[QUOTE=anosmia]
I think the honest answer about smell reception at this point in time is that it remains poorly understood. Various “Shapist” (lock-and-key) models are the most widely accepted, but their predictive value has proven to be completely dismal – another thorn in side of the fragrance and flavour R&D budgets.
“Vibrationist” theories have been proposed from time to time, most recently in the form of an inelastic electron tunnelling model devised by the biophysicist Luca Turin. The guys is not well liked in smell research circles, and vibration theory has not been well received, but he has done a terrific job of enumerating all of the many inadequacies of current lock-and-key theories of smell reception.
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Interesting. I’m not an expert in olfaction either, but I hadn’t heard the vibration model. There’s an interesting article in Nature neuroscience by Keller and Vosshall published in 2004 that seems to systematically eviscerate the model though. Here are the major findings–
[QUOTE=Vosshall and Keller 2004]
Turin predicts that the smell of a mixture of guaiacol and benzaldehyde has a vanilla character not found in its components because the combined molecular vibrations of benzaldehyde and guaiacol approximate the vibrations of vanillin.
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At neither concentration did the mixture of benzaldehyde and guaiacol have a stronger vanilla character than that of its individual components. A similar result was obtained when odor pairs were rated on an odor similarity rating scale (Supplementary Fig. 1 online, panel b).
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A second prediction of vibration theory as proposed by Turin is that aldehydes with an even number of carbon atoms have a different odor than those with an odd number.
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Contrary to Turin’s prediction, pairs consisting of two odd or two even numbered aldehydes were not perceived as more similar than pairs consisting of an odd and an even numbered aldehyde (Fig. 1c). We found instead, as suggested in previous studies, that the carbon chain length of these molecules is the salient feature sensed by the olfactory system.
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A third prediction of Turin’s vibration theory is that acetophenone (AP) and completely deuterated acetophenone (AP-d8), which have the same shape but different molecular vibrations, should have distinguishable smells.
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Subjects easily distinguished the enantiomers but could not distinguish AP from AP-d8 (Fig. 2b). Finally, we used a duo-trio test in which two stimuli were presented and the subject was asked to identify the one identical to a third reference smell. In a separate session, we tested six subjects who had successfully distinguished AP from AP-d8 to determine whether their correct selections reflected chance performance or true discrimination of these two odorants. None of the six subjects was able to distinguish the two smells. The proportion of correct choices ranged from 43% to 67% (mean, 53%; standard error plusminus 14%; Fig. 2c).
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I find this article pretty convincing at first glance. I think the vibrational model is cool, but it kind of defies the way signal transduction appears to work in any other sensory modality. As far as I can tell, the lock-and-key model is a better (though certainly imperfect) model. The fact is, structural biology is hard, and I suspect we simply don’t know enough about odorant receptors to design molecules well against them at this point.
I could be wrong.
