Saturday science subject: Quantum olfaction

— 4:19 PM on December 23, 2006

Physicists at the University College London have built upon a ten-year-old theory about the way the sense of smell works, and according to them, that theory is entirely plausible. The theory suggests quantum mechanisms—not the widely-accepted "lock and key" interaction between molecules and olfactory receptors—are responsible for allowing our noses to tell different smells apart from each other.

[Luca] Turin's more controversial theory, put forth in 1996 and now the subject of two popular books, holds instead that odorant receptors sense the way a molecule's atoms jiggle. The shape of the molecule still comes into play, Turin says, because it determines the odorant's overall vibrational frequency.

The [University College London] researchers imagined that the odorant fits into a spot between a site that donates an electron and one that receives the electron. In this model, the receptor switches on when an electron hops from donor to acceptor. The group calculated that an electron could "tunnel" through the barrier imposed by the odorant, an effect made possible by quantum mechanics, they wrote in a preprint accepted for publication in Physical Review Letters.

Before it tunnels, the electron distorts the odorant molecule's electrical field. When it tunnels, it effectively disappears, causing that electrical field to wobble like a plucked string, Turin explains. Tunneling is likely to take place if the plucking matches the molecule's natural mode of vibration.

The theory would explain why similar molecules, such as ethanol and ethane thiol, have completely different smells. (The former smells like Vodka, the latter like rotten eggs.) According to Professor Marshall Stoneham, one of the University College London physicists, "The way the numbers worked out it looks like a perfectly credible mechanism. The reason we're not 100 percent certain is we don't know the detailed structure of the receptors."
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