Sense of Smell Tied To Quantum Physics?

SpaceAdmiral writes "A controversial theory that proposes that our sense of smell is based not on the shape of the molecules that enter our nose but on their vibrations was given a boost recently when University College London researchers determined that the quantum physics involved makes sense. The theory, proposed in the mid-1990s by biophysicist Luca Turin, suggests that electron tunneling initiates the smell signal being sent to the brain. It could explain why similarly shaped molecules can have very different smells, and molecules with very different structures can smell similar." Turin has now formed a company to design odorants using his theory, and claims an advantage over the competition of two orders of magnitude in rate of discovery. The article concludes, "At the very least, he is putting his money where his nose is."

Comments

Raised eyebrows...

[BWJones]

I am going to be very skeptical of this and would not be tossing any money into a private company to study this just yet. The olfactory system is well capable of distinguishing many small molecules, even those that are very similar using a variety of well known and well understood processes just as in the immune system. Look, a Nobel prize was awarded back in the 30's for the discovery that IGGs can recognize even racemic molecules such as L and D forms of glycine even and the olfactory literature is just as rich. The biggest problem however, with the UCL approach is that it completely ignores years of cortical, subcortical and psychophysics data. Furthermore, there is no effort or model in their work that might explain how the signals would be transduced into cortical/subcortical signals or how they account for potential noise in the system. Their claim that signals can be translated through tunneling in a biological system which likely swamps those potential signals with noise is what really troubles me.

I am not saying that they should not do it, or that they are absolutely wrong, as it is possibly interesting. Rather all I am saying is my eyebrows are raised at their claims.

Re:Raised eyebrows...

[alkaloids]

IGGs can recognize even racemic molecules such as L and D forms of glycine Ah, glycine is um, not chiral. Therefore you can't have an L or a D form, nor can you have a racemate... Close though! You were really unlucky, as glycine is the only AA that's not chiral.

As to the rest of the comment, I'll raise my eyebrows at it. I'm thoroughly skeptical that tunneling would be involved in smell though, but it would be amazing if it were. We'll find out soon enough I'm sure.

Re:Raised eyebrows...

[CapsaicinBoy]

I am a chemosensory psychophysicist, but I work in taste/chemesthesis, not smell. That having been said, I was in the room when Keller and Vosshall presented the following at the Association for Chemoreception Sciences meeting in 2004.

A PSYCHOPHYSICAL TEST OF THE VIBRATION THEORY OF OLFACTION
Keller A., Vosshall L.B. Laboratory of Neurogenetics and Behavior,
Rockefeller University, New York, NY

At present no satisfactory theory exists to explain why a given
molecule has a particular smell. A recent book about the physiologist
Luca Turin has generated new interest in the theory that the smell of a
molecule is determined by its intramolecular vibrations rather than by
its shape. We present the first psychophysical experiments in humans
that test key predictions of this theory. The results suggest that
molecular vibrations alone cannot explain the perceived smell of a
chemical. Specifically, we have found that: (i) in a component
identification task no vanilla odor character was detected in the mixture
of benzaldehyde and guaiacol (ii) odor similarity ratings did not reveal
that even and odd numbered aldehydes form two odor classes and (iii)
naive subjects who could easily discriminate the smell of two molecules
that differ in shape but not in molecular vibration failed to discriminate
two molecules with similar shape but different molecular vibrations in
three different experimental paradigms (similarity rating, duo-trio test,
triangle test). Taken together our findings are consistent with the idea
that the smell of a molecule is determined by its shape but we found no
evidence that the smell of a molecule is influenced by its vibrational
properties.

They subsequently published their findings in Nature Neuroscience.

Keller A, Vosshall LB. A psychophysical test of the vibration theory of olfaction. Nat Neurosci. 2004 Apr;7(4):337-8.

At present, no satisfactory theory exists to explain how a given molecule results in the perception of a particular smell. One theory is that olfactory sensory neurons detect intramolecular vibrations of the odorous molecule. We used psychophysical methods in humans to test this vibration theory of olfaction and found no evidence to support it.

The short version is that the data do not support Luca Turin's speculation.

Re:Been rooting for this guy!

[Otter]

Here's a good discussion of Turin's work as it stood a few months ago. I agree with Lowe that Nature Neuroscience's trashing of him was excessive and obnoxious, particularly because, as you say, there's no question that he behaves like a responsible scientist pushing a wildly controversial idea should.

What about Axel and Buck theory?

[dockingman]

I'm a graduate student in Computer-Aided Drug Design, and as part of my degree I did a research proposal on prediction of smell with computers.

Richard Axel and Linda Buck received their Nobel Prize in 2004 for Physiology or Medicine for "for their discoveries of odorant receptors and the organization of the olfactory system". Note that this is not *only* for the discovery of the receptors, but also for the *way they work*. There are hundreds of receptors in mammals (almost 1,000 in mice, about 330 in humans) that have different selectivities for different odorant molecules and act combinatorially, that is, that the signal perceived by the brain is the result of the combination of receptors activated by the odorant. Given the large number of receptors, and that any number can be activated by an odorant, the variety of smells is huge, and on the other hand the promiscuity of the receptors allows for a chance of 2 dissimilar molecules having the same smell...

Some literature I suggest for someone interested:
- Nobel Prize illustrated presentation: http://nobelprize.org/nobel_prizes/medicine/laurea tes/2004/illpres/
(see also the Nobel Lectures therein)
- Unpredictability of smell: Sell, C. S. Angew. Chem., Int. Ed. 2006, 45, 6254-6261.

I really think that the system of smell is already quite strongly explained by this theory, that also follows the classical binding+activation of receptors that drives traditional biochemistry and drug design.

I'm still surprised that some theoretical chemist/physicist didn't do QM calculations to prove the tunneling, and publish it in a leading peer-reviewed journal, if the theory is so sound...