170

Bioelectromagnetism

 

both molecules are explained by the RPM (Hore and Mouritsen, 2016; Rodgers and Hore, 2009), and the

EcPL photocycle which also provides a framework for the discussion of DmCRY (Maeda et al., 2012). In

the natural favin system, e.g., AtCRY1, EcPL, and DmCRY, the singlet radical pair was generated from

the favin-excited singlet state, and then the spin transition from the singlet to triplet states occurred at

a rate depending on the strength and angle of the magnetic feld (Maeda et al., 2012).

Meanwhile, in the artifcial favin system, the magnetic feld efects on the FAD in an aqueous solution

have been reported in the process, in which the isoalloxazine ring (favin ring) and adenine form a stack­

ing structure, and the intramolecular electron transfer occurs from adenine to favin under acidic condi­

tions (Murakami et al., 2005). Tat is, one of favin semiquinone radicals FADH^• has a broad absorption

band at 500–730 nm (somewhat environment-dependent), and in particular, the magnetic feld efects

on the FADH^• showed the transient absorption at 580 nm during photoexcitation of FAD under acidic

conditions (Murakami et al., 2005). From this result and the transient absorption of the cation form of

favin³FH⁺ (exited triplet state) at 650 mn, it is estimated that the triplet-born radical pair formation

occurred from the triplet state of favin by exposure to a magnetic feld (Murakami et al., 2005).

Horiuchi et al. (2003) studied the magnetic feld efect on the electron transfer reactions from indole

derivatives to favin derivatives in micellar solutions. In this study, the hydrophobic nature of the favin

derivatives and the dynamics of the radical pair were studied by observing the infuence of the magnetic

feld efect on the transient absorption (Horiuchi et al., 2003). Te magnetic feld efect on the transient

absorption is sensitive to the restricted difusion process (Horiuchi et al., 2003). It is a nice tool for the

analysis of the incorporation and the difusion process of the radical pair generated by the photochemi­

cal reaction of favin and indole derivatives (Horiuchi et al., 2003).

Horiuchi et al. (2003) presented that the magnetic feld efects on the transient absorption spectra

were observed in the ribofavin-indole system (Figure 4.13). Initially, the T-T absorption band was

observed around 650 nm (Horiuchi et al., 2003). Synchronized with the decay of this band, the absorp­

tion observed around 500–600 nm is recognized (Horiuchi et al., 2003). Tis absorption band has been

assigned to the neutral radical of ribofavin (Dudley et al., 1964; Müller et al., 1972). When the external

magnetic feld (B = 0.2 T) was applied, the absorption changed (Horiuchi et al., 2003). Action spectra of

the magnetic feld efect (Ali et al., 1997; Murakami et al., 2002) were obtained at various delay times

afer pulsed laser irradiation by plotting the change of the transient absorption versus monitoring wave­

length and are shown in Figure 4.13b (Horiuchi et al., 2003). Te action spectra of the magnetic feld

efect clearly distinguish between the contribution of the radical species and the overlapped spectrum

of the triplet excited states (Horiuchi et al., 2003). Te observed action spectra of the magnetic feld efect

are also assigned to the neutral radical of ribofavin (Dudley et al., 1964; Müller et al., 1972).

Te magnetic feld efect on the free radical yields observed by transient absorption refected efec­

tively the association of the donor and acceptor molecules with the micelles (Horiuchi et al., 2003). In

the ribofavin-indole system, the magnetic feld efect increased rapidly with an increasing concentra­

tion of sodium dodecyl sulfate (SDS) higher than the critical micellar concentration (Horiuchi et al.,

2003). In contrast, in the favin mononucleotide-indole system, the increase of magnetic feld efect was

very slow even at higher concentrations of SDS (Horiuchi et al., 2003). Tis result showed that ribofavin

was well associated with the SDS micelle and the difusion process of the radical pair was restricted by

the micellar cages (Horiuchi et al., 2003).

Horiuchi et al. (2003) presented the magnetic feld efects on the time profles observed at 510 nm in

the ribofavin-indole system (Figure 4.14). It is difcult to analyze the rising kinetics of the absorption

band of the radical species because of the overlap with strong fuorescence and the contribution of T-T

absorption. However, subtraction of the time profles of the magnetic feld efect, A(B = 0.2 T) − A(B = 0 T),

showed that the magnetic feld efect of favin radical is positive and grows with a similar timescale as

the decay of the T-T absorption observed at 690 nm.

Te diference in the magnetic feld efect is not only due to the association with micelles in the

ground state but also due to the dynamic process of the radical pair. Te comparison of the rap­

idly decaying components (within 1 µs) between the ribofavin-indole system (Figure 4.14a) and the