★ Rogue theory of smell gets a boost

    Published online： 6 December 2006

    Rogue theory of smell gets a boost

    1.A controversial theory of how we smell，which claims that our fine
sense of odour depends on quantum mechanics ，has been given the thumbs
up by a team of physicists.

    2.Calculations by researchers at University College London（UCL ）

    show that the idea that we smell odour molecules by sensing their
molecular vibrations makes sense in terms of the physics involved.

    3.That's still some way from proving that the theory，proposed in
the mid-1990s by biophysicist Luca Turin，is correct.But it should make
other scientists take the idea more seriously.

    4."This is a big step forward ，" says Turin，who has now set up his
own perfume company Flexitral in Virginia.He says that since he published
his theory，"it has been ignored rather than criticized."

    5.Most scientists have assumed that our sense of smell depends on
receptors in the nose detecting the shape of incoming molecules ，which
triggers a signal to the brain.This molecular 'lock and key' process is
thought to lie behind a wide range of the body's detection systems： it
is how some parts of the immune system recognise invaders ，for example
，and how the tongue recognizes some tastes.

    6.But Turin argued that smell doesn't seem to fit this picture very
well.Molecules that look almost identical can smell very different— such
as alcohols ，which smell like spirits，and thiols，which smell like rotten
eggs.And molecules with very different structures can smell similar.Most
strikingly，some molecules can smell different— to animals ，if not necessarily
to humans — simply because they contain different isotopes （atoms that
are chemically identical but have a different mass）。

    7.Turin's explanation for these smelly facts invokes the idea that
the smell signal in olfactory receptor proteins is triggered not by an
odour molecule's shape，but by its vibrations ，which can enourage an
electron to jump between two parts of the receptor in a quantum-mechanical
process called tunnelling.This electron movement could initiate the smell
signal being sent to the brain.

    8.This would explain why isotopes can smell different ： their vibration
frequencies are changed if the atoms are heavier.Turin's mechanism，says
Marshall Stoneham of the UCL team ，is more like swipe-card identification
than a key fitting a lock.

    9.Vibration-assisted electron tunnelling can undoubtedly occur— it
is used in an experimental technique for measuring molecular vibrations."The
question is whether this is possible in the nose，" says Stoneham's colleague，
Andrew Horsfield.

    10.Stoneham says that when he first heard about Turin's idea，while
Turin was himself based at UCL，"I didn't believe it".But ，he adds ，
"because it was an interesting idea ，I thought I should prove it couldn't
work.I did some simple calculations ，and only then began to feel Luca
could be right." Now Stoneham and his co-workers have done the job more
thoroughly，in a paper soon to be published in Physical Review Letters

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