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Quantum Compass of Migratory Birds

 

Tis is a method of directly detecting the spin polarization state (non-Boltzmann distribution), and in

the case of the spin dynamics of radical pair, both absorption (A) and emission (E) signals of opposite

phases appear. Furthermore, the singlet-born radical pair from the excited singlet state of the favin

induced by a magnetic feld showed the spectral shape of E/A spin polarization (emission [E] in the low

magnetic feld side and absorption [A] in the high magnetic feld side) (Weber et al., 2010). In contrast,

triplet-born radical pair from the excited triplet state of the favin induced by a magnetic feld indicated

the spectral shape of A/E spin polarization (Weber et al., 2010).

In the in vitro biological systems, the magnetic feld-induced E/A spin polarization has been detected

in Xenopus laevis CRY-DASH (XlCRY-DASH) (Biskup et al., 2009). In addition, the magnetic feld-

induced E/A spin polarization has also been detected in the natural form of CRY-DASH of the cyano­

bacterium Synechocystis sp. (ScyCRY-DASH), but in the case of the mutant in which Trp was replaced by

phenylalanine (Phe), these signals were not detected for some sequences (Biskup et al., 2011).

Te singlet-born radical pair from the excited singlet state of the favin induced by a magnetic feld

was observed in natural favoproteins such as FAD (Biskup et al., 2009, 2011), but not observed in arti­

fcial systems. Te triplet-born radical pair from the excited triplet state of the favin induced by a mag­

netic feld was detected in artifcial systems (Horiuchi et al., 2003). Te distance between Trp and favin

is close enough to the state of the favin, and the electron transfer reactions from Trp occur in the excited

singlet state. In contrast, in the conventional artifcial systems, the distance between Trp and favin is

so far, and therefore, the electron transfer reactions from Trp occur greatly in the excited triplet state

with a long life.

In the case of the migratory bird CRY, its structure has not been clarifed yet. In contrast, the molecu­

lar dynamics structures of Arabidopsis thaliana cryptochrome 1 (AtCRY1) have been determined in

detail (Brautigam et al., 2004; Huang et al., 2006; Kattnig et al., 2016, Figure 4.15).

Figure 4.15 gives an impression of the positions of the two radicals in AtCRY1 in the average structure

and in the two extreme conformations, r = rmin and r = rmax (Kattnig et al., 2016). Te variation in the radi­

cal separation is mainly attributable to the greater mobility of the TrpH^•

C

+ radical compared to the more

snugly bound FAD^•− radical located in the center of the protein (Kattnig et al., 2016).

Recently, it has been reported that the magnetic compass of migratory birds has a “quantum efect,”

which may be related to the detection of the magnetic direction required for migration. Te quantum

efect also could be related to the photosynthesis of green sulfur bacteria, in which multiple pathways

responsible for photosynthesis simultaneously achieve diferent energy states (Lee et al., 2007; Engel

et al., 2007; Ishizaki and Fleming, 2009, 2011; Ishizaki et al., 2010; Sarovar et al., 2010; Ishizaki and

Fleming, 2011).

FIGURE 4.15 Molecular dynamics structures of Arabidopsis thaliana cryptochrome 1 (AtCRY1) in the region of

the FAD and Trp triad (Kattnig et al., 2016). Te positions of the two components of the radical pair (FAD^•− and

TrpHC

•+), the proximal and medial Trp (TrpHA and TrpHB), and the backbone in the average structure (rav = 1.89 nm)

are shown. Te positions of the two radicals, and the backbone in the region of TrpHC

•+, are shown for the structures

with, respectively, the largest (rmax = 2.43 nm) and smallest (rmin = 1.69 nm) radical separations. Te curved arrow

indicates the range of motion of TrpHC

•+. (Reproduced with permission from Kattnig et al., 2016, Copyright 2016,

IOP Publishing.) It is licensed under the Creative Commons Attribution 3.0 International.