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Bioelectromagnetism

the ability to form radical pairs (Giovani et al., 2003). Actually, an in vitro experimental study frstly

suggested that bird CRY1a is excited by blue light (420–470 nm), and forms long-lived radical pairs blue

light irradiation (Liedvogel et al., 2007; Muheim and Liedvogel, 2015).

Maeda et al. (2008) demonstrated that a radical-pair-based chemical reaction could respond to a weak

magnetic feld in the strength of the GMF in a chain-like molecular triad (CPF triad) as a completely

artifcial system using transient absorption measurements. Furthermore, Maeda et al. (2012) found that

a radical-pair-based chemical reaction could also respond to magnetic felds of several tens of mT, which

magnitudes are larger than that of the GMF in an experimental system that reconstructs a natural pro­

tein CRY. Tis CRY was derived not from the migratory birds but from a plant Arabidopsis thaliana.

From these results, the RPM models became realistic and pioneered that CRY actually functions as a

high-sensitivity magnetic sensor (Maeda et al., 2012).

In fact, concerning the question of whether the RPM models really function as a magnetic sensor in

the retina of migratory birds, the following reports have been made: Te frst is a report on the local­

ization of CRY and neural activity in the retina of migratory birds under magnetic felds. Te second,

though not a report on magnetic feld efects, was the detection of a relatively long-lived radical pair

formation in the order of milliseconds at room temperature in the CRY of a migratory bird called gar­

den warbler (Sylvia borin) using transient absorption measurements. From the experimental results, in

which the radical pair formation and the spin dynamics of radical pairs were detected by these biophysi­

cal methods and numerical simulations, the RPM models are regarded as promising candidates for the

magnetic sensor of migratory birds.

4.4.1 Chemical Characteristics of Cryptochrome

FAD, which is a functional molecule and chromophore of cryptochrome (CRY), is known to have vari­

ous electronic states depending on the external environment and to be involved in many redox reactions

in the living systems. From Arabidopsis thaliana cryptochrome 1 (AtCRY1), an anion radical is obtained

afer 450 nm photoexcitation of FAD via an electron transfer from a tryptophan (Trp) residue found

nearby the protein cavity (Maeda et al., 2012). Giovani et al. (2003) proposed a scheme of light-induced

reactions in purifed AtCRY1 (Figure 4.9).

Light-induced intra- or interprotein electron transfer has been proposed to trigger conformational

changes of CRY that allow binding of a signaling partner for further signal transduction (Yang et al.,

2000). Giovani et al. (2003) consider the following products of the electron transfer reactions as potential

FIGURE 4.9 Scheme of light-induced reactions in Arabidopsis thaliana cryptochrome 1 (AtCRY1) (Giovani et al.,

2003). Solid arrows indicate reactions observed directly. Broken arrows indicate additional reactions. Protonation­

deprotonation accompanying electron transfer is not explicit because it was not kinetically resolved. FAD*, singlet

excited FAD afer absorption of blue light; D, external electron donor; A, external electron acceptor. (Reproduced

with permission from Giovani et al., 2003, Copyright 2003, Springer Nature.)