Vibration theory of olfaction
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The '''Vibration theory''' of [[olfaction|smell]] proposes that a molecule's smell character is due to its [[vibrational spectroscopy|vibrational frequency]] in the infrared range. The theory is opposed to the more widely accepted [[shape theory of olfaction]], which proposes that a molecule's smell character is due to its shape.
==Introduction==
The current vibration theory has recently been called the "swipe card" model, in contrast with "lock and key" models based on shape theory.<ref name="titleAccess : Rogue theory of smell gets a boost : Nature News">{{cite web |url=http://www.nature.com/news/2006/061204/full/news061204-10.html |title=Access : Rogue theory of smell gets a boost : Nature News |accessdate=2008-04-11 |format= |work=}}</ref> As proposed by [[Luca Turin]], the odorant molecule must first fit in the receptor's binding site. Then it must have a vibrational energy mode compatible with the difference in energies between two energy levels on the receptor, so electrons can travel through the molecule via inelastic [[electron tunneling]], triggering the [[signal transduction]] pathway.<ref name="pmid8985605">{{cite journal |author=Turin L |title=A spectroscopic mechanism for primary olfactory reception |journal=Chem. Senses |volume=21 |issue=6 |pages=773–91 |year=1996 |pmid=8985605 |url=http://www.flexitral.com/research/chemical_senses_complete.pdf |doi=}}</ref>
The odour character is encoded in the ratio of activities of receptors tuned to different vibration frequencies, in the same way that [[color|colour]] is encoded in the ratio of activities of [[cone cell]] receptors tuned to different frequencies of light. Although vibration theory explains odour character, it does not explain intensity: why some odours are stronger than others at the same concentrations.
Some studies support vibration theory while others challenge its findings.
==Major proponents and history==
The theory was first proposed by Malcolm Dyson in 1937 and expanded by Robert H. Wright in 1954, after which it was largely abandoned in favor of the competing shape theory. A 1996 paper by Turin revived the theory by proposing a mechanism, speculating that the [[G-protein-coupled receptors]] discovered by [[Linda Buck]] and [[Richard Axel]] were actually measuring molecular vibrations using inelastic electron tunneling, rather than responding to molecular keys that work by shape alone.<ref name="pmid8985605">{{cite journal |author=Turin L |title=A spectroscopic mechanism for primary olfactory reception |journal=Chem. Senses |volume=21 |issue=6 |pages=773–91 |year=1996 |pmid=8985605 |url=http://www.flexitral.com/research/chemical_senses_complete.pdf |doi=}}</ref> The theory remains controversial.
==Support==
===Explaining differences in stereoisomer scents===
[[Carvone]] presented a perplexing situation to vibration theory. Carvone has two [[isomer]]s, which have identical vibrations, yet one smells like [[Mentha|mint]] and the other like [[caraway]] (for which the compound is named).
An experiment by Turin filmed by the [[BBC Horizon]] documentary "A Code in the Nose" 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 with the trained perfumers used as subjects, who perceived that a mixture of 60% butanone and 40% mint carvone smelled like caraway.
===The sulfurous smell of boranes===
According to Turin's original paper in the journal ''Chemical Senses'', the well documented smell of [[borane]] compounds is intensely sulfurous, though these molecules contain no [[sulfur]]. He proposes to explain this by the similarity in frequency between the vibration of the B-H bond and the S-H bond.<ref name="pmid8985605">{{cite journal |author=Turin L |title=A spectroscopic mechanism for primary olfactory reception |journal=Chem. Senses |volume=21 |issue=6 |pages=773–91 |year=1996 |pmid=8985605 |url=http://www.flexitral.com/research/chemical_senses_complete.pdf |doi=}}</ref>
===Isotope effects===
A major prediction of Turin's theory is the [[isotope]] effect: that the normal and [[deuterium|deuterated]] versions of a compound should smell different, although they have the same shape. A 2001 study by Haffenden et al showed humans able to distinguish [[benzaldehyde]] from its deuterated version.<ref name="Haffenden">{{cite journal |author= Haffenden LJ, Yaylayan VA, Fortin J|title=Investigation of vibrational theory of olfaction with variously labelled benzaldehydes |journal=Food Chem. |volume=73 |issue=1 |pages=67–72 |year=2001|doi=10.1016/S0308-8146(00)00287-9}}</ref><ref name="titleDavid MacKay: Smells: Summary">{{cite web |url=http://www.inference.phy.cam.ac.uk/mackay/smell/ |title=David MacKay: Smells: Summary |accessdate=2008-04-11 |format= |work=}}</ref> In addition, tests with animals have shown fish and insects able to distinguish isotopes by smell
