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February 18, 2011
Smell The Vibrations? Fruit Flies Might Be Able To. . .
A few years ago, I wrote here about Luca Turin and his theory that our sense of smell is at least partly responsive to vibrational spectra. (Turin himself was the subject of this book, author of this one (which is quite interesting and entertaining for organic chemists), and co-author of Perfumes: The A-Z Guide, perhaps the first attempt to comprehensively review and categorize perfumes).
Turin's theory is not meant to overturn the usual theories of smell (which depend on shape and polarity as the molecules bind into olfactory receptors), but to extend them. He believes that there are anomalies in scent that can't be explained by the current model, and has been proposing experiments to test them. Now he and his collaborators have a new paper in PNAS with some very interesting data.
They're checking to see if Drosophila (fruit flies) can tell the difference between deuterated and non-deuterated compounds. The idea here is that the size and shape of the two forms are identical; there should be no way to smell the difference. But it appears that the flies can: they discriminate, in varying ways, between deuterated forms of acetophenone, octanol, and benzaldehyde. Deuterated acetophenone, for example, turns out to be aversive to fruit flies (whereas the normal form is attractive), and the aversive quality goes up as you move from d-3 to d-5 and d-8 forms of the isotopically labeled compound.
The flies could also be trained, by a conditioned avoidance protocol, to discriminate between all of the isotopic pairs. Most interestingly, if trained to avoid a particular normal or deutero form of one compound, they responded similarly when presented with a novel pair, which seems to indicate that they pick up a "deuterated" scent effect that overlays several chemical classes.
There's more to the paper; definitely read it if you're interested in this sort of thing. Reactions to it have been all over the place, from people who sound convinced to people who aren't buying any of it. If Turin is right, though, it may indeed be true that we're smelling the differences between C-H stretching vibrations, possibly through an electron tunneling mechanism, which is a rather weird thought. But then, it's a weird world.
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