Chemical magnetoreception in birds: the radical pair mechanism - PubMed (original) (raw)

Chemical magnetoreception in birds: the radical pair mechanism

Christopher T Rodgers et al. Proc Natl Acad Sci U S A. 2009.

Abstract

Migratory birds travel vast distances each year, finding their way by various means, including a remarkable ability to perceive the Earth's magnetic field. Although it has been known for 40 years that birds possess a magnetic compass, avian magnetoreception is poorly understood at all levels from the primary biophysical detection events, signal transduction pathways and neurophysiology, to the processing of information in the brain. It has been proposed that the primary detector is a specialized ocular photoreceptor that plays host to magnetically sensitive photochemical reactions having radical pairs as fleeting intermediates. Here, we present a physical chemist's perspective on the "radical pair mechanism" of compass magnetoreception in birds. We outline the essential chemical requirements for detecting the direction of an Earth-strength approximately 50 microT magnetic field and comment on the likelihood that these might be satisfied in a biologically plausible receptor. Our survey concludes with a discussion of cryptochrome, the photoactive protein that has been put forward as the magnetoreceptor molecule.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

A simple reaction scheme that could form the basis of a compass magnetoreceptor. The spin-correlated radical pair is depicted in red for the singlet state and blue for the triplet. See Key Features of a Radical Pair Magnetoreceptor: Chemistry for further details.

Fig. 2.

Fig. 2.

Quantum mechanical spin dynamics simulations for the reaction in Fig. 1, performed as described in ref. . (A) In the absence of the magnetic field and with no recombination (green), the fraction of radical pairs that exist in the triplet state, _p_T(t), oscillates at the frequency of the hyperfine coupling (here 14 MHz). (B) When a weak magnetic field is introduced (green), _p_T(t) shows an additional, slower, modulation at the frequency of the Zeeman interaction (here 1.4 MHz). The radical pair reactions cause _p_T(t) to be exponentially damped (red) and allow the reaction product (species C in Fig. 1) to accumulate (blue). The applied magnetic field (50 μT) results in an increased transient conversion of the radical pair into the triplet state, causing C to be formed more rapidly and in higher yield. Faster recombination than shown here would allow scant time for the slow modulation arising from the Zeeman interaction to alter _p_T(t); the yield of C would then be much less affected by the field. For details of the calculation, see

SI Appendix

.

Fig. 3.

Fig. 3.

Spin dynamics simulations of anisotropic reaction yields for model radical pairs performed as described in refs. and using the reaction scheme of Fig. 1. (Upper) Polar plots. (Lower) The corresponding signal modulation patterns for a bird looking directly along the Earth's magnetic field vector. The heights of the vertical scale bars in the upper images correspond to singlet yields of 2% (black) or 0.2% (red). (A and B) The simulations demonstrate that a relatively simple orientation dependence of the reaction yield (A) can be obtained from radical pairs containing a small number of hyperfine interactions or from more complex radicals when a few symmetry-related hyperfine interactions dominate (B). (C and D) More intricate anisotropy patterns (C) can be dramatically simplified if the radical pairs are axially rotationally disordered (D). The signal modulation pattern for C is identical to that for D and is only shown once. Note that in all cases the reaction yield is invariant to exact reversal of the magnetic field vector, i.e., the response is that of an inclination compass rather than a polarity compass. See

SI Appendix

for details of the calculations.

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