Olfactory design: smell and spectroscopy
by Jonathan Sarfati
Our sense of smell is actually a complex system designed to detect thousands of
chemicals. It helps warn of us of danger, e.g. rotting food—we can sense one
component of rotten meat, ethyl mercaptan, at a concentration of 1/400,000,000th
of a milligram per litre of air.1
Smell also helps us distinguish types of foods and flowers. The sense of smell is
actually responsible for most of the different ‘tastes’ of foods. In
many animals, this sense is even more important than in humans—it helps bees
find nectar, for example.
The nose contains millions of receptors, of 500–1000 different types. They
are in the yellow olfactory epithelium, that covers about 2.5 cm2
on each side of the inner nose. The different types of receptors are proteins folded
so a particularly shaped odour molecule can dock. Each receptor is coupled to a
g-protein. When the odour molecule docks, the g-protein is released. This
sets off a second messenger to stimulate a neuron to send a signal. This is transmitted
by olfactory nerve fibres which enter either of two specialized structures (olfactory
bulbs), stemlike projections under the front part of the brain. They sort
the signals, and transmit them to the brain for processing.1,2
Recently, Luca Turin, a biophysicist at University College, London, proposed a mechanism
where an electron tunnels from a donor site to an acceptor site on the receptor
molecule, causing it to release the g-protein. Tunnelling requires both the starting
and finishing points to have the same energy, but Turin believes that the donor
site has a higher energy than the receptor. The energy difference is precisely that
needed to excite the odour molecule into a higher vibrational quantum state. Therefore
when the odour molecule lands, it can absorb the right amount of the electron’s
energy, enabling tunnelling through its orbitals.3
This means the smell receptors actually detect the energy of vibrational quantum
transitions in the odour molecules, as first proposed by G.M. Dyson in 1937.4 This energy decreases with increasing mass of the
atoms, and increases with increasing bond strength. It also depends on the symmetry
of the molecule. For a diatomic molecule,5
the fundamental transition energy is:
E = h⁄2π(k/µ)½
Where h is Planck’s constant; k is the force constant of the bond; and µ
is the reduced mass, which is related to the masses of the two atoms by:
µ = m1m2⁄(m1 + m2)
A transition can sometimes be caused by incident electromagnetic radiation of the
right frequency (ν, the Greek letter nu). This frequency is related
to the energy by:
E = hν
The vibrational spectrum is normally measured in wavenumbers
 ,
the reciprocal of the wavelength, so its units are cm-1 (reciprocal centimeters).
Wavenumber is related to energy by:
= E⁄hc
As this energy is in the infrared region, infrared absorption spectroscopy
is a common tool for measuring vibrational energies and bond strengths (together
with the complementary technique of Raman spectroscopy).
This means certain groups of atoms have similar energies, so have similar vibrational
spectra. For example, chemicals with sulfur-hydrogen bonds tend to vibrate at about
2500 cm-1 and this is often perceived as a ‘rotten’ smell—rotten
eggs produce chemicals like hydrogen sulfide (H2S), and ethyl mercaptan
produced by rotting meat is C2H5SH.
Turin supports his theory by noting that decaborane (B10H14)
smells very similar to S–H compounds, and it has nothing in common with them
apart from similar vibrational energies. Although boron has a much lower atomic
mass than sulfur, B–H bonds are much weaker than S–H bonds, and these
effects happen to cancel out.
Further support was provided by the analogous compounds ferrocene and nickelocene.
These have a divalent metal ion (iron and nickel respectively) sandwiched between
two cyclopentadienyl anions (C5H5–). The
main vibrational difference between them is that the metal ring bond in ferrocene
vibrates at 478 cm-1, while in nickelocene it is 355 cm-1.
Ferrocene smells rather spicy, while nickelocene smells like the aromatic hydrocarbon
rings. Turin proposes that below a threshold of 400 cm-1, the vibrational
signal is swamped by ‘background noise’, so is not detected by the nose.
As different isotopes have different masses but similar chemical properties, they
affect the vibrational energy. It can be seen from the formula for reduced mass
that the biggest difference results from replacing hydrogen (Ar = 1)
with deuterium (Ar = 2)—the numerator is doubled. Indeed, deuterated
acetophenone smells fruitier than ordinary acetophenone (C6H5COCH3).
It also smells slightly of bitter almonds, just like many compounds containing the
cyanide or nitrile group (C≡N)—both C–D and C≡N bonds vibrate
at about 2200 cm-1.
One challenge to Turin’s theory is the different smells of some enantiomers
(optical isomers), as they have identical vibrational spectra. For example, R-carvone
smells like spearmint, and S-carvone like caraway. The answer is: the spectra are
identical only in an achiral medium, as in solution or gas phase. But the
smell receptors are chiral and orient the two enantiomers differently.
This means that different vibrating groups lie in the tunnelling direction in each
enantiomer. Turin thinks that the caraway S-carvone is oriented so a carbonyl (C=O)
group lies in that direction, so is detected; in the minty R-carvone, it lies at
right angles, so is ignored. Turin supported this by manufacturing a caraway scent
by mixing the minty carvone with the carbonyl-containing butanone (C2H5COCH3).
If Turin’s theory were true, then infrared and Raman spectroscopy would be
essential tools for the perfume industry! Turin is also using inelastic tunnelling
spectroscopy—‘inelastic’ refers to the energy loss before
tunnelling, as with the proposed sensory mechanism.
The precise chemistry of olfaction is still little understood. But Turin believes
he has found a sequence of amino acid residues that could function as the electron
donor together with NADPH. He has also found five residues coordinated to a zinc
atom that could be the acceptor site. One warning sign of zinc deficiency is loss
of the sense of smell, and zinc is often involved in biological electron-transfer
reactions.
Whether or not Turin’s idea is correct, the olfactory system exhibits what
the biochemist Michael Behe calls irreducible complexity, and is therefore
evidence of design.6 This means the
system requires many parts for it to work, and would not function if any were missing.
The chemical sensing machinery needs proteins with just the right shape to accommodate
the odour molecules. And under Turin’s model, the right energy levels as well.
And even if the sensors were fully operational, the chemical information gathered
by the nose would be useless without nerve connections to transmit it and the brain
to process it.
Related articles
References
- Sensory reception: smell (olfactory) sense. Britannica CD,
Version 97. Encyclopædia Britannica, Inc. 1997. Return to text
- Hill, S., 1998. Sniff’n’shake. New Scientist
157(2115):34–37. Return to text
- Turin, L., 1996. A spectroscopic mechanism for primary olfactory
reception. Chemical Senses 21:773. Cited in Hill, Ref.
2 and Sell, Ref. 4. [Note added subsequently: Dr Turin himself wrote (email 9 February
2000): ‘Dear Dr Sarfati, I write to congratulate you on your lucid and accurate
description of my spectroscopic theory of smell ….’ However, he said
he didn’t necessarily agree with my conclusion that a Creator was responsible.
But he continued, ‘I entirely agree, however that if true, my theory is one
more example of the wonderful design of living things,’ but he left the question
of the cause of this design open.] Return to text
- Sell, C., 1997. On the right scent. Chemistry in Britain,
33(3):39–42. Return to text
- For more complicated molecules, see Wilson, E.B., Decius, J.C.
and Cross, P.C., 1955. Molecular Vibrations: the Theory of Infrared and Raman vibrational
spectra, McGraw-Hill, New York. Return to text
- Behe, M. J., 1996. Darwin’s Black Box: The Biochemical
Challenge to Evolution, The Free Press, New York. See Product information,
above right. Return to text
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