Vibration theory of olfaction: Difference between revisions

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===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 |vauthors=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 |access-date=2008-04-11 }}</ref> However, this study has been criticized for lacking [[double-blind]] controls to eliminate bias and because it used an anomalous version of the [[duo-trio test]].<ref name=Vosshall2004>{{cite journal |vauthors=Keller A, Vosshall LB |title=A psychophysical test of the vibration theory of olfaction|journal=Nature Neuroscience|volume=7|issue=4|pages=337–338|year=2004|doi =10.1038/nn1215|pmid=15034588}}</ref> In additionanother study, tests with animals have shown fish and insects able to distinguish isotopes by smell.<ref name="Havens">{{cite journal |vauthors=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|series=Developments in Food Science |isbn=9780444820136 }}</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=10.1007/BF01946534}}</ref><ref>[http://www.nature.com/news/2011/110214/full/news.2011.39.html#B1 Flies sniff out heavy hydrogen] Nature 14 February 2011.</ref><ref name=Franco>{{cite journal |author=Franco, M. I. | author2= Turin, L. |author3= Mershin, A.|author4=Skoulakis, E. M. C. |name-list-style=amp |title=Molecular vibration-sensing component in Drosophila melanogaster olfaction |journal=Proceedings of the National Academy of Sciences of the USA |volume=108 |issue=9 |pages=3797–3802| year=2011 |pmid=21321219 |pmc=3048096|doi=10.1073/pnas.1012293108 |url=http://www.pnas.org/content/early/2011/02/08/1012293108.abstract|bibcode = 2011PNAS..108.3797F }}</ref>

Deuteration changes the heats of adsorption and the boiling and freezing points of molecules (boiling points: 100.0&nbsp;°C for H₂O vs. 101.42&nbsp;°C for D₂O; melting points: 0.0&nbsp;°C for H₂O, 3.82&nbsp;°C for D₂O), p''K''_a (i.e., [[dissociation constant]]: 9.71×10⁻¹⁵ for H₂O vs. 1.95×10⁻¹⁵ for D₂O, cf. [[Heavy water]]) and the strength of hydrogen bonding. Such [[Kinetic isotope effect|isotope effects]] are exceedingly common, and so it is well known that deuterium substitution will indeed change the binding constants of molecules to protein receptors.<ref>{{cite journal | last1 = Schramm | first1 = V. L. | year = 2007 | title = Binding isotope effects: boon and bane | journal = Curr. Opin. Chem. Biol. | volume = 11 | issue = 5| pages = 529–536 | doi=10.1016/j.cbpa.2007.07.013 | pmid=17869163 | pmc=2066183}}</ref> Any binding interaction of an odorant molecule with an olfactory receptor will therefore be likely to show some isotope effect upon deuteration, and the observation of an isotope effect in no way argues exclusively for a vibrational theory of olfaction.