Smell that? It's the scent of mounting credibility for a controversial theory of smell that puts molecular vibrations front and center. Physicists have now analyzed the proposed mechanism and deemed it plausible.
The new calculations by no means prove the theory, but they give it added legitimacy, says biophysicist and perfumer Luca Turin, who developed the idea. "Most people would probably feel that if it can be done at all, evolution has managed to make use of it."
The question: What property of an odor molecule (or odorant) do the receptors in our noses pick up? The reigning but still unproved explanation of smell supposes that the shape is the thing, with receptors fitting like a lock into the molecule's key. But the shape theory doesn't explain why some nearly identically shaped molecules smell vastly different, such as ethanol, which smells like vodka, and ethane thiol (rotten eggs).
Turin's more controversial theory, put forth in 1996 and now the subject of two popular books, holds instead that odorant receptors sense the way a molecule's atoms jiggle. The shape of the molecule still comes into play, Turin says, because it determines the odorant's overall vibrational frequency. But he didn't know how all the details fit together.
Physicist Marshall Stoneham and his colleagues at University College London report they have constructed a specific mechanism based on the properties of so-called G-protein coupled receptors, which project from olfactory cells inside the nose.
The researchers imagined that the odorant fits into a spot between a site that donates an electron and one that receives the electron. In this model, the receptor switches on when an electron hops from donor to acceptor. The group calculated that an electron could "tunnel" through the barrier imposed by the odorant, an effect made possible by quantum mechanics, they wrote in a preprint accepted for publication in Physical Review Letters.
Before it tunnels, the electron distorts the odorant molecule's electrical field. When it tunnels, it effectively disappears, causing that electrical field to wobble like a plucked string, Turin explains. Tunneling is likely to take place if the plucking matches the molecule's natural mode of vibration.
The effect is more like a "swipe card" than a lock and key, says Stoneham, who adds he was skeptical of the idea when he first met Turin a decade ago. "The way the numbers worked out it looks like a perfectly credible mechanism. The reason we're not 100 percent certain is we don't know the detailed structure of the receptors."
"This provides the first step" to devising better tests of Turin's theory, says biochemist Shuguang Zhang of M.I.T., whose lab is exploring the idea. "At least it shows the alternative theory is possible."
Turin says the strongest tests of his theory so far come from studies in which researchers replace an odorant atom with an isotope of that atom, which has a slightly different weight and changes the molecule's frequency of vibration. In animals his predictions hold up, but the evidence is mixed in people, he says.
A study published in 2004 in Nature Neuroscience, for example, found that people could not discriminate between two such odorants. An accompanying editorial remarked on "the extraordinary--and inappropriate--degree of publicity that the theory has received from uncritical journalists."
Of course it also concluded, "In some sense it does not matter whether the public believes in the vibrational theory of olfaction; the truth will eventually come out." The new model may hasten that day.