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Bioelectromagnetism
2012; Lau et al., 2012; Wiltschko and Wiltschko, 2014, Bolte et al., 2016; Kerpal et al., 2019; Wiltschko
et al., 2021). In this CRY, a favoprotein, “favin adenine dinucleotide (FAD)” is a photofunctional mol
ecule, and it is bound to the site buried in the helix domain by intermolecular interaction. When FAD
is excited by blue light, electron transfer occurs from nearby tryptophan (Trp) that is charge-separated,
and the consequent radical pair induces the efciency of reaction to be detected, albeit with a weak
magnetic feld. Using such highly sensitive magnetic receptors (magnetoreceptors), migratory birds are
assumed to be able to migrate in their intended proper direction.
Here, the historical background study of the “radical pair (recombination) mechanism (RPM)” mod
els for quantum-assisted magnetic sensing and recent studies on both natural and artifcial in vitro
systems related to favoproteins such as FAD are introduced. Te forming processes of radical pairs dif
fered between natural favoproteins and artifcial systems, which were focused in this review. Te latter
system was expected to provide advantages for precisely controlling the experimental condition. Te
avian magnetic compass is currently being actively investigated in the feld of “Quantum Biology” (Ball,
2011; Al-Khalili and McFadden, 2014; Solov’yov et al., 2014).
4.2 Historical Background Study of the Radical
Pair Mechanism Models
As historical background studies of spin chemistry, it has been generally considered difcult to con
trol chemical and biological reactions using magnetic felds. Te reason is that the electronic energy
involved in the magnetic felds is extremely smaller than that in the chemical reactions. For example,
the Zeeman splitting of electron spins by 1 T magnetic feld is 0.935 cm−1, while the thermal energy is
about 200 cm−1, and the activation energy of chemical reactions is usually 3.000 cm−1 or more (Hayashi,
1982). In spite of this historical background, however, it has been mainly published by Japanese research
ers that when radical pairs are included in the reaction process, even relatively weak magnetic felds in
the feld range below 1 T could afect the chemical reaction rate and yield (Hata, 1976, 1978, 1985, 1986;
Tanimoto et al., 1976; Hayashi and Nagakura, 1978; Hata et al., 1979; Sakaguchi et al., 1980a, b, 1981;
Sakaguchi and Hayashi, 1982; Hata and Yagi, 1983; Hata and Nishida, 1985).
4.2.1 Magnetic Field Effects on Chemical Reactions
via Radical Pair Recombination
Even in the 1 T magnetic feld, it is difcult to thermodynamically change the chemical reaction at
room temperature for the above-mentioned reasons, i.e., the Zeeman splitting energy in the magnetic
feld. Regarding the chemical magnetic feld efects, from the 1970s to the 1980s, chemical reactions
through unstable radical pairs or biradicals have been proved to be infuenced by an external magnetic
feld (Kaptein, 1972; Sagdeev et al., 1973; Hata, 1976, 1978, 1985, 1986; Tanimoto et al., 1976; Hayashi and
Nagakura, 1978; Hata et al., 1979; Turro and Chow, 1979; Sakaguchi et al., 1980a, b, 1981; Hayashi, 1982;
Sakaguchi and Hayashi, 1982; Hata and Yagi, 1983; Hata and Nishida, 1985). Te magnetic feld efect
on chemical reactions was interpreted in terms of the fact that a magnetic feld enhances or reduces
the singlet-triplet (S-T) conversion of intermediate radical pairs or biradicals (Kaptein, 1972; Hayashi
and Nagakura, 1978) through the electronic Zeeman and electron-nuclear hyperfne coupling (HFC)
interactions (Hayashi et al., 1966; Itoh et al., 1969). In particular, as one of the pioneer studies, Japanese
researchers observed and found the magnetic feld efects on the reaction yield and rate of the chemical
reaction proceeding via a radical-pair intermediate in the solution from both experimental and theo
retical perspectives (Hata, 1976, 1978, 1985, 1986; Tanimoto et al., 1976; Hayashi and Nagakura, 1978;
Hata et al., 1979; Sakaguchi et al., 1980a, b, 1981; Hayashi, 1982; Sakaguchi and Hayashi, 1982; Hata and
Yagi, 1983; Hata and Nishida, 1985).
Photochemical reaction proceeding via a radical-pair intermediate in the solution can generally be
expected to show an external magnetic feld efect which arises from an electric Zeeman interaction