''And I can see, hear, smell, touch, taste
And I've got one, two, three, four, five
Senses working overtime
Trying to take this all in''
Some of you might recognise the above lyrics as being from the wonderful song Senses Working Overtime by XTC. This was released in the early eighties, a musical decade which, in retrospect, looks increasingly good in comparison to those that have succeeded it. And of the five senses mentioned in the song, this month's column will concentrate on smell - for the simple reason that nobody quite knows how it works.
Perhaps I should qualify that last statement. There is a widely accepted theory that says that the shape of a molecule determines its smell; specific receptors in our noses bind molecules of a particular shape, and this then generates a signal which is interpreted by the brain as a specific smell. Therefore, molecules of different shapes should have different smells.
However, an alternative theory, expounded primarily by Luca Turin, a scientist from an institute in Greece, contends that it is not the shape of a molecule, but rather the way that it vibrates, which determines its smell. And a paper published in January by Dr Turin and co-workers elaborates further on this theory.
As with all good scientific theories, Dr Turin's is testable - in other words, experiments can be devised which can potentially give results at odds with the theory. Should such contrary results be obtained, the theory must then be either discarded or revised. In this case, Dr Turin's theory, very broadly speaking, predicts that certain rather specialised molecules having the same shape should smell different. And here's where the chemistry comes in.
Let's assume we have a sample of hydrogen (H) atoms. If we measured the mass of each atom in turn, we'd find that approximately 1 atom in every 6600 would be twice as heavy as the others. These heavier atoms are an isotope of hydrogen, called deuterium (D). In most respects, hydrogen atoms and deuterium atoms behave chemically similarly - they occupy the same volume and have the same ''shape'' - but the mass difference between the two has significant consequences where molecular vibrations are concerned. When two atoms are bonded together, the bond between them can vibrate, just like a spring, when it absorbs infrared radiation. The frequency at which it vibrates is determined primarily by the mass of the two atoms, and therefore changing the mass of one of the atoms should alter this vibrational frequency.
Hydrogen atoms are often found bonded to carbon (C) atoms in molecules, and the upshot of the above paragraph is that, while C-H bonds and C-D bonds should behave chemically similarly, they should have different vibrational frequencies. This means that if we prepare two molecules which are identical, except for the presence of H atoms in one and D atoms in the other, then the two molecules will occupy the same volume and have the same shape, but will exhibit different vibrational frequencies. Therefore, according to Dr Turin, such molecules should smell different, despite having the same shape. This should be relatively straightforward to test - or so you might think.
Dr Turin and co-workers' latest paper looked at two different molecules, one of which contained significantly more C-H bonds than the other. They prepared the C-D versions of both molecules and then used human subjects to analyse the smells of these. Interestingly, they found that the C-H and C-D versions of the smaller molecule smelled identical, whereas those of the heavier molecule smelled quite different. Despite this, they claimed the results were consistent with their vibrational theory, and that, as always, further work was required.
It is fascinating to realise that there are still questions regarding something as fundamental as our sense of smell. One day, perhaps sometime soon, those questions will undoubtedly be answered.
- Dr Blackman is an associate professor in the chemistry department at the University of Otago.