Dubbed 鈥渢he Emperor of Scent鈥�, biophysicist Luca Turin has stirred up a heated debate about the inner workings of our sense of smell. In the mid-1990s he revived the argument that the human nose detects the presence of a compound from its molecular vibrations rather than its shape, as most researchers believe. He has been busy piecing together evidence to support the vibrational theory, and his latest book, The Secret of Scent, chronicles his quest. Mick O鈥橦are talks with Turin about which smell convinced him that his theory is correct and what the future might hold for the world of perfumery.
Wasn鈥檛 the mystery of smell solved by the time Linda Buck and Richard Axel won the Nobel prize for physiology or medicine in 2004?
Their work was hugely important. Until they discovered human smell receptors the field was a free-for-all and people could come up with any bullshit idea. Their work put a constraint on acceptable theories of how we smell 鈥� we are now bound by the fact that the receptors exist. However, the mechanism of how the receptors detect and smell an odour molecule remains a mystery. Buck and others believe it is connected with the shapes of molecules, or specifically parts of their shapes. This theory says two molecules smell the same because they have bits of shape that the receptors perceive to be the same. But which bits? This explains things away rather than actually explaining. It gets you out of a pickle but you cannot use it to predict smells as you can with vibrational theory.
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Why is vibrational theory better than shape theory at explaining why molecules smell different?
Two examples. The first is, if you have an alcohol molecule containing an OH group [oxygen and hydrogen] and a thiol molecule of the same structure where the O is replaced by an S [a sulphur atom] to give an SH group, you might think that an olfactory system that depends on shape would become confused by these. They are very similar in shape 鈥� O and S are next to each other in the same column of the periodic table. Yet this is not the case. Pure OH ethanol smells like vodka and SH ethanethiol smells of rotten eggs. So why? Well, nothing else smells of rotten eggs like SH except for one thing 鈥� the boranes, a group of chemical compounds of boron and hydrogen. And borane has nothing in common with SH except for molecular vibration. They are the only two things in the world that smell of rotten eggs and they are the only two that have a molecular vibration of 2500 wave numbers. It鈥檚 too marvellous to be a coincidence.
The second example is when you take two molecules that are 鈥渆nantiomers鈥�, or the mirror image of each other. Most enantiomer pairs smell similar. But if smell recognition was based on shape, a specific receptor would be able to detect one enantiomer but not the other, because it would not fit. As far as the receptor is concerned the two enantiomers are as different as different can be. Yet they smell the same because they have something in common. And again, it鈥檚 their vibrational spectrum. Shape recognition cannot explain this.
Yet vibrational theory is still a minority view. Why do others disagree?
It鈥檚 unproven as yet. I鈥檓 not for a minute saying people should slap their foreheads and say 鈥渉e鈥檚 right!鈥�. What I鈥檓 saying is that shape simply does not explain how the receptors work. Every scientist has two jobs 鈥� demolition and construction. I鈥檝e done a good demolition job 鈥� people agree with that. Fragrance chemists have always agreed with me after two beers, which makes them honest. Demolition is OK, but what do you construct instead? Empirically the easiest test of vibrational theory would be if different isotopes of the same element smelled different. For example, if you replace hydrogen with heavy hydrogen, the molecular shape is identical but the vibration is different. So do isotopes smell different or not? There鈥檚 no consensus.
But in your experience do different isotopes smell different?
They do, but to be honest I don鈥檛 like the fact that they don鈥檛 smell as different as I鈥檇 expect. The effect is there, but it鈥檚 small. Dogs and insects can tell isotopes apart, but humans can only sometimes. Then again, humans have a poor sense of smell, although they referee scientific papers and insects don鈥檛.
Why do some scientists still hang onto shape as a mechanism for smell?
It鈥檚 a one-party state. If you don鈥檛 believe in vibration you are stuck with shape, but nobody can explain how it works. I understand why people won鈥檛 take me on faith. We have to wait for scientific proof. However, the practical work I have been doing with my company offers a striking demonstration that something is working 鈥� using my theories we have created successful molecule after successful molecule. I think people thought we got lucky after the first two, but now we are up to 20. And we have a 1 in 10 success rate of predicting smell 鈥� far better than the industry standard of 1 in 1000.
If proved, what significance will vibration theory have for the perfume industry and in general?
I hope it will make my company a lot of money. Then one day the big perfume companies will start doing it. What we do has a big impact on what they do. But perfumery is a B-list scientific endeavour. It鈥檚 frivolous. However, the broader question of how receptors work is very interesting. For example, in pharmacology, the dogma of shape is also a little shaky. The jury is still out over what is involved in drug/receptor reactions. The sort of thing I and others are proposing may be the tip of the iceberg: it could apply more broadly to drugs and similar fields.
How did you earn your 鈥淓mperor of Scent鈥� sobriquet, and is it a millstone?
I had no control over it. It was chosen by publisher Random House as the title for Chandler Burr鈥檚 book. It鈥檚 a joke, and I鈥檒l never live it down. In some ways Chandler鈥檚 book makes me seem like a prize arsehole. But I鈥檝e got used to it. It was a rather corrupt Napoleonic promotion. First I was nothing, then overnight I became an emperor.
What is your favourite perfume?
Mitsouko, the last survivor of the golden age of fragrance, created in 1919 by Jacques Guerlain in France. It鈥檚 based on a mixture of bergamot, oakmoss and labdanum created in 1917. Guerlain added a touch of synthetic peach. It is superb, it smells old-fashioned, like Brahms in a world of hip-hop.
鈥淢y favourite perfume smells like Brahms in a world of hip-hop鈥�
Perfumery鈥檚 wonderful attraction is the fact that you can write a chemical poem. You start out with things that don鈥檛 smell bad in the first place, such as rose essence or lavender, and by mixing them together you get something that is far more than the sum of its parts. It鈥檚 not a trivial skill. I have a friend, a genius perfumer who doesn鈥檛 take herself at all seriously, and I frequently tell her, 鈥淵ou鈥檙e one of the greatest composers of the 20th century.鈥�
Profile
Born in Lebanon in 1953 to Italian parents and raised in France, Luca Turin was a biophysics lecturer at University College London and worked for the CNRS, the French national research institute. He became interested in olfaction when he began collecting fragrances, and in 1992 wrote a guide to perfumes. In 2003 he was the subject of biographer Chandler Burr鈥檚 book The Emperor of Scent (Random House). Turin now works for US start-up company Flexitral producing new smell molecules. His latest book, The Secret of Scent (Faber & Faber), is on sale in the UK and was published in the US on 7 November.