Luca Turin and the mischievous secret of scent

Tessa Evans muses about the time she thought she might become a perfume chemist, inspired by Luca Turin, the man who (may have) made sense of scent.

 
Illustration by Meg Gough-Brooks

Illustration by Meg Gough-Brooks

 

Model Specimens is a monthly column that explores the role models who inspired today's scientists. This month, our Physical Science Editor Tessa Evans shares how Luca Turin inspired her to become a perfume chemist, at least for a little while.

There was a time when I flirted with the idea of becoming a perfume chemist. Mixing vials of rose geranium, ambergris and lily of the valley seemed an occupation that exuded a certain elegance.

Now, I’ve never been someone who could ever be described as ‘elegant,’ and the grubby marigold, mandarin and violet concoctions I brewed under the house and stored in glass jars when I was seven could never be worn in an attempt to woo a lover. Simply steeping flowers in water does not perfume make.

The interest, however, came much later. Well into my chemistry degree, my parents gave me a book called The Emperor of Scent, which was what prompted me to muse about such an illustrious career path. The book is by a journalist, Chandler Burr, who had become entranced with an erratic, eccentric, discipline-crossing scientist by the name of Luca Turin.

Originally a biophysicist by trade, Turin always had an affinity for smells, which led his work to traverse across biology and chemistry and back again.

Familiarity with too many trivial perfumes can at length turn one into a libertine of smell: cynical, unmoved, cruelly practical. But every old roué hides at his core an unblemished memory of the boy he once was, ready to fall for the first creature whose intelligence revives his freeze-dried heart. I felt something give way when I smelled 1740: the shimmering classical accord of leather, immortelle, spice, rich pipe tobacco, and a sort of lived-in buttery warmth is simply irresistible. And why resist?

— Luca Turin describing 1740 by Histoire de Parfums in The Little Book of Perfumes

Turin describes scents in the same manner Justin Schmidt describes the painful stings of insects in his pain scale — though Turin’s are more sass to Schmidt’s severity. In his book The Little Book of Perfumes: The Hundred Classics, he describes Coty's 1917 perfume Le Chypre de Coty with this: “Fragrances before Chypre had flesh; Coty invented bones … Smelling it unadorned is like watching an actress take her makeup off while eyeing you in the mirror, slipping some jeans on, putting her arm in yours, and offering to set out into the night unrecognized, paler, and more plainly beautiful than ever.”

Through his obsession with perfume, Turin popularised a somewhat odd way to think about how we smell (based on earlier work by Malcolm Dyson and Robert H. Wright), going up against the scientific status quo, and – well, losing.

As far as our senses go, we still don’t know exactly how scent works — or more precisely, the mechanism of how our nose interprets smell. The main theory of olfaction (smelling) is based on a lock and key mechanism, and assumes that the 3D shape of a molecule determines what it will smell like. It is the shape of molecules (keys) that bind to receptors (locks) in our noses — through intermolecular forces — which cause the receptor to change the molecule's structure (unlock) and transmit the response to our brain. This kind of action is common in biochemistry.

The most expensive perfume in the world - Amouage Gold by Amouage. In his book with Tania Sanchez, The Little Book of Perfumes, Turin notes Amouage Gold is the product of famous perfumer Guy Roberts being told by the Omanian royal family which finances Amouage: "Put whatever you like in it, no matter how much it costs." Turin describes it as being "all about texture rather than structure, a hundred flying carpets of scent overlapping each other. It’s as if Joy had eloped with Scheherazade for a thousand and one nights of illicit fun.” yosoynuts/Flickr (CC BY-NC-SA 2.0)

The most expensive perfume in the world - Amouage Gold by Amouage. In his book with Tania Sanchez, The Little Book of Perfumes, Turin notes Amouage Gold is the product of famous perfumer Guy Roberts being told by the Omanian royal family which finances Amouage: "Put whatever you like in it, no matter how much it costs." Turin describes it as being "all about texture rather than structure, a hundred flying carpets of scent overlapping each other. It’s as if Joy had eloped with Scheherazade for a thousand and one nights of illicit fun.” yosoynuts/Flickr (CC BY-NC-SA 2.0)

Only there are a number of holes with this theory. We don't have evidence that the receptors actually change the shape of the molecules when they bind, or know how this would translate to a signal from the receptor to the olfactory bulbs behind our noses. It also can’t explain why molecules which have completely different structures can smell the same (there are huge numbers of differently shaped molecules which smell ‘musky’), or how molecules that have very similar shapes can smell completely different. Those working in the field know of these flaws, but it’s believed to be the most accurate based on experimental evidence.

Turin’s theory about how olfaction works is based on molecular vibration. Each of the bonds between two atoms in a molecule vibrates in a certain way, which is easily measured with infrared light. (Give me, or any chemist, an IR spectrum (graph), and we could find out what kinds of bonds exist in the molecule measured.) Cumulatively, Turin thinks the vibration of a molecule is like a symphony, with the sum of a molecule's vibrations creating the smell.

Like in the shape theory, the smell molecule must bind to receptors in our nose, with the intensity of smell dependent on how long the molecule is bound. His mechanism, of how this works is where things differ. His theory states that an electron can travel between the molecule and receptor using inelastic tunneling, but only when the conditions are right. This means there would be several different receptors tuned to different vibrational energies, with tunneling only occuring when a molecule with the right vibrational energy is present. The movement of this electron then carries the signal along the pathway to your brain, telling you what the smell is. Turin believes that with this in mind, you should be able to design custom-made scents using vibrations to design molecules.

“Maurice Roucel has a knack for putting together perfumes that feel haunted by the ghostly presence of a woman… It did constrain the woman inside Envy to be at once seraphic and suburban, complete with the sort of suppressed anger that such a creature would feel at being reincarnated as a florist in New Jersey.”
— Luca Turin describing Envy by Gucci in The Little Book of Perfumes

Shifts in knowledge require huge leaps of faith. To say that Turin’s theory was not well-received would be to put it lightly. Too enmeshed in their own beliefs, the ‘shapists,’ as they are called in Burr’s book, prevent the little guy (Turin) when he tries to publish in Nature.

At least, that’s what Burr’s book would have you believe.

This formed my view of the scientific world for a while. The injustice of the staunchly conservative science heavyweights stamping out fresh revolutionary ideas. I was, after all, 21 when I read the book.

Perfumes have always been made with a mix of the natural and synthetic. Now, chemists search for new molecules with exciting smells in laboratories, much like the Fragonard Perfume factory, in Grasse, France. John Rohan/Flickr (CC BY-NC 2.0)

Perfumes have always been made with a mix of the natural and synthetic. Now, chemists search for new molecules with exciting smells in laboratories, much like the Fragonard Perfume factory, in Grasse, France. John Rohan/Flickr (CC BY-NC 2.0)

After his theory had eventually been published, Turin enjoyed a stint at MIT. While there, a paper came out proving that people could tell the difference in smell of deuterated and non-deuterated compounds, one of the cornerstones of Turin’s theory. Turin states that normal (hydrogen) and deuterated (where all the hydrogens on a molecule are replaced with hydrogen’s isotope, deuterium) versions of molecules should smell different, as despite being the same shape and identical in every other chemical way, they will have slightly different vibrational frequencies. The paper proved Turin’s theory was right, at least for the bitter-almond scented benzaldehyde.

Fed up with Turin’s sensationalist theory, ‘shapist’ neurobiologists Andreas Keller and Leslie Vosshall recreated the experiment with a different molecule, acetophenone, and "found no evidence to support" the theory. They published their results in Nature Neuroscience, and an editorial accompanying the paper said the only reason the study was done was because of the “extraordinary—and inappropriate—degree of publicity that the theory has received from uncritical journalists.”

The editorial goes on to say that “despite the forcefulness of his assertions, most scientists in the field were unconvinced by his proposal. Thus his paper was rejected by Nature, and it was eventually published (without review, according to Turin's own account) by Chemical Senses in 1996.”
 

“There is so much comfort in unrepentant sweetness that ambery oriental fragrances often tumble into the honey pot headfirst and die a slow, sticky death."

— Luca Turin describing Emeraude by Coty (1921) recreated by the Osmothèque in The Little Book of Perfumes

One of the things that makes understanding this mechanism so hard is that evidence of what something smells like is inherently anecdotal. You can't quantify, or really qualify someone's perception of a smell. And, being the new, revolutionary theory on the block, the vibrational theory of smell not only has to prove that it is the vibration of molecules which allows us to smell, but also that it isn’t their shape.

Tessa's copies of Luca Turin's books. Note that one is a little worse for wear after being dropped in the bath. Author provided

Tessa's copies of Luca Turin's books. Note that one is a little worse for wear after being dropped in the bath. Author provided

Now I'm more strongly in the evidenced-based science camp. Ideas are great, wonderful even, but you have to be able to back them up. For this theory, there still isn't a whole lot of evidence. It’s sort of odd to think that one of my scientific heroes might not be all that scientific. In spite of this, he has taught me a lot about science: about not all science being cut and dry, about the publishing world, and about belief in your ideas.

A part of me really wants it to be true. When you read his (or Chandler Burr’s) book, it’s easy to get swept up into the vibrational theory of scent. As a chemist, I can see that his steps are logical and he speaks about techniques and machines I have used. Sure, he throws some complicated physics into the mix (which has been found to be legit, or at least consistent with known physics), but he likens olfaction to the way hearing and vision work, so it all seems plausible.

That isn’t to say that it’s definitely not true. It still might be. There have been a number of papers back and forth from both sides, with Turin more recently finding the reverse of Keller and Vosshall: humans could tell the difference between deuterated and non-deuterated acetophenone. And a new angle of attack studying the receptors started long arguments back and forth in letters to PNAS.

“The enormous artistic edge that chemistry gives perfumers is the ability, familiar to the gods of Olympus and to fairy godmother when putting together a titanically gifted baby, to compose a personage from disparate inherited virtues: the rosy, grassy freshness of lily-of-the-valley, the rasp of the lily proper, the mushroom note of gardenia, the lemon of magnolia, the banana of ylangylang, the deep woody velvet of violets, the boozy sweetness of rose, the soapy edge of cyclamens, etc. Marshalling all these molecular genes into producing something viable, even beautiful, is far from easy… Not sure about Beyond, but definitely Paradise.”  
— Luca Turin describing Beyond Paradise by Estee Lauder in The Little Book of Perfumes

The world of perfume is where Turin excels. He may or may not have cracked the secret of scent, but he sure knows a lot about perfumes. In his book The Secret of Scent, he explains the difficulties of using natural products to make perfumes, illustrating why my homemade attempts were never any good: “In a way, the very existence of perfumery is due to the raw materials being such poor replicas of the real thing. If rose oil really smelled of roses, the perfumer would merely hang her head in shame and give up. Instead, her task is to mix these gnarled, cooked, mangled bits of dried-up living things, and, much like an embalmer, give them the bloom of life once again.”

My interest in perfume chemistry slowly died, and I became obsessed with writing instead. Occasionally I do still think about learning the alchemical art of combining different molecules in an attempt to create some stunning symphony of scent. But maybe I’m better off. If I did, I’d have to take sides in the shape vs vibration debate, and I don’t think I’m ready for that.

Edited by Nicola McCaskill