Making sense of a smelly theory

Stink: Recent findings are casting doubt on a popular but disputed explanation behind what causes odor.

Medicine & Science

March 29, 2004|By Michael Stroh | Michael Stroh,SUN STAFF

The sugary tang of dew-moist mulch. Steak spitting atop flaring coals. The laundry-fresh fragrance of a breezy April afternoon.

Springtime aromas are easy enough to name - but almost impossible for scientists to explain. Why do certain substances smell the way they do? Long after researchers have unraveled many of the central mysteries of vision and the other four senses, they continue to be stumped by smell.

Now scientists at Rockefeller University in New York have taken a small step toward solving the puzzle - not by determining what causes odor, but by showing what doesn't.

In the April issue of Nature Neuroscience, scientists have for the first time cast doubt on a provocative theory of odor that has caused a major stink among olfactory researchers in recent years.

Most scientists subscribe to the so-called shape theory of odor: A substance's smell is directly related to the shape of the molecules that compose it. But even supporters acknowledge shape theory isn't perfect.

One problem is that scientists know of only 350 smell-sensitive receptors in the nose, yet humans are capable of discerning thousands of odors.

More problematic is that perfume makers and others who sculpt scents for a living are unable to predict what something will smell like from its chemical structure.

Enter biophysicist Luca Turin. Drawing on the knowledge that all molecules vibrate at particular frequencies, Turin argues it's not the shape of a molecule but how it quivers that determines its odor.

Since he first proposed his vibration theory several years ago, Turin and his ideas have drawn an unusual amount of attention: The scientist was the subject of both a recent British Broadcasting Corp. documentary and a 2002 book by Chandler Burr, The Emperor of Scent, issued last month in paperback (Random House).

Other scientists haven't been so enthralled. Many panned Turin's theory, but none of his critics bothered to test it.

Until now. Leslie Vosshall and Andreas Keller at Rockefeller recruited nearly two dozen volunteers and subjected them to a battery of sniff tests.

In one, researchers mixed a smoky-smelling chemical called guaiacol with benzaldehyde, which reeks of bitter almonds. Vibration theory predicts the resulting concoction should smell like vanilla, the substance whose molecular vibration is closest to the sum of two component chemicals.

But after giving the mixture a good whiff, none of the volunteers detected even a hint of vanilla.

In another test, Rockefeller scientists asked subjects to smell acetophenone, known for its sweet, pungent orange blossom odor, and deuterated acetophenone, which has a nearly identical molecular shape but markedly different molecular vibration.

If vibration theory were true, the scientists reasoned, the substances should also have markedly different odors.

The volunteers couldn't tell the difference.

The Rockefeller researchers caution that their findings aren't a complete defeat for Turin's idea. "We didn't disprove the vibration theory," Vosshall says. "We just didn't find anything to support it."

So, for the time being, a dab of Mystere behind a woman's ear will remain just that.

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