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The Vibration theory of smell proposes that the sensation of smell arises not only from the recognition, by olfactory receptor membrane proteins in the cells of the nose, of the shape of odorant molecules in accordance with the widely posited lock and key mechanism of molecular biology, as proposed by the orthodox shape theory of olfaction, but that it may also strongly reflect the vibrational spectrum of frequencies of vibrations of odour molecules in the infrared range.
On this view it is suggested that a critical part of the relevant signal transduction process in a cell, to complete the overall charge transduction chain, may often be the inelastic electron tunneling of a charge from one part of the receptor to a different energy level in another part of the receptor protein. It is proposed that for strong tunnelling to be possible, it would therefore be 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.
Some evidence supports this model, while some evidence exclusively supports the shape mechanism.
Support
Explaining differences in stereoisomer scents
Carvone presented a perplexing situation to this 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.
Challenging shape theory
- 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 disperate shaped amber odorants)
- Hiding functional groups does not hide the group's characteristic odor
Challenges
Three predictions by Luca Turin on the nature of smell, using concepts of vibration theory, have been found to be false (Keller and Vosshall, 2004).
References
- Turin, Luca, (1996). A spectroscopic mechanism for primary olfactory reception. Chemical Senses. 21(6):773-791
- 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).
"Vibration theory of olfaction"