Crystalline contact insecticides are used for controlling malaria vectors. As such, activity depends upon the interface between the crystals and mosquitoes. Crystals made from molecules are generally polymorphic: they can be made to crystallize with a variety of structures, each with a characteristic set of lattice and surface energies, as well as unique chemical and physical properties. A systematic study of the crystallography of contact insecticides, a critical but overlooked aspect in the consideration of bioavailability, was initiated. Increasing bioavailability (rate of uptake from crystals) can thwart metabolic resistance development where chemical reaction rates are determinative and overcome barriers to penetration. An inverse correlation between the thermodynamic stability of crystal phases and insecticidal activity has not yet been violated by experiment. This correlation exists in pyrethroids as well as organochlorines and neonicotinoids. The higher the free energy of a crystal and thus, the higher the chemical potential energy of the toxicant molecules, the more rapidly molecules are liberated and absorbed by the tarsi of insects contacting the crystal surface.

Polymorphism in the service of infectious disease prophylaxis

ACS Spring Meeting 2021

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