Press cutting: Rogue theory of smell gets a boost

Calculations by researchers at UCL show that the idea that we smell odour molecules by sensing their molecular vibrations makes sense in terms of the physics involved.

That's still some way from proving that the theory, proposed in the mid-1990s by biophysicist Luca Turin, is correct. But it should make other scientists take the idea more seriously. …

Most scientists have assumed that our sense of smell depends on receptors in the nose detecting the shape of incoming molecules, which triggers a signal to the brain. …

But Turin argued that smell doesn't seem to fit this picture very well. Molecules that look almost identical can smell very different - such as alcohols, which smell like spirits, and thiols, which smell like rotten eggs. And molecules with very different structures can smell similar. …

Turin's explanation for these smelly facts invokes the idea that the smell signal in olfactory receptor proteins is triggered not by an odour molecule's shape, but by its vibrations, which can encourage an electron to jump between two parts of the receptor in a quantum-mechanical process called tunnelling. This electron movement could initiate the smell signal being sent to the brain.

This would explain why isotopes can smell different: their vibration frequencies are changed if the atoms are heavier. Turin's mechanism, says Professor Marshall Stoneham [UCL Physics & Astronomy], is more like swipe-card identification than a key fitting a lock.

Vibration-assisted electron tunnelling can undoubtedly occur - it is used in an experimental technique for measuring molecular vibrations. "The question is whether this is possible in the nose," says Stoneham's colleague, Dr Andrew Horsfield [UCL Physics & Astronomy]. …

Stoneham says that when he first heard about Turin's idea, while Turin was himself based at UCL, "I didn't believe it". But, he adds, "because it was an interesting idea, I thought I should prove it couldn't work. I did some simple calculations, and only then began to feel Luca could be right." Now Stoneham and his co-workers have done the job more thoroughly, in a paper soon to be published in 'Physical Review Letters'. …

Philip Ball, 'Nature'