<ref name="Havens">{{cite journal |author=Havens BR, Melone CD |title=The application of deuterated sex pheromone mimics of the american cockroach (Periplaneta americana, L.), to the study of wright's vibrational theory of olfaction |journal=Dev. Food. Sci. |volume=37 |issue=1 |pages=497–524 |year=1995|doi=10.1016/S0167-4501(06)80176-7 |url=http://flexitral.com/research/Meloan_1995.pdf}}</ref><ref name="pmid862794">{{cite journal |author=Hara J |title=Olfactory discrimination between glycine and deuterated glycine by fish |journal=Experientia |volume=33 |issue=5 |pages=618–9 |year=1977 |pmid=862794 |doi=}}</ref>
===Consistency with physics===
Biophysical simulations published in [[Physical Review Letters]] in 2006 suggest that Turin's proposal is viable from a physics standpoint.<ref name="pmid17358733">{{cite journal |author=Brookes JC, Hartoutsiou F, Horsfield AP, Stoneham AM |title=Could humans recognize odor by phonon assisted tunneling? |journal=Phys. Rev. Lett. |volume=98 |issue=3 |pages=038101 |year=2007 |pmid=17358733 |doi=}}</ref><ref name="titleRogue Odour Theory Could Be Right">{{cite web |url=http://www.sciencedaily.com/releases/2007/02/070204162541.htm |title=Rogue Odour Theory Could Be Right |accessdate=2008-04-11 |format= |work=}}</ref>
===Correlating odor to vibration===
A 2004 paper published in the journal ''Organic Biomolecular Chemistry'' by Takane and Mitchell shows that odor descriptions in the olfaction literature correlate more strongly with vibrational frequency than with molecular shape.<ref name="pmid15534702">{{cite journal |author=Takane SY, Mitchell JB |title=A structure-odour relationship study using EVA descriptors and hierarchical clustering |journal=Org. Biomol. Chem. |volume=2 |issue=22 |pages=3250–5 |year=2004 |pmid=15534702 |doi=10.1039/B409802A}}</ref>
===Lack of antagonists===
Turin points out that traditional lock-and-key receptor interactions deal with [[agonist|agonists]], which increase the receptor's time spent in the active state, and [[antagonist|antagonists]], which increase the time spent in the inactive state. In other words, some [[ligand|ligands]] tend to turn the receptor on and some tend to turn it off. As an argument against the traditional lock-and-key theory of smell, no olfactory antagonists have yet been found until 2004. A japanese research group published that an
oxidation product of [[eugenol]] is able to antagonize, i.e. prevent, the smell of eugenol.<ref name="pmid15574817">{{cite journal |author=Oka Y, Nakamura A, Watanabe H, Touhara K |title=An odorant derivative as an antagonist for an olfactory receptor |journal=Chem. Senses |volume=29 |issue=9 |pages=815–22 |year=2004 |pmid=15574817 |doi=10.1093/chemse/bjh247}}</ref>
===Additional challenges to 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 [[sulfur]] atoms and the disparate shaped [[amber]] odorants)
*Hiding [[functional group]]s does not hide the group's characteristic odor
==Challenges to vibration theory==
Three predictions by Luca Turin on the nature of smell, using concepts of vibration theory, were addressed by experimental tests published in [[Nature Neuroscience]] in 2004 by Vosshall and Keller. The study failed to support the prediction that isotopes should smell different, with human subjects unable to distinguish [[acetophenone]] and its deuterated counterpart.<ref name="pmid17358733">{{cite journal |author=Brookes JC, Hartoutsiou F, Horsfield AP, Stoneham AM |title=Could humans recognize odor by phonon assisted tunneling? |journal=Phys. Rev. Lett. |volume=98 |issue=3 |pages=038101 |year=2007 |pmid=17358733 |doi=}}</ref><ref name="pmid15048113">{{cite journal |author= |title=Testing a radical theory |journal=Nat. Neurosci. |volume=7 |issue=4 |pages=315 |year=2004 |pmid=15048113 |doi=10.1038/nn0404-315}}</ref><ref name="titleRogue Odour Theory Could Be Right">{{cite web |url=http://www.sciencedaily.com/releases/2007/02/070204162541.htm |title=Rogue Odour Theory Could Be Right |accessdate=2008-04-11 |format= |work=}}</ref>
In addition, Turin's description of the odor of long-chain aldehydes as alternately (1) dominantly waxy and faintly citrus and (2) dominantly citrus and faintly waxy was not supported by tests on untrained subjects, despite anecdotal support from fragrance industry professionals who work regularly with these materials. Vosshall and Keller also presented a mixture of [[guaiacol]] and [[benzaldehyde]] to subjects, to test Turin's theory that the mixture should smell of [[vanillin]]. Vosshall and Keller's data did not support Turin's prediction.
==References==
{{reflist}}
* {{cite book | author=Burr, Chandler | title=The Emperor of Scent: A Story of Perfume, Obsession, and the Last Mystery of the Senses | year=2003 | location=New York | publisher=Random House | id=ISBN 0-375-50797-3}}
*Flexitral website [http://flexitral.com]
*[http://www.rockefeller.edu/pubinfo/news_notes/rus_032604_b.php Newsletter report], Rockefeller University Scientist.
*Zyga, Lisa. Quantum mechanics may explain how humans smell. ''PhysOrg.com'' (2007). [http://www.physorg.com/news89542035.html]
[[Category:Olfactory system]]