148
Bioelectromagnetism
External magnetic feld efects on the photosubstitution reaction (1→3) in cyclohexane are shown by
Hata and Nishida (1985). Here, 1, 4-methyl-2-quinolinecarbonitrile; 3, 2-cyclohexyl-4-methylquinoline. In
the case of the chemical yield of 2-cyclohexyl-4-methylquinoline 3, the results are plotted against the feld
strength in either the absence or presence of 1,3-pentadiene. In either case, the conversion was almost inde
pendent of the feld strength (19%–21%). In the absence of 1,3-pentadiene, as shown by curve (a), the chemi
cal yield of 3 was ca. 65% at the zero feld. However, it decreased steeply upon the application of a magnetic
feld of 40 mT to be ca. 54% (the magnetic feld efect due to HFI mechanism). Further increase in the feld
strength resulted in the quadratic recovery in the chemical yield to reach a constant value of ca. 63% (the
magnetic feld efect due to Δg mechanism). However, the addition of 1,3-pentadiene as shown by curve
(b) caused a complete disappearance of these magnetic feld efects. Consequently, the chemical yield of 3
became independent of the feld strength to show ca. 67%. Tis means that both HFI and Δg magnetic feld
efects observed in this reaction can be assigned to the feld dependence of the chemical yield of the T1-born
cage product. Also, the fact that product 3 was obtained in a high feld (ca. 67%), even in the of 1,3-pentadi
ene, suggests strongly that the photosubstitution reaction proceeds from the S1 state as well as the T1 state.
Tanimoto et al. (1976) examined the photodegradation reaction at room temperature under magnetic
felds of up to 4.2 T to determine the relative magnetic feld change of the cage product. Tis study is
the frst study to confrm the magnetic feld efect on the reaction yield of the chemical reaction in the
solution. Dissipative product yields and chemical reactions from triplet precursors can be treated in the
same way (Hayashi and Nagakura, 1978; Sakaguchi et al., 1980a, b).
Furthermore, regarding the rate of the chemical reaction proceeding, it is theoretically estimated that
the singlet-triplet (S-T) conversion rate (kST) (in the coherent mixing between singlet and triplet spin
states) may be decreased by the magnetic feld (Hayashi and Nagakura, 1978; Sakaguchi et al., 1980a,
b). In order to measure kST experimentally, the research team devised a method of measuring the time
change of the absorption intensity of the electron spectrum in the radical pair (Sakaguchi et al., 1980a, b,
1981; Sakaguchi and Hayashi, 1982). Tey used a pulsed laser as the excitation light source to examine the
magnetic feld efects on kST in photochemical reactions in a micelle for the frst time (Sakaguchi et al.,
1980a, b, 1981; Hayashi, 1982; Sakaguchi and Hayashi, 1982).
When benzophenone (BP) in a sodium dodecyl sulfate (SDS) micelle is laser excited, the triplet excited
state (3BP*) abstracts hydrogen from the micelle molecule (RH) to form a triplet radical pair (Sakaguchi
et al., 1980a, b).
3
*
3
BP + RH ° (K• •R)
(4.1)
Here, BP
= (C H
)2 CO, K• = (C H )2 COH, R• = •C H SO − . kST can be obtained from the time change
6
5
6
5
12
24
4
of the absorption intensity of K•. Since these kST values are similar to T1
−1 of electrons in radicals, kST could
be afected by T1. Magnetic feld strength dependence of the S-T conversion constant (kST) at remote pairs
of 4,4′-difuorobenzophenone ketyl radical, (C6H4F)2 COH, and alkyl radical, C H SO −, in an SDS
12
24
4
micelle is shown by Hayashi (1982), modifed from Sakaguchi and Hayashi (1982). Te experimental
results suggested magnetic feld strength dependence of the S-T conversion constant. Surprisingly, the
kST value at 13.4 kg (1.34 T) is about 1/10 of the kST value at 0 kg, and such a large magnetic feld efect on
kST has not been reported so far (Hayashi, 1982). Te reason is thought to be that radical-pair interme
diates can exist for a long time in micelles (Hayashi, 1982). Since the reaction in the micelle is similar
to the reaction in the living systems, the magnetic feld efects on the biological reaction can be greatly
expected (Hayashi, 1982).
4.2.2 Magnetic Field Effects on Biological Systems
via Radical Pair Recombination
With regard to magnetic feld efects on radical pair recombination in biological systems, the yield of
excited triplet electronic states in the photosynthetic reaction center was frst shown to be magnetic