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Quantum Compass of Migratory Birds

FIGURE 4.8 Schematic illustration of the avian radical pair-based compass (Pedersen et al., 2016).

Magnetoreceptive molecules in the bird’s eyes host a pair of radicals (R1, R2), and endow the bird with capabilities

to sense the GMF. In the most simplifed case, each radical pair is associated with a coordinate frame such that

internal magnetic interactions are considered isotropic in the xy-plane, while the anisotropy defnes the z-axis.

Te radical pairs participate in spin-dependent chemical reactions that are sensitive to the angle Θ between this

z-axis and the direction of the GMF B, which, in turn, could be related to the direction of bird motion, denoted by

v. (Reproduced with permission from Pedersen et al., 2016, Copyright 2016, Springer Nature.) It is licensed under

the Creative Commons Attribution 4.0 International.

Tese spin-correlated radicals could form electronically entangled singlet and triplet states, which are

respectively characterized by an anti-parallel and parallel alignment of the unpaired electron spins of

the radicals (Pedersen et al., 2016). Te core of the radical pair mechanism (RPM) of avian magnetore­

ception relies on the possible engagement of the radicals in biochemical reactions that could be afected

by magnetic felds even though the Zeeman interaction of an unpaired electron spin with the GMF is

more than six orders of magnitude smaller than the thermal energy available inside the biological sur­

roundings (Pedersen et al., 2016). Hence, from a classical perspective, a magnetic sensitivity should not

arise from the biochemical reactions, but arise from the quantum reactions (Pedersen et al., 2016). Such

quantum reactions enter the stage through the RPM, which so far is the only known way an external

magnetic feld can infuence a chemical reaction (Steiner and Ulrich, 1989; Brocklehurst, 2002; Timmel

and Henbest, 2004; Rodgers, 2009; Solov’yov et al., 2014; Hore and Mouritsen, 2016). Te RPM has been

studied for about half a century by now and has been successfully applied to various phenomena such

as spin polarization (Muus et al., 1977) and magnetic isotope efects (Salikhov, 1996). Te anisotropy of

the internal magnetic interactions in the radical pair, i.e., the hyperfne interactions, defnes a molecular

coordinate system, that, in turn, determines the orientation between the radical pair and the magnetic

feld using just a single angle Θ (Pedersen et al., 2016, Figure 4.8).

Here, taking more advanced parameters into account, Pedersen et al. (2016) linked the microscopic

proposition of the chemical compass to the macroscopic scale through the angular probability distribu­

tion R(Θ), obtained from the spin dynamics of a radical pair. Using a simple model, they have simulated

fight trajectories of a large number of birds assuming their navigation to rely on chemical compass

distributions, R(Θ), of diferent precision (Pedersen et al., 2016). It was revealed that the precision of the

chemical compass has a great impact on not just the spread of birds but also largely infuences the time

it takes to make the trip (Pedersen et al., 2016). Hence a precise chemical compass is of great importance,

and it should once again be emphasized that a spin chemical mechanism with a fast spin-dependent

reaction would be able to provide the needed precision (Pedersen et al., 2016).

4.4 Radical Pair-Based Magnetoreceptor and Cryptochrome

Perhaps most importantly, Ritz et al. (2000) hypothetically proposed that cryptochrome (CRY) in

the bird retina is a strong candidate for function as a radical pair-based magnetoreceptor as a “CRY­

based RPM theory.” Te CRY is a class of photopigments known from plants and related to photolyases

(Sancar, 2003), and they are assumed to possess chemical properties crucial for the model, including