The Neonicotinoid Electronegative Pharmacophore Plays the Crucial Role in the High Affinity and Selectivity for theDrosophilaNicotinic Receptor:  An Anomaly for the Nicotinoid Cation−π Interaction Model

Abstract

Cation-pi interaction, a prominent feature in agonist recognition by neurotransmitter-gated ion channels, does not apply to the anomalous action of neonicotinoids at the insect nicotinic acetylcholine receptor (nAChR). Insect-selective neonicotinoids have an electronegative pharmacophore (tip) in place of the ammonium or iminium cation of the vertebrate-selective nicotinoids, suggesting topological divergence of the agonist-binding sites in insect and vertebrate nAChRs. This study defines the molecular and electronic basis for the potent and selective interaction of the neonicotinoid electronegative pharmacophore with a unique subsite of the Drosophila but not of the vertebrate alpha4beta2 nAChR. Target site potency and selectivity are retained when the usual neonicotinoid N-nitroimine (=NNO(2)) electronegative tip is replaced with N-nitrosoimine (=NNO) or N-(trifluoroacetyl)imine (=NCOCF(3)) in combination with an imidazolidine, imidazoline, thiazolidine, or thiazoline heterocycle. X-ray crystallography establishes coplanarity between the heterocyclic and imine planes, including the electronegative substituent in the trans configuration. The functional tip is the coplanar oxygen atom of the N-nitrosoimine or the equivalent oxygen of the N-nitroimine. Quantum mechanics in the gas and aqueous phases fully support the conserved coplanarity and projection of the strongly electronegative tip. Further, a bicyclic analogue with a nitro tip in the cis configuration but retaining coplanarity has a high potency, whereas the N-trifluoromethanesulfonylimine (=NSO(2)CF(3)) moiety lacking coplanarity confers very low activity. The coplanar system between the electronegative tip and guanidine-amidine moiety extends the conjugation and facilitates negative charge (delta(-)) flow toward the tip, thereby enhancing interaction with the proposed cationic subsite such as lysine or arginine in the Drosophila nAChR.

Atypical nicotinic agonist bound conformations conferring subtype selectivity

“…Neonicotinoids are selective agonists for the insect versus vertebrate nAChR subtypes. The distinct neonicotinoid nitro or cyano pharmacophore may be recognized by a specific site existing only in the insect subtype (14)(15)(16). Sequence alignments of the nAChR ␤-subunits from diverse species of insects and vertebrates suggest a purported site for interaction with the nitro/ cyano electronegative tip, that is, a basic residue (arginine or lysine) adjacent to the loop D tryptophan in various insect ␤-subunits.…”

Mapping the elusive neonicotinoid binding site

“…In contrast to the nicotinoids, the neonicotinoids are coplanar between the nitro-or cyano-substituted heterocyclic guanidine/amidine planes (13, 18) providing electronic conjugation, which facilitates partial negative charge (␦ Ϫ ) flow toward the tip. However, the equivalent region of positive charge to balance the partial negative charge on the tip is not localized on any specific atom but rather is dispersed into the whole heterocyclic moiety (13,19). indicating their proximity and reactivity.…”

“…Protonation of these agonists at physiological pH is important for binding site interactions at the superfamily of neurotransmitter-gated ion channels (5,7,(9)(10)(11). Neonicotinoids, on the other hand, are uncharged with an electronegative nitro or cyano pharmacophore (12,13). The first step in understanding neonicotinoid binding sites was identification of the molecular determinants for target site resistance and sitedirected mutagenesis (14)(15)(16), but such studies do not provide a complete description of the binding site.…”

Modes of Action, Resistance and Toxicity of Insecticides Targeting Nicotinic Acetylcholine Receptors

“…Other nicotine-like compounds (nicotinoids) were identified as insecticide candidates including synthetic compounds containing a 3-pyridylmethylamine group and a basic amino nitrogen atom [28]. Dihydronicotyrine and N,N di-substituted 3-pyridinylmethylamines were also generated but not taken forward as commercial insecticides [13].…”

“…Following observations by Japanese fishermen that crustaceans became paralysed when placed in the same containers as ragworms, nereistoxin was isolated from the marine annelid Lumbriconereis heteropoda and became the basis for several related synthetic compounds, most notably cartap [10,11,12]. Other nicotine-like compounds (nicotinoids) were identified as insecticide candidates including synthetic compounds containing a 3-pyridylmethylamine group and a basic amino nitrogen atom [13] as well as molecules of plant origin such as charatoxins [14].…”