Lily of the valley fragrance: Electronic surface
structure determines interactions with scent receptors.
The scent of lily of the valley hangs in the air
for readers of the journal Angewandte Chemie: just rub the journal's
cover and enjoy a lily-of-the-valley scent. Lily-of-the-valley scent
components can also be found within the pages of the journal: an
interdisciplinary team headed by Reinhold Tacke (Inorganic Chemistry,
University of Würzburg), Philip Kraft (Scent Research, Givaudan
Schweiz Inc.), and Hanns Hatt (Cell Biology, University of Bochum)
have attempted a "scent prediction" to test their computer model of
lily-of-the-valley fragrance receptor hOR17-4. This molecule was
characterized in detail as the first human scent receptor by Hatt and
his co-workers, who also discovered it in sperm.
A fragrance is usually composed of a mixture of
many different scented substances. Each of these individual substances
can react with several of the approximately 347 scent receptors in our
nose and show a complex scent. "When developing new fragrances, we
have to rely on correlations between structure and effect derived from
model substances as well as intuition," says Kraft. With the help of
sperm, however, it is possible to study the lily-of-the-valley
receptor in virtual isolation and simulate the primary process of
scent sensing by computer.
An olfactory receptor responds to a scent molecule
when it fits into the receptor's binding cavity. If the structure of
the cavity is known, it should be possible to use computer models to
predict whether a scent activates the receptor in question and to what
degree. To prove this theory, the scientists investigated how the
replacement of one carbon atom with a silicon atom affects the scent
of lily-of-the-valley fragrance components lilial and bourgeonal, and
whether this subtle alteration, which has minimal influence on the
molecular shape, can also be predicted quantitatively. The human nose
was indeed fooled. Tacke says, "All four of the synthesized compounds
had the typical floral aldehydic lily-of-the-valley fragrance, but
didn't smell completely identical." However, near their threshold
levels, it was no longer possible to tell these scents apart. "Only
the most sensitive lily-of-the-valley receptor is activated at these
concentrations," explains Hatt.
On the basis of calculated binding energies, the
team had made a prediction of the scent intensities as well as the
sensitivity of sperm to the test molecules. These predictions
corresponded very closely to the experimentally observed results. As
expected, the odor thresholds were significantly higher for the
silicon analogues than for lilial and bourgeonal.
"Our computer calculations are exclusively based on
the surface shape of the scent molecules, which is defined by their
electrons," explain the researchers. "These results thus unambiguously
prove that it is this electronic surface structure of a molecule that
determines the interaction between a scent molecule and its olfactory
receptors—and thus defines its fragrance."
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