductive skew theory seeks to explain how reproduction is divided among group members in animal societies. Existing theory is framed almost entirely in terms of selection, though nonadaptive processes must also play some role in the evolution of reproductive skew. Here I propose that a genetic correlation between helper fecundity and breeder fecundity may frequently constrain the evolution of reproductive skew. This constraint is part of a wider phenomenon that I term "caste load," which is defined as the decline in mean fitness caused by caste-specific selection pressures, that is, differential selection on breeding and nonbreeding individuals. I elaborate the caste load hypothesis using quantitative and population genetic arguments and individual-based simulations. Although selection can sometimes erode genetic correlations and resolve caste load, this may be constrained when mutations have similar pleiotropic effects on breeder and helper traits. I document evidence for caste load, identify putative genomic adaptations to it, and suggest future research directions. The models highlight the value of considering adaptation within the boundaries imposed by genetic architecture and incidentally reaffirm that monogamy promotes the evolutionary transition to eusociality.

Diastereoselective metabolism of a novel cis-nitromethylene neonicotinoid paichongding in aerobic soils

Many pesticides are chiral but used as racemic mixtures, even though their stereoisomers are often degraded stereoselectively in soils. Evaluation of degradation of chiral compounds is mostly focused on the enantioselectivity rather than diastereoselectivity/epimer preferences. In this study, we explored the diastereoselective transformation of paichongding (IPP), a novel chiral neonicotinoid with broad-spectrum insecticidal activity, to several degradation intermediates in different soils. (14)C-Labeling coupled with LC-MS/MS and high resolution MS were used to track residues of IPP and identify major transformation metabolites. The stereoisomers of IPP known as 5R, 7R-IPP (RR-IPP), 5S, 7S-IPP (SS-IPP), 5S, 7R-IPP (SR-IPP), and 5R, 7S-IPP (RS-IPP) showed diastereoselective/epimer-selective persistence in all soils except an acidic clay soil. Moreover, IPP was transformed to a range of degradation intermediates (M1-M6), which also showed significant diastereoselective and soil preferential formation. Depropylation, nitrosylation, denitration, demethylation, dehydroxylation, and ketonization contributed to IPP transformation. The diastereoselective degradation of the parent compound and formation of incomplete intermediates implies that diastereomers/epimers should be regarded as different chemicals. The approach of coupling (14)C and MS may be used as an effective tool to understand the environmental processes and risks of other man-made chiral compounds.

Soil microbial effects on the stereoselective mineralization, extractable residue, bound residue, and metabolism of a novel chiral cis neonicotinoid, paichongding

Fate characteristics of the four stereoisomers of paichongding [IPP, 1-((6-chloropyridin-3-yl)methyl)-7-methyl-8-nitro-5-propoxy-1,2,3,5,6,7-hexahydroimidazo[1,2-a]pyridine] in aerobic sterilized and nonsterilized fluvio-marine yellow loamy soil were investigated using a (14)C tracer technique combined with HPLC and LC-MS/MS. Results showed that the mineralization and bound residue (nonsterile/sterilized soil, % of applied amount) of four stereoisomers of IPP were 1.76-6.10/0.33-0.82 and 12.01-31.20/6.58-20.81 at 100 days after treatment. Seven and five incomplete intermediates of IPP were detected in nonsterilized and sterilized soil, respectively, and a possible degradation pathway was proposed. Degradation mainly occurred on the tetrahydropyridine ring, including oxidation and elimination of the methyl, propyl, and nitro groups. All of these results suggest that soil microbial activity greatly contributes to the epimeride-selective mineralization, formation of bound residue, and degradation of IPP in loamy soil. The identified transformation intermediates could be used for further study on their toxicity to target and nontarget species.

Investigating the mode of action of sulfoxaflor: a fourth-generation neonicotinoid

BACKGROUND

The precise mode of action of sulfoxaflor, a new nicotinic acetylcholine receptor-modulating insecticide, is unclear. A detailed understanding of the mode of action, especially in relation to the neonicotinoids, is essential for recommending effective pest management practices.

