Page 6 - Periodic Issue 04

A Quantum Needle

in a Haystack

Peter J. Hore, Fellow, Corpus Christi, Professor

and Tutor in Chemistry

How could a molecular interaction ten million times
smaller than k T make a significant difference to the yield
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of a chemical reaction? The answer is not obvious, but it
seems likely that migratory birds use such effects to sense
the Earth’s magnetic field.

It has been known for half a century that birds navigate
with the help of a light-dependent magnetic compass,
but clues to the mechanism of this extraordinary sense
have only emerged in the last ten years. It is thought that
birds perceive the geomagnetic field using photochemical
reactions in their eyes. The research groups of Christiane
Timmel, Stuart Mackenzie and David Manolopoulos in the
Chemistry Department, together with my own, are trying
to discover whether this is indeed the case and how, in
detail, such a sensor could work.
The primary magnetoreceptor is believed to be
cryptochrome, a blue-light photoreceptor protein found in
a variety of different cell types in the avian retina. Photo-
induced electron transfer within the protein produces
pairs of radicals in which the unpaired electron spins
are correlated and far removed from their equilibrium
configuration. As a consequence, and because the radical
recombination reactions conserve spin, weak magnetic
interactions can affect the yield of a conformation of the
protein that could act as a signalling state. Because the radical pair can respond to a magnetic field as weak as
spins are not at equilibrium, the interactions can be small the Earth’s. Using specifically developed spectroscopic
compared to k T and still have this effect. * techniques, the Timmel and Mackenzie groups have
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shown that cryptochrome photochemistry is indeed
Experimental and theoretical evidence is accumulating in sensitive to weak magnetic fields in vitro and that the
support of this mechanism although much of it remains proteins appear to be fit for purpose as magnetic compass
frustratingly circumstantial. There are many fundamental sensors. The Manolopoulos group is developing theoretical
puzzles, some of which we are attempting to unravel. The methods that allow the spin dynamics of realistic radical
Timmel group has done experiments on a molecular triad pairs, including the aforementioned triad molecule, to be
composed of covalently linked carotenoid, porphyrin, accurately modelled in silico.
and fullerene molecules to establish the principle that a

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Periodic The Magazine of the Department of Chemistry