STRUCTURAL FACTORS CONTRIBUTING TO INSECTICIDAL AND SELECTIVE ACTIONSOF NEONICOTINOIDS

Nicotinoids and neonicotinoids are characterized by the presence of th e 3-pyridylmethylamine moiety in their structure. In the former, the a mino nitrogen atom is ionized, while in the latter the corresponding n itrogen atom is not ionized but bears a partial positive charge, Both types of insecticides interact with nicotinic acetylcholine receptor ( nAChR) of insect origin. The poor interaction of neonicotinoids with v ertebrate nAChR was shown by its poor binding affinity to the nAChR fr om Torpedo electric organ and rat brain and poor activation with nAChR expressed in Xenopus oocytes. The full positive charge was essential to interact with the vertebrate nAChR, while the 3-pyridylmethylamine moiety with a partial positive charge was enough to interact with the insect nAChR. For penetration into the insect central nervous system, hydrophobicity seemed to play an important role, as indicated by the b inding of the injected compounds to the housefly head nAChR. The ioniz ation reduced hydrophobicity and limited the penetration of nicotinoid s, resulting in less insecticidal activity. Among neonicotinoids, nitr omethylene type compounds, though far higher in binding affinity, were less hydrophobic than the corresponding nitroimine type, and the net result was better or inferior insecticidal activity. A chlorine atom a t the 6 position of the 3-pyridyl group found in commercialized neonic otinoids contributes to increased binding affinity and more importantl y hydrophobicity, thus increasing insecticidal activity, N-Me-imidaclo prid was found to be a propesticide of imidacloprid. (C) 1998 Wiley-Li ss, Inc.