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Magnetoreception in Plants

 

 

FIGURE 5.3 Te classical RPM entails three essential steps. In the case of cryptochrome, F is the favin and W

is tryptophan. Te spin-selective reaction channel is charge recombination (rate constant kb) within the singlet

state of the RP which regenerates the ground, or resting state of the protein. Te magnetosensitive RP state also

undergoes proton transfer reactