Page 6 - PERIODIC 2016
P. 6

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
                          B
              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
                          B
                                                                 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






          6
              Periodic       The Magazine of the Department of Chemistry
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