The science of smell

Luca Turin on the vibration theory of scent

Analyzing the science of scent has long presented a challenge to researchers. In the exploration of the olfactory system, the debate comes down to two contending theories: shape and vibration.

Traditionally, the shape theory has proposed a “lock and key” system, in which the molecules of a given scent bind directly to nasal receptors. In contrast, the vibration theory presents the mechanism of scent as a function of the vibrational frequency of a molecule in the infrared range. According to the vibration theory, the scent molecule must be energetically compatible with the receptor that it binds to, ensuring that electrons can travel through the molecule via inelastic electron tunneling and trigger a signal transduction pathway.

I had the opportunity to speak with Luca Turin, a major theorist and promoter of the vibration concept. Turin is currently working at MIT on DARPA’s RealNose project, which aims to simulate the mammalian olfactory system and applies the vibration theory.

The Tech: How did you become interested in the science of scent?

Turin: I think everyone (scientist or not) at some point wonders how smell works. The Italian physicist Giorgio Careri told me that Enrico Fermi in his presence once sniffed the air while frying onions and said “wouldn’t it be nice to know how that works ?” I started reading up on smell and it gradually became clear to me that there were big gaps in our knowledge, so I started thinking about it.

TT: Your vibration theory of olfaction presents the process of smell as a function of the vibrational spectroscopic properties of molecules. What led you to investigate and propose this idea?

Turin: This is a very old idea first proposed in the late 1920’s by Malcom Dyson, an extraordinary man mostly remembered these days for having devised the IUPAC nomenclature of molecules almost singlehandedly. Dyson eventually lost interest and the idea was resurrected by the Canadian chemist and entomologist R.H. Wright in the ’60s. Clifton Meloan at Kansas State University did some crucial experiments in the ’80s. Then my turn came to push this wonderful idea along a little further. I initially thought it was plum crazy, but then I realized that although no classical mechanism could explain it, a quantum one might and I decided to work on it.

TT: What is the relationship between the human sense of smell, and that of other primates and mammals? Is the vibration theory consistent in many species?

Turin: Mammals and especially primates are so closely related that there is likely no difference at all. There is some good evidence that insects can smell vibrations too, so it may be fairly general.

TT: Your book, Perfumes: The Guide, is a fantastic guide to the science and components of many critically acclaimed perfumes: from refined classics to unusual fragrances. What sparked your interest in the art and chemistry of perfumes?

Turin: [Regarding art:] I love fragrance and find it easy to write and talk about it, and I found an ideal co-author in Tania Sanchez who is teching me to write English properly. [Regarding chemistry:] My theory allows the smell of a molecule to be predicted far better than before, and for nine years my day job was CTO of a fragrance discovery company called Flexitral.

TT: What do you feel is the most exciting implication that your work may have?

Turin: That other receptors may also work by something other than lock-and-key.

TT: What topics or ideas are you currently researching?

Turin: I am interested in devising experiments that could find the connection, if any, between quantum mechanics and neuroscience.