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The Vibration theory of smell is that the quality of a particular odour arises from olfactory receptors' responding to frequencies of vibrations of odour molecules in the infrared range. The theory is opposed to the more widely accepted shape theory of olfaction in which the shape of odorant molecules allow them to fit into membrane proteins in the olfactory receptors.

In vibration theory, the vibration frequency of odour molecules is transduced via inelastic electron tunneling of a charge from one part of the receptor to a different energy level in another part of the receptor protein. For strong tunnelling to be possible, it is necessary not only for the shape of the odorant molecule to be compatible with the shape of the receptor, but also for the odorant molecule to have a vibrational energy mode compatible with the difference in energies between the two energy levels on the receptor. The quality of a particular odour is encoded in the ratio of activities of receptors tuned to different vibration frequencies, in the same way that colour is encoded in the ratio of activities of cone cell receptors tuned to different frequencies of light.

Some evidence supports vibration theory; some evidence supports shape theory. Although vibration theory explains the quality of odours, it does not explain intensity of odours, why some odours are stronger than others at the same concentrations.

Contents 1 Support 1.1 Explaining differences in stereoisomer scents 1.2 Challenging shape theory 2 Challenges 3 References

Carvone presented a perplexing situation to vibration theory. Carvone has two isomers, both of which vibrate the same yet smell different. One smells like mint and the other like caraway (for which it is named).

An experiment reported in a book regarding Luca Turin's research consisted of mixing the mint isomer with butanone, on the theory that the shape of the G-protein-coupled receptor prevented the carbonyl group in the mint isomer from being detected by the "biological spectroscope". The experiment succeeded: a mixture of 60% butanone and 40% mint carvone smells like caraway, a success for the vibration theory of olfaction.

• Similarly shaped molecules with different molecular vibrations have different smells (metallocene experiment and deuterium replacement of molecular hydrogen)
• Differently shaped molecules with similar molecular vibrations have similar smells (replacement of carbon double bonds by sulphur atoms and the disparate shaped amber odorants)
• Hiding functional groups does not hide the group's characteristic odor

Three predictions by Luca Turin on the nature of smell, using concepts of vibration theory, have been claimed to be false (Keller and Vosshall, 2004).

• Turin, Luca, (1996). A spectroscopic mechanism for primary olfactory reception. Chemical Senses. 21(6):773-791
• Burr, Chandler (2003). The Emperor of Scent: A Story of Perfume, Obsession, and the Last Mystery of the Senses. New York: Random House. ISBN 0-375-50797-3. 
• Keller, A and Vosshall, LB. (2004). A psychophysical test of the vibration theory of olfaction. Nature Neuroscience 7:337-338. See also the editorial on p. 315. (Newsletter report, Rockefeller University Scientist).