Structure-activity relationship of a series of sensory irritants

The relative potencies for a series of sensory irritants, with structures based on toluene, were determined by measuring the airborne concentrations that caused a 50% decrease in respiratory rate in Swiss-Webster mice. This concentration is referred to as the RD50. Toluene, a relatively nonirritating compound, and compounds with chlorine, two chlorines, bromine, and iodine atoms substituted on the alpha carbon of toluene were tested. The RD50s for these compound types were determined to be 4900, 27, 27, 5.2, and 4.3 ppm, respectively. In addition, compounds with chlorine substituted at the ortho, meta, and para positions on the toluene ring were also tested. The RD50s were determined to be 4.9, 13, and 14 ppm, respectively. The structure-activity relationships of the compounds studied are explained by a model (Abraham et al., 1990; Nielsen and Alarie, 1982) that relates the interaction of sensory irritants with a receptor protein in a lipid bilayer. The trends in the RD50s, and thus sensory irritation, for the compounds studies are related to the development of a partial positive charge on the toluene alpha carbon by the positioning of a ring chlorine and the bond dissociation energies of the alpha carbon-halogen bond for the iodo, bromo, and chloro isomers of benzyl halide.