教養学部報
第627号 
Animal magnetism : watching magnetic fields influence the chemistry of living cells
Jonathan R. Woodward
Since the discovery of the magical attractive effect of lodestones in the ancient world, human beings have been fascinated by the power of invisible magnetic fields, and in particular if and how they might affect human beings. Indeed the Sushruta Samhita, an ancient Hindu medical text describes how magnetite was used to remove arrows trapped inside the body. In modern science, the interest in magnetic fields and biology persists, and there are two particular aspects on which biologists and medics focus. The first is that we now know that many animals are capable of sensing the Earth's very weak magnetic field and using it for navigation. Arctic Terns, for example, use their built-in magnetic compass to help them navigate worldwide round trips of up to 90,000 km every year. The second is that epidemiological studies suggest a weak association between environmental electromagnetic fields and childhood leukemia, meaning that since 2001, the World Health Organization classifies extremely low frequency magnetic fields as possibly carcinogenic to humans.
These observations beg the question of the physical mechanism by which a very weak magnetic field may be able to influence biological processes. In recent years, a mechanism which I became fascinated with as an undergraduate student has become the frontrunner for explaining the magnetic compass ability of animals. This mechanism is known as the radical pair mechanism and involves chemical reactions that proceed through short-lived (typically a few hundred billionths of a second) intermediates known as radical pairs. Radicals possess a quantum mechanical property known as spin, which makes them behave like tiny magnets, and remarkably, when these pairs of radicals come together, their possibility of reacting together depends on the relative orientation of these tiny magnets. Applying even a weak magnetic field can influence how the radical pairs change these orientations and can lead to the reaction going faster or slower. Returning to the bird magnetic compass, the idea is that proteins known as cryptochromes are excited by blue light and generate magnetically sensitive radical pairs in the eyes of migratory birds, essentially allowing the birds to 'see the magnetic fields' in their environments. Thus the mechanism is almost more astonishing than the compass ability itself and is an example of what has recently been referred to as 'quantum biology.'
Until now, there has been no direct evidence for the radical pair mechanism operating in real living systems. Recently, PhD student Noboru Ikeya and I were able to demonstrate the effect of a magnetic field on radical pair-based photochemistry in living cells for the first time. All cells possess a property known as autofluorescence, which is the emission of light by certain molecules naturally present in the cells. We studied the natural light emitted by individual cells when exposed to blue light and suspected that the autofluorescence in this case was due to molecules called flavins, which we already knew showed magnetically sensitive photochemistry. Our idea was to use this autofluorescence to find out if the flavin molecules were undergoing radical pair based chemical reactions in the cells. We were able to successfully demonstrate a clearly measurable effect in which applying a magnetic field caused a reduction in the amount of light emitted by the cells. Furthermore, we were able to confirm that the effect came from natural flavin molecules in the cells and determine some magnetic properties of the radical pairs involved. This discovery has thus now firmly established the connection between the radical pair mechanism and biological magnetoreception, taking us another step closer to really understanding how those magical and mysterious magnetic fields affect biology.
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