How does the sense of smell work? Today two competing camps of scientists are at war over this very question. And the more controversial theory has just received important new experimental confirmation.
At issue is whether our noses use delicate quantum mechanisms for sensing the vibrations of odor molecules (aka odorants). Does the nose, in other words, read off the chemical makeup of a mystery odorant—say, a waft of perfume or the aroma of wilted lettuce—by “ringing” it like a bell? Chemistry and forensics labs do this all the time with spectrometers—machines that bounce infrared light off mystery materials to reveal the telltale vibrations that the light provokes. Olfaction might, according to the vibration theory of smell, do the same using tiny currents of electrons instead of infrared photons (see previous coverage of the vibration theory here).
The predominant theory of smell today says: No way. The millions of different odorants in the world are a little more like puzzle pieces, it suggests. And our noses contain scores of different kinds of receptors that each prefer to bind with specific types of pieces. So a receptor that is set to bind to a molecule called limonene sends a signal to our brains when it finds that compound, and that's one of the cues behind the smell of citrus. Likewise that same receptor wouldn't bind to hydrogen sulfide—which smells of rotten eggs.
So, the promoters of the standard theory say, the familiar chemical interactions between receptor and odorant are all that's needed to explain olfaction. No fancy quantum vibration theory is necessary.
Yet here's a twist: odorant molecules typically contain many hydrogen atoms. And hydrogen comes in multiple forms, each very chemically similar to the others. But those different isotopes of hydrogen do strongly affect how a molecule vibrates. So deuterium, containing a hydrogen nucleus that has both a proton and a neutron (as opposed to plain-old-hydrogen that has just a proton), might help scientists discriminate between the proposed vibration and standard chemical binding theories of olfaction.
According to new research published today in PLoS ONE, human noses can sniff out the presence of at least some kinds of deuterium. Specifically, experimenters found regular musk molecules smelled different from ones that contain deuterium. "Deuterated" musks, says researcher Luca Turin of the Alexander Fleming Biomedical Sciences Research Center in Greece, lose much of their musky odor and instead contain overtones of burnt candle wax.
The finding represents a victory for the vibration theory, Turin says. And, he adds, it makes some sense, when you consider the purpose of our olfactory ability—whatever its mechanism is. The natural world contains millions of types of molecules. Some are good for us, and some are bad. The nose helps to distinguish one from the other. "Olfaction is trying to be like an analytical chemist," Turin says. "It's trying to identify unknowns." Chemists identify unknowns using spectrometers. Olfactory receptors, according to the vibration theory, act like little wetware spectrometers.
Adding to Turin's quiver is a 2011 finding in Proceedings of the National Academy of Sciences indicating that drosophila flies, too, can smell the difference between a molecule called acetophenone (which to humans smells sweet) and its deuterated cousin.
That’s all well and good, says Eric Block, professor of chemistry at the University at Albany in New York State. But, he says, it hardly proves the vibration theory, which faces some contrary evidence. For one, he points out that Turin once claimed humans, like drosophilia, could sniff out a deuterated version of the molecule acetophenone from the regular stuff, yet in 2004 Nature Neuroscience published a contrary claim, that human noses can't smell the presence of deuterium in acetophenone (Scientific American is part of Nature Publishing Group). And, Turin himself says in his new paper that he has confirmed the negative 2004 finding, although he thinks he has an explanation for the failure: deuterated acetophenone has relatively few deuteriums in it and thus may generate a weak vibrational signal that is too weak for humans to detect. Block says Turin can't have it both ways: either noses can smell deuterium or they can't.
Meanwhile, smell biologist Tim Jacob of Cardiff University in Wales, says that rotten egg smell is a good example of the vibration theory's appeal. Sulfur is a chemical hallmark of rotting organic material—something that is dangerous for us to eat. And molecules containing sulfur almost always smell horrible to us, he says—just as should be the case if evolution worked properly to favor our survival.
But there's no single shape or simple chemical property that sulfur universally confers to every kind of odorant molecule. On the other hand, sulfur does add signature vibrations to a molecule that a molecular vibration–sensitive nose might detect. "I do all my research without needing to know which model most accurately describes what's going on," Jacob says. But, he says of the vibration theory, "from a biological point of view it has great interest."
And that keeps fans of this fight watching and wondering: Which side will ultimately score the knockout punch? And who will need the smelling salts?