The Mysterious Science of Smell
By LIDA TUNESI
The science of smell is still largely a mystery. In general, researchers think that the way we experience scent has to do with the shape of odorant molecules—say, the features of the molecules in a whiff of rose—but they also know that these shapes aren’t that straightforward.
A study published in Chemistry-A European Journal provides new insights. Professors Kevin Ryan and Mark Biscoe, along with Ph.D. alumni Min Ting Liu, Mihwa Na and Yadi Li (The Graduate Center, City College of New York), were authors on the study.
The authors performed an experiment with octanal, a common component of citrus oils. Like many molecules, octanal can take on a variety of conformations—it doesn’t have just one shape. (Conformation is a term in chemistry that refers to the way in which atoms in a molecule can rotate their bonds to create a new spatial arrangement.) These results suggest, however, that each of octanal’s shapes play a role in the way it smells.
Odorant molecules bind to receptors in our noses, and receptors send signals to our brains that tell us what we’re smelling. But many molecules are not rigid, and change their shape by rotating the bonds that connect their constituent atoms: Imagine taking an uppercase E and rotating the top third so that one bar sticks out to the left while two stick out to the right. This rotation makes it hard to say which conformation is responsible for a molecule’s scent.
The researchers tackled this problem by making three different versions of the octanal molecule. Each version looked like one of octanal’s conformations, but locked into place and unable to rotate. They then tested each of the three molecules’ interactions with a rodent olfactory receptor on genetically modified neurons.
One octanal variant activated the receptor to a large degree, one caused less activation, and another prompted no activation at all. This shows that receptors do respond differently to a molecule’s different conformations. In fact, the authors think that the message we receive in our brains when we smell an odorant is likely a time-averaged response to each of the molecule’s shapes. Knowing that one conformation causes more activation than the others could be useful for fragrance designers, the authors say, as using the locked-in version of shape could create a stronger scent.