Rabu, 16 Februari 2011
Fly sniffs molecule's quantum vibrations
How does a nose generate the signals that the brain registers as smell? The conventional theory says it's down to the different shapes of smelly molecules. But fruit flies have now distinguished between two molecules with identical shapes, providing the first experimental evidence to support a controversial theory that the sense of smell can operate by detecting molecular vibrations.
The noses of mammals, and the antennae of flies, are lined with different folded proteins that form pocket-shaped "receptors". It has been generally assumed that a smell arises when an odour molecule slides into a receptor like a key in a lock, altering the receptor's shape and triggering a cascade of chemical events that eventually reach the brain. But this "shape" theory has limitations. For one, it can't easily explain why different molecules can have very similar smells.
In 1996, Luca Turin, a biophysicist now at the Massachusetts Institute of Technology, proposed a solution. He revived a theory that the way a molecule vibrates can dictate it odour, and came up with a mechanism to explain how this might work.
His idea was that electrons might only be able to pass across a receptor if it was bound to a molecule that vibrated at just the right frequency. Ordinarily, the energy needed for the electron to make this journey would be too great, but the right vibrational energy could prompt a quantum effect in which the electron "tunnels" through this energy barrier, and this would then be detected and registered as a particular smell (see diagram).
If this is correct, animals should be able to distinguish between molecules of the same shape but with bonds that vibrate at different frequencies. That is the case for chemicals in which atoms of deuterium – a hydrogen isotope whose nucleus contains a neutron as well as the normal proton – replace ordinary hydrogen atoms. The extra neutrons don't change the molecule's shape, but they double the mass of the hydrogen atoms and so alter the frequencies at which the molecule vibrates.
Past tests on humans failed to turn up strong evidence that people can distinguish normal odour molecules from their "deuterated" counterparts. But now Turin has teamed up with Efthimios Skoulakis of the Alexander Fleming Biomedical Sciences Research Center in Vari, Greece, to test the idea on fruit flies, which can easily be trained to recognise different odours.
Their team initially placed fruit flies in a simple maze that let them choose between two arms, one containing a fragrant chemical such as acetophenone, a common perfume ingredient, the other containing a deuterated version. If the flies were sensing odours using shape alone, they should not be able to tell the difference between the two. In fact, the researchers found that flies preferred ordinary acetophenone. They also showed a preference for ordinary versions of octanol and benzaldehyde over deuterated versions.
The team also found they could use mild electric shocks to either reinforce or reverse this preference for non-deuterated molecules in general. This suggests the flies may be able to sense the vibrations characteristic of the bonds linking deuterium to carbon atoms.
"At the outset, I thought this could never work," Skoulakis says. "During the course of the experiment we convinced ourselves."
Turin sees the results as a "vindication" of his theory, at least in flies. "My theory was described as impossible physically, implausible biologically, not supported by evidence," he says. "This is a clear indication that some component of fruit fly olfaction is sensing vibrations."
The experiment "really supports this idea that fruit flies have the ability to be quantum detectors", says Gregg Roman of the University of Houston in Texas, whose lab just started studying isotope detection in fruit flies.
How large a role molecular vibration sensing plays is unclear. Leslie Vosshall of Rockefeller University in New York City agrees that the experiment suggests fruit flies can distinguish one isotope from another but says the assumption that this is due to vibrations is an "over-interpretation".
Turin's original tunnelling idea was based on a type of odour receptor in humans that fruit flies don't appear to have. "The logic of using the fly to test the vibration theory escapes me," she says.
Turin and Skoulakis are now planning genetic studies that might help pinpoint the amino acids on receptors that play a key role in isotope detection. This could help piece together a specific tunnelling mechanism for flies.
Could humans also differentiate between isotopes? In 2004, Vosshall and Andreas Keller found that people could not distinguish between acetophenone and its deuterated cousin. But Skoulakis says flies might be more sensitive to the effects of quantum vibrations. He says that giving mild shocks to humans, which wasn't done in the previous experiment, may help their brains pick up on differences.
Experiments are planned in another type of mammal. Several years ago John Sagebiel of the University of Nevada, Reno and Mary Cablk of the Desert Research Institute in Reno, Nevada, found that their pet dog, an Australian shepherd, seemed to be able to tell apart ordinary acetophenone and a deuterated version. They are now applying for funding to see if these informal results hold up in other dogs.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1012293108