Scientists have long been puzzled by how smells are triggered.
Why has a molecule a pungent smell, a related one being inodorous?
A team at the London Center for Nanotechnology (LCN) at University College London has reanalyzed an intriguing 10-year-old theory of smell and proved it to be more veridical than imagined.
Odorant molecules activate several types of receptors in our noses, but why nearly identical shaped molecules smell nothing alike? The puzzle exists because scientists do not know how the odorant molecules interact with the nasal receptors at atomic-scale.
The LCN physicists assumed that electrons in the receptor structures can be turned on to tunnel between energy states, if the odorant molecule’s vibration frequency fits the energy difference
of these states. When the LCN group checked this 10 years old hypothesis, it was found that a general model of this electron tunneling matches with physics principles as well as with known features of smell.
Quantum mechanical tunneling, often used in technology, is triggered when a particle passes through a barrier despite being forbidden by classical physics. Only small particles, like electrons, can do it because they behave waves. When an odorant molecule’s vibrations (phonons) trigger electrons in an olfactive receptor to tunnel between energy states, olfactive impulses are sent to the brain.
Different odorants have different frequencies, picked up by different receptors triggering different smells. “I personally was surprised that our answer seems so robust"we don't need to fudge or to take special helpful values of parameters,” said Marshall Stoneham, one of the researchers. “At the start, we could not have been sure at all. In fact, when I first heard of the ideas 10 years ago, I did not expect them to work out.”
The old chemical mechanism of smell was a “lock and key” model, with different shaped molecules assigned to different receptors, but the LCN team comes with a “swipe card” model. Perhaps a combination of these models occurs: an odorant molecule would be “read” by receptors that receive its vibration spectrum, along with matching its shape. “The major other theories of how receptors generate signals that are specific to certain molecules are all ones depending on molecular shape, mainly ‘lock and key’ mechanisms,” said Stoneham. “As we say, this popular model fails badly for these small scent molecules (similar molecules smell different, differently-shaped molecules smell the same, the actuation process is ill-defined).”
The non-mechanical actuation is physically plausible: the charge transfer rate corresponds to the observed time scale; the inelastic electron impulse is decipherable; and the molecule’s vibration spectrum appeared linked to its smell.
There is more to be found, but the swipe card model explains how selectivity acts for human (and mammalian) smell. “On the possibility of whether or not we might completely understand smell in the near future, there are many levels of understanding,” Stoneham explained.
Image credit: Marshall Stoneham, et al.
