Quantum mechanics stinks
Our sense of smell has been in the news recently, with some new research published into how our noses detect different substances’ odours. Our other senses – sight, touch, taste and hearing – are well understood, but the mechanism of smell is still a mystery.
Molecules all possess their own shapes: the more atoms in the molecule, the more complex the shapes can become. There are countless different possible shapes of molecules and it has generally been thought that these shapes dictate what substances smell of. We have a patch of receptors in our noses that match certain molecular shapes. When the right shaped molecule fits into the right receptor, like a key in a lock, a signal is sent to the brain to tell us what we are smelling.
However, Dr Luca Turin has published work on a more controversial theory for the mechanism of smell. He takes a different approach, using the vibrations of chemical bonds and quantum effects to describe how our smell receptors detect different molecules.
To test his theory, he took a simple molecule and swapped all the hydrogen atoms for deuterium. This maintained the shape of the molecule, but changes how its chemical bonds vibrate, because deuterium is heavier than hydrogen. Turin was able to show that people could tell the difference between the odours of the molecule containing hydrogen and the molecule containing deuterium, which he believes is strong evidence that the classic ‘lock and key’ theory could be flawed.
Dr Jennifer Brookes is a Sir Henry Wellcome Fellow working on smell detection at University College London and Harvard University. She also works on the quantum mechanics of how we smell, though using theoretical data rather than experimental results. Her theories back up Turin’s work, challenging the ‘lock and key’ model.
For example, sulphurous compounds often have very strong smells, one of the more unpleasant being hydrogen sulphide, which smells of rotten egg. Hydrogen sulphide is a tiny molecule consisting of a central sulphur atom, with two hydrogen atoms sticking out from it in a V-shape. However, it is not the only compound that gives this distinctive smell. Decaborane smells very similar, but it has a basket-like shape of ten boron atoms, completely different to hydrogen sulphide’s shape. The ‘lock and key’ model would struggle to explain this, but the two molecules do vibrate at a similar energy, fitting with the quantum theory proposed by Turin.
Furthermore, Brookes describes two very similarly shaped compounds, ferrocene and nickelocene, which both have a central metal atom (iron in ferrocene and nickel in nickelocene), and identical structures surrounding them. Their shapes and sizes are indistinguishable but ferrocene smells spicy while nickelocene smells oily. Because the central metal atom is different, their vibrations are also different, again fitting with Turin’s theory.
Brookes doesn’t dismiss that shape is important: the molecules need to fit in to the receptor in order for the vibrations to be read, much like a swipe-card: the card must be the right shape to fit into the card reader, but it is the information taken from the card that is important.
Although these examples give some evidence against the ‘lock and key’ model, there is still a lot of scepticism surrounding the ‘swipe-card’ theory. With the existing theory embedded in the scientific community, it is controversial and exciting when new research challenges it. Brookes hopes that now the ‘swipe-card’ model has had some positive press, other researchers in the field may start to look into it more. It may even make people start to question the mechanisms of other biological systems.
Brookes JC, Horsfield AP, & Stoneham AM (2012). The swipe card model of odorant recognition. Sensors (Basel, Switzerland), 12 (11), 15709-49 PMID: 23202229
Gane S, Georganakis D, Maniati K, Vamvakias M, Ragoussis N, Skoulakis EM, & Turin L (2013). Molecular vibration-sensing component in human olfaction. PloS One, 8 (1) PMID: 23372854