Most scientists believe that the shape of a molecule defines its odour, and that we smell using receptors in our noses that bind selectively to molecules of a specific shape. However, this theory cannot explain why very differently shaped molecules can smell the same, or why similarly shaped molecules with different masses have very different smells.

Some scientists have tried to explain these contradictions by pointing out that every molecule has a distinct set of vibrations – and these could also be detected by the receptors in our nose. However, this theory has suffered from the lack of a plausible mechanism for converting vibrations into a signal than could be sent to the brain.

Now, the UCL researchers have calculated that electron tunnelling could provide the link between smell and molecular vibrations. Their work builds on a theory first proposed in 1996 by Luca Turin, who was then at UCL. Turin suggested that a receptor acts like an electrical switch that allows a current to flow when bound to a molecule with specific vibration properties. He also suggested that the switching mechanism is electron tunnelling, which is a purely quantum effect that is known to be affected by vibrations in a process called phonon-assisted tunnelling.

Stoneham and colleagues have taken Turin’s idea a step further by calculating the rates of electron movement that would be expected in a hypothetical receptor. The calculations show that electron flow would increase significantly when an odour molecule with the correct vibrational frequency is bound to the receptor.

Stoneham and colleagues are now examining data from experiments on how receptors respond to different molecules and hope that their calculations will encourage other physicists to devise further experiments to test their theory. “We are also in contact with an experimental group working on determining the atomic structure of olfactory receptors,” adds Andrew Horsfield, who is one of the UCL group.