Saturday, September 22, 2012

Yet Another Whack at the Vibration Theory of Smell

University of Illinois physicist Klaus Schulten and colleagues have added another 11 pages to the pile of theoretical physics papers claiming that the vibration theory of smell is theoretically possible. In an amusing twist, Schulten talks trash about the other vibration theory fan-boys:
Those who suggested that molecular vibration played a role in odorant recognition in previous studies “didn’t know about [Rudolph] Marcus’ theory and they didn’t do quantum chemical calculations,” Schulten said. “They argued very much on principle (that it was possible). So we are saying now, yes, it is really possible even when you do the most complete and reliable calculations.”
Those idiots! They like so totally forgot the quantum chemical calculations.

Here at FirstNerve we don’t know a Huang-Rhys factor from a hybrid Becke-type three parameter exchange functional, but we sure as hell enjoy watching theoretical physicists get into a food fight.

For everyone else observing from the sidelines, here are three important take away points:

First, in advocating a “vibrationally assisted” mechanism of olfaction the authors concede that a molecule’s shape and size do matter. This is a big walkback from pure vibration theory.

Second, the authors conclude only that the new model is “energetically feasible,” not that it is biologically realistic.

Finally, since they acknowledge that “receptor structure presently is still unknown,” their conclusions are largely a matter of conjecture. Until there is some convincing biology, their 19 different equations are not worth the pixels they are printed in.

The study discussed here is “Vibrationally assisted electron transfer mechanism of olfaction: myth or reality?,” by Ilia A. Solov’yov, Po-Yao Chang, & Klaus Schulten, published online July 17, 2012 in Physical Chemistry Chemical Physics. It is available here.


Persolaise said...

Thanks for letting us know :-)

I'm no scientist, but I do find this whole debate fascinating. It's interesting that the current thinking seems to be moving in the direction of 'vibration AND shape'.

Avery Gilbert said...


Thanks for the encouragement. Glad to know that non-scientists are observing the walkback.

The “debate”, such as it is, exists almost entirely in the public mind. We can thank the breathless “Emperor” book for that. In scientific circles, there’s Turin and a chorus of physicists chanting “Yes, it’s possible, yes, it’s possible”. And then there is everybody else—from neurophysiologists to molecular biologists to behavioral neuroscientists—scratching their heads and saying “WTF?”

For an idea that is supposedly so shit-hot, remarkably few scientists see any implications worth exploring in their own work. That tells us something. (And it’s not “OMG they’re too threatened to acknowledge the genius of Turin.”)

Honestly, someone should write a comic novel about it.

EdC said...

I'm pretty sure everyone in the field knows that a molecule's size, shape and vibrational modes are all interdependent. Also charge distributions on the molecular surface, hydrogen bonding, mass and all other properties. In particular, electron tunneling is a sufficiently short range effect that a molecule will have to have the right shape for its vibrations to assist. I don't recall Turin ever arguing that shape was irrelevant.

That problem that "receptor structure presently is still unknown," seems like to big barrier to further work in shape or vibrational theories. Or even the Rube Goldberg weights and levers mechanism that just occurred to me :-)

Avery Gilbert said...


You've put your finger on it. Given that an odor molecule's size, shape, charge distribution, etc. are all at play in activating a receptor, then what exactly is left for vibration to do?

Not a helluva lot.

Leave a vibrationist alone with a Chevy V8 and he'll discover "The Camshaft Theory" of the internal combustion engine.

Anonymous said...

Shape...vibrational...but how those "theories" can explain that certain "molecules" can smell differently when concentration changes? There are examples around...
Alirio Rodrigues

Avery Gilbert said...

Alirio Rodrigues:

Excellent question. There are many examples of molecules whose odor changes character with concentration.

The standard view is this: there are roughly 400 different types of olfactory receptor in the human nose. Some respond to only a narrow range of odor molecules. Others are activated by a broad range of molecules.

At low concentrations the variable-character molecule activates one or a few types of olfactory receptor--i.e., only those most attuned to it.

At higher concentrations more of the less-specific receptors become activated. Because a larger suite of sensory neurons is sending signals to the olfactory bulb and brain, the resulting odor perception changes.

In a nutshell, higher odor concentration recruits a wider variety of olfactory receptors and thus changes the sensory signal to the brain. While this may be true to some extent for all odor molecules, it is presumably exaggerated in the case of some, and these are the concentration-dependent character shifters.