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."

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:Been rooting for this guy!

[Otter]

Here's a good discussion [corante.com] 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.

Re:Raised eyebrows...

[Vreejack]

The article is about olfactory receptors, not neurons. All the interactions described here are taking place where the external part of the olfactory receptor meets passing molecules. The actual news here is that the olfactory receptors might actually be capable of detecting quantum-level effects, unlike brain neurons which lack anything near the sensitivity required for that.

Re:Quantum Chemistry

[diqrtvpe]

Now, IANAQP, but I am a Physics student, and I have had reasonable experience with quantum tunneling. From what I've learned, quantum tunneling is most easily described in terms of electons hopping across barriers. The electron has a non-zero probability of being found outside the potential well created by its parent atom/molecule, and (skipping over most of the science and math) this means that there will be a non-zero rate of tunneling from that well to the other wells nearby. Now, in many cases that rate is infinitessimally small, but in a case like this it would be conceivable that the rate could go up to something non-trivial. The molecules would have to get pretty darn close, but if they're bound then that solves the problem. If this were the actual paper, instead of a popular article, you would certainly expect to see a whole lot of nigh-incomprehensible gibberish that explained what exactly they thought was going on. As this was written for a less specialized audience, they simplified it using, as far as I know, one of the standard ways of describing what we think is actually going on.

Re:Raised eyebrows...

[Mr. Underbridge]

The notion that things with similar structures having different smells - well, things with different structures often have different chemistries. Often a slight change in structure has significant effect on shape, size, polarity, electronegativity, etc, and these things can have enormous impacts on the ability of an odorant to fit correctly with a G-coupled protein receptor, which are the proteins responsible for olfaction.

The notion that things with different structures smelling the same is irrelevant - it's been shown that a similar *perception* can be caused by a very different combination of actual receptor activations. The conclusion there, not surprisingly, is that perception owes more to the backend processing done in the nasal epithelium and the brain *after* the signals are sent downstream from the receptors.

I'm not saying it plays no role at all, but it's danged questionable. The only evidence at all is the isotopic effect, but there may be other alternative effects going on, including something as mundane as the difference in vapor pressure. The olfactory sensors I worked on could distinguish H20 fromD20, and they most certainly did NOT work on a principle of electronic tunnelling. Sometimes when people hear hoofbeats, they assume camels and zebras.

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:Penrose and Quantum Consciousness

[chreekat]

Sure, I'll take a swing at it (my credentials are shaky -- a BS in computational physics). This theory says that tunneling, a quantum mechanical process, lets an electron jump into the nervous system. That's equivalent to saying that a quantum mechanical process causes an electric current... something the nervous system uses extensively. I don't know if a single electron would be enough to trigger a signal, but two possibilities for the theory are (1) it *is* enough, (2) more than one electron tunnels.

Please excuse my undergraduate hand-waving. ;)

Re:Quantum Chemistry

[me_mi_mo]

You keep repeating that things like photons, electrons and the like are "merely models". I have to take issue with this, as they happen to be effective models.

I would *love* to see how you would *begin* to explain how light and matter interact at a *fundamental* level, without using the concept of electrons and photons.

These guys are not cranks - the (free, as in beer) preprint [arxiv.org] seems to be a pretty typical quantum transport paper, albeit with a slightly "sexed up" angle.

Models are good, if they work.

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!

[linuxscrub]

There was a book written on this guy, about 4 years ago:

The Emperor of Scent: A True Story of Perfume and Obsession by Chandler Burr

While not a technical book, it does cover the mass-spectrometer-in-your-nose thing at some level. It's a good read, as it covers the guy, his idea, the fairly radical nature of the idea, and it's fairly small effect thus far (up to the point the book was written).

ls

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/ [nobelprize.org]
(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...