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Your sense of smell depends on olfactory receptors. These receptors bind to oderant molecules and identify them by structure or shape. But what about molecules that are similar in shape? Those should smell enough alike that we can’t tell the difference. But here’s the catch - we can.
For example, ethanol and ethanethiol share the same shape. The only difference is that ethanol has an oxygen atom in place of a sulphur atom. But ethanethiol smells like rotten eggs, whereas ethanol smells slightly like alcohol. If smell depended on shape alone, these two molecules would smell the same. Scientists believe to have discovered why they don’t: they vibrate differently.
By applying an electric current to a molecule, we can cause it to vibrate. If we measure those vibrations, then we can distinguish molecules with similar shapes, but differing vibrations.
Deuterium atoms are simply hydrogen atoms with an added neutron in the nucleus. This doesn’t change the chemical composition of the atom, it only makes the atom heavier. A heavier atom vibrates at a different frequency than a lighter one, so a molecule with deuterium in place of hydrogen should smell different to a vibration dependant olfactory sense.
Four researchers from the Massachusetts Institute of Technology in the United States and the Biomedical Sciences Research Centre “Alexander Fleming” in Greece tested these similar-shaped compounds on fruit flies, Drosophila melanogaster. After training the flies to avoid the heavier, deuterated acetophenone, the researchers placed them in a maze containing both types of acetophenone. The researchers then counted how many of the flies swarmed towards the deuterated acetophenone and how many swarmed the plain acetophenone.
Approximately 30 percent more flies preferred the plain acetophenone to the deuterated acetophenone, indicating that there is in fact a vibration-sensing component to our sense of smell. The group also repeated the experiment using deuterated octanol and regular octanol on the same flies with similar results.
Deuterated octanol and nitriles are chemically unrelated. However the molecular vibrations between carbon and nitrogen in nitriles are similar to those between carbon and deutrium in deuterated octanol. Flies conditioned to avoid deuterated octanol also avoided the nitrile when exposed to it. Likewise, when exposed to deuterated octanol, flies trained to avoid the nitrile also avoided deuterated octanol.
These experiments indicate that we do in fact depend on the vibrations of molecules in order to smell them, at least in part. This result could also help in developing better “artificial noses”, with far-reaching effects.