Gas leaks, garlic breath, skunks in the neighborhood—ah, the scent of thiols. The human nose is particularly sensitive to these sulfur-containing compounds, which is no surprise given that they are often associated with things to avoid. But how exactly are our nostrils (and those of other mammals) so adept at sniffing out thiols when other odors, such as bleach or vinegar, require higher thresholds of molecules in the air for us to detect them?
The hypersensitivity comes down to the metal copper, according to a team of chemists from the U.S. and China. As they reported this fall in the Journal of the American Chemical Society, the researchers discovered that the same receptors in the nose that pick up these unpleasant-smelling molecules also bind with particles of copper that reside in nasal mucus. The metallic partner amplifies a thiol's intensity by up to 1,000 times. And in experiments in which the scientists created thiol receptors that could not bind with copper, sensitivity to the odorant all but disappeared.
Evolutionarily, it pays to have a nose that can pick up the minutest presence of thiols, says study co-author Eric Block, a chemist at the University at Albany, State University of New York. The sulfur compounds are released by rotting food, for instance, and some predators give off olfactory cues to their presence in this form.
This is the first time a metal has been implicated in smell, and the study will likely encourage other scientists seeking to better understand olfaction to look for metallic sidekicks, says Robert Crabtree, a chemist at Yale University. Crabtree first speculated about copper's role in thiol olfaction in 1977.