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Bioelectromagnetism

FIGURE 5.2 Phenomenological cases of MFD of reaction yields. (Adapted from Steiner and Ulrich (1989).)

Dotted lines indicate sign inversion of efects when changing the precursor multiplicity in the RPM.

According to the RPM, the singlet-triplet conversion rate is slowed down at low felds (hyperfne

mechanism) and increased in high felds (Δg mechanism). When an MF exceeds that of the hyper­

fne interaction, the triplet states T+,− are separated from the remaining states energy levels due to the

Zeeman interaction:

Ez = µBg B m

j ext

j

(where μB is the Bohr magneton, gj is the Landé g-factor, B

is the applied MF, and mj is the magnetic

moment). Terefore, the spin-dependent kinetics becomes MF-dependent.

Te Zeeman interaction with the external MF modifes the energy levels of the pair, changing the

extent and frequency of S ↔ T interconversion, and so leads to magnetic feld efects (MFEs) on the

lifetime of the radical pair and on the yields of the reaction products formed from it (Zollitsch et al.,

2018). Te MFE develops during the lifetime of the spin-correlated radical pair (RP1) and is inherited by

downstream products. As such, the time- and wavelength-dependence of the MFE can provide informa­

tion on F•−, even when its signature in the spectral deconvolution of the ΔA data is difcult to observe

(Zollitsch et al., 2018).

One of the most studied MFE is the favin-based RP system. In these proteins, photoexcitation of the

favin adenine dinucleotide (FAD) cofactor in its fully oxidized redox state, FAD+, produces a singlet

excited state, 1FAD*, which is then reduced on a picosecond timescale by consecutive electron transfers

along a chain of three tryptophan or guanosine residues, generating the singlet radical pair, 1[FAD•—

Trp•+] (Evans et al., 2015). In radical pairs formed by photo-induced electron transfer reactions in sensor

proteins, the magnetic sensitivity arises from a combination of the coherent quantum spin dynamics

and the spin-selective reactivity of a pair of spin-correlated radicals, which cause the yield of a signal­

ing state to depend on the intensity and direction of the external MF. An RP-based sensor may also be

ext