RESULTS

Radiolabel binding experiments using a tritiated analogue of sulfoxaflor ([(3) H]-methyl-SFX) performed on membranes from Myzus persicae demonstrate that sulfoxaflor interacts specifically with the high-affinity imidacloprid binding site present in a subpopulation of the total nAChR pool. In competition studies, imidacloprid-like neonicotinoids displace [(3) H]-methyl-SFX at pM concentrations. The effects of sulfoxaflor on the exposed aphid nervous system in situ are analogous to those of imidacloprid and nitenpyram, and finally the high-affinity sulfoxaflor binding site is absent in a Myzus persicae strain (clone FRC) possessing a single amino acid point mutation (R81T) in the β-nAChR, a region critical for neonicotinoid interaction.

CONCLUSION

The nicotinic acetylcholine receptor pharmacological profile of sulfoxaflor in aphids is consistent with that of imidacloprid. Additionally, the insecticidal activity of sulfoxaflor and the current commercialised neonicotinoids is affected by the point mutation in FRC Myzus persicae. Therefore, it is suggested that sulfoxalfor be considered a neonicotinoid, and that this be taken into account when recommending insecticide rotation partnering for effective resistance management programmes.

New insights on the molecular features and electrophysiological properties of dinotefuran, imidacloprid and acetamiprid neonicotinoid insecticides

Structural features and hydrogen-bond interactions of dinotefuran (DIN), imidacoloprid (IMI) and acetamiprid (ACE) have been investigated experimentally through analyses of new crystal structures and observations in structural databases, as well as by Density Functional Theory quantum chemical calculations. Several conformations are observed experimentally in the solid state, highlighting the large flexibility of these compounds. This feature is confirmed by the theoretical calculations in the gas phase, the numerous and different energetic minima of the three neonicotinoids being located within a 10kJ/mol range. Comparisons of the observed and simulated data sheds light on the hydrogen-bond (HB) strength of the functional group at the tip of the electronegative fragment of each pharmacophore (NO(2) for DIN and IMI and CN for ACE). This effect originates in the 'push-pull' nature of these fragments and the related extensive electron delocalization. Molecular electrostatic potential calculations provide a ranking of the two fragments of the three neonicotinoid in terms of HB strength. Thus, the NO(2) group of DIN is the strongest HB acceptor of the electronegative fragment, closely followed by the cyano group of ACE. These two groups are significantly more potent than the NO(2) group of IMI. With respect to the other fragments of the three neonicotinoids, the nitrogen atom of the pyridine of IMI and ACE are stronger HB acceptors than the oxygen atom of the furanyl moiety of DIN. Finally, compared to electrophysiological studies obtained from cockroach synaptic and extrasynaptic receptors, DIN appears more effective than IMI and ACE because it strongly increases dose-dependently the ganglionic depolarisation and the currents amplitudes. These data suggest that DIN, IMI and ACE belong to two subgroups which act differently as agonists of insect nicotinic receptors.

The mode of action of a nitroconjugated neonicotinoid and the effects of target site mutation Y151S on its potency

Neonicotinoid insecticides, such as imidacloprid, are selective agonists of the insect nicotinic acetylcholine receptors (nAChRs) with -NO(2) or -CN group in trans-configuration. Previously we reported the excellent insecticidal activity of a series of nitroconjugated neonicotinoids with -NO(2) or -CN group in cis-configuration by replacing nitromethylene pharmacophore with a nitroconjugated system. To understand the action mode of these nitroconjugated neonicotinoids, a representative member IPPA152201 was chosen to perform toxicity and pharmacology studies. IPPA152201 showed a comparable toxicity with imidacloprid against Nilaparvata lugens in a susceptible strain and had no significant cross-resistance in an imidacloprid resistant strain. IPPA152201 showed good efficacies on the isolated cockroach neurons (pEC(50) = 5.91 ± 0.14) and the evoked responses by IPPA152201 could be blocked by the typical nAChRs antagonists methyllycaconitine citrate (MLA) and dihydro-β-erythroidine (DHβE), with pIC(50) of 6.56 ± 0.07 and 6.89 ± 0.12. The efficacy of IPPA152201 on hybrid receptors Nlα1/β2 in Xenopus oocytes and response inhibition by MLA and DHβE were also observed. These data demonstrate that IPPA152201 acts on insect nAChRs as an agonist. In addition, the influence of a Nlα1 mutation (Y151S), which has been linked to the lab-generated neonicotinoid resistance in N. lugens, has been examined. Compared to the wildtype Nlα1/β2, this mutation reduced I(max) for IPPA152201 to 63.2% and caused a 1.5-fold increase in EC(50), which is much smaller than the effects on imidacloprid. The high insecticidal activity and little influence by Y151S mutation make IPPA152201 to be a potential insecticide to manage N. lugens.

cis-Configuration: a new tactic/rationale for neonicotinoid molecular design

Resistance development and limited lepidopteran activities call for the discovery of "super-neonicotinoids" solving these problems. Compounds with the cis-configuration offer an opportunity for further optimization. Fixing the nitro group in the cis-configuration provided a new approach for neonicotinoid molecular design. Introductions of the heterocycle or a bulky group are two synthesis concepts to fix the cis-configuration of the nitro group. The design, synthesis, bioactivity, and preliminary modes of action of five types of cis-neonicotinoids are reviewed. cis- and trans-neonicotinoids have some differences in bioactivities and modes of action. This study focused, especially, on the reaction diversities of nitromethylene analogues of imidacloprid with various aldehydes.

China: forward to the green pesticides via a basic research program

The 973 Program is China's keystone national research program established to support basic research in natural and physical sciences. In addition to promoting the development of core technology and scientific infrastructure needed to enable China to meet the social and economic challenges of the 21st century, the training and mentoring of the new generation of China's young scientists are also important objectives of this national program. The green chemical pesticide research program is a part of the 973 Program. The main objectives of stage 1 of the green chemical pesticide research program (2003-2008) are to establish China's capability to conduct basic research in the discovery of "green" crop protection chemicals that are not only novel in mode of action and highly selective to pest species that are unique to China's agricultural situation but also possess favorable environmental and human hazard and risk potentials. The target-based discovery strategy was selected as the main discovery platform. This strategy not only provided this research program the best chance to discover new products but also provided members of this research team opportunities to establish core technologies in chemoinformatics/computation-aided pesticide design using QSAR, QAAR, sensitive and selective bioassay methodology, combinatorial synthesis, hit to lead optimization, and biological targets that were derived from resistance-AChE, IGR, nAChR, etc. On the basis of the learning from stage 1, stage 2 (2010-2014) of this program will focus on the molecular target-oriented innovation of green chemical pesticides. This commentary presents key learnings and accomplishments from the stage 1 of China's green chemical pesticide research program. It is hoped that this information will stimulate further constructive collaborations between pesticide scientists from China and abroad.

Photodegradation of novel nitromethylene neonicotinoids with tetrahydropyridine-fixed cis configuration in aqueous solution

Photodegradation of 1-((6-chloropyridin-3-yl)methyl)-7-methyl-8-nitro-1,2,3,5,6,7-hexahydroimidazo[1,2-a]pyridin-5-ol (IPPA152001) and 1-((6-chloropyridin-3-yl)methyl)-7-methyl-8-nitro-5-propoxy-1,2,3,5,6,7-hexahydroimidazo[1,2-a]pyridine (IPPA152004) was studied in distilled water; high-pressure mercury lamp and xenon lamp were used as light sources. The photolytic rate constants of the two compounds and their half-lives were calculated. Compared with imidacloprid, the order of photodegradation rate of three compounds was as follows: imidacloprid > IPPA152001 > or = IPPA152004. Mercury lamp light intensity was measured with chemical actinometers. The quantum yields for photodegradation of IPPA152001 and IPPA152004 in distilled water were also calculated. Photodegradation products of IPPA152004 were preliminarily identified on the basis of the spectral data of GC-MS, LC-TOF-ESI, and LC-MS-MS in positive mode. The photodegradation pathways of IPPA152004 were proposed according to the structures of photoproducts.