Structure-dependent volatility of juvenile hormone receptor agonists and its consequences for activity assays: When...

Volatility, the propensity to dissipate from solid or liquid state to the air, is an important property of certain pesticides and small molecules mediating chemical communication. Volatility is the function of a compound's vapor pressure, which depends on intra- and intermolecular forces given by the chemical structure. The sesquiterpenoid juvenile hormones (JHs), vital to insect development and reproduction, have been known for vapor-borne activity in vivo. The same applies to some synthetic JH mimics (juvenoids), which serve as biorational insecticides of the insect growth regulator (IGR) class. However, neither volatility of the natural JHs nor the mode of vapor-phase JH or juvenoid action have been experimentally defined. Here, we employ a reporter assay based on the insect JH receptor (JHR) to determine to what extent diverse JHR ligands - five natural JH homologs and six synthetic juvenoids of two types (aliphatic and aromatic) - exert JHR agonist activity over distance. Our microtiter plate volatility assay showed that specific vapor-borne activity and effective concentrations of the agonists agreed with their empirical or calculated vapor pressure values. While most of the natural JHs and the aliphatic juvenoids produced moderate to extensive vapor-borne activity across the assay plate, the aromatic juvenoids did not. These experiments have resolved volatility of juvenile hormones and their mimics at the molecular action level. The data bear significance for proper designing of JH signaling assays and screens for JHR modulating compounds, where any contaminating vapor-borne activity is detrimental. Physiological effect of JH as an airborne endocrine signal is discussed.