The Scent of a New Theory

Fresh evidence favours the controversial idea that quantum effects may be responsible for our sense of smell

Traditionally, it has been held that our sense of smell depends on the shape of molecules. This lock and key mechanism proposes that molecules wafting into our nose bind with one of the receptor molecules present in the nose and this is what we smell. In 1996, biophysicist Luca Turin proposed a controversial new theory to account for the fact that in some cases molecules very similar in shape smell very different. He suggested that receptors in our nose could actually detect the vibration of molecules through quantum effects and that is what we sense as smell. He even wrote about this theory in a fascinating book The Secret of Scent, but many scientists dismissed the theory as speculative. Now, a new paper that he has co-authored offers fresh evidence for his theory.

Published last month in the prestigious journal Proceedings of the National Academy of Sciences, the paper details a study on fruit flies exposed to two versions of the molecule acetophenone, which is often used in fragrances that resemble jasmine, almonds and cherries. One version of the molecule was normal acetophenone, while the other version replaced a hydrogen atom in the molecule by its isotope deuterium, a heavier version of hydrogen. This made no difference to the shape of the molecule, but it did change the vibration properties, with the heavier version of the molecule vibrating more slowly.

The researchers found that fruit flies were not only able to distinguish between the two molecules, they actually showed a preference for the normal version, while avoiding the heavier molecule. Fruit flies bred to ensure they had no sense of smell were unable to distinguish between the two molecules, suggesting that it was their sense of smell that helped flies distinguish between the two, and clearly this sense relied on something more or other than the shape of the molecule.

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