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October 1, 2009

"Designer Genes" By Luca Turin

"Designer Genes" By Luca Turin

The Second Law of thermodynamics stipulates that whenever you see your cup of café crème spontaneously separate into cream and black coffee, you must buy a lottery ticket. A more subtle omen of the End of Days for those who do not stare at coffee endlessly: you spray a fragrance on your wrist; it starts with a muted patchouli-sandalwood accord, gradually gets more floral as time goes on and wakes you up with a start at 2AM with a cymbal crash of citrus. Every perfumer and fragrance chemist I’ve met dreams of such an Apocalypse Eau Fraîche working in inverse time.

One day, when we really understand olfaction, we will find out whether it is even possible. My guess is that a top note musk may be forever beyond reach. However, as CTO of a molecule design company, I once made an attempt at a drydown citrus which worked wonderfully for a time but died a dramatic death a few months later.

Here’s what happened. What makes a molecule last long is mostly its weight. Adding a carbon (weight 12) usually doubles the time a molecule stays on your skin. Adding three would slow things down eightfold. What makes the problem interesting is that adding even one carbon usually changes the odor completely, never mind three. I and others had noticed, however, that replacing a double bond between carbons with a little carbon triangle called a cyclopropane leaves the odor reasonably unchanged. Armed with this idea and some clever methods for calculating odor, I tried to see whether other triangular arrangements would do better.

A triangle of two carbons and oxygen, while stable and easy to make, gives everything a bright metallic smell. But to my surprise calculations said that replacing the oxygen with a sulfur (to give a triangle the chemists call a thiirane) would not change the smell and add fully 32 weight units, a touch short of three carbons.

Dihydromyrcenol seemed a suitable candidate: very popular dry citrus-lime topnote, sold by the ton, nice carbon-carbon double bond just begging to be messed with, stable as a rock otherwise. We made the thiirane. To my delight it smelled very close indeed to the original, but went on forever. I took it to a trade show and showed it to two perfumer colleagues working for one of the big five. They declared it to be pure magic. We made a bit more and decided to show it at a meeting with one the largest firms in the business.

The day came: ten perfumers, their boss and some technical staff sat around the table. I passed smelling strips. All but two of the perfumers loved it; the last two said it smelled of skunk, not lemon. What we had discovered was not just a long-lasting citrus, but a new genetic variation among humans: the ability to break open thiiranes into sulfides, the smell of skunk and rotten eggs. Still, eight out of ten perfumers were impressed.

Luca Turin works at the MIT; he lives in Boston.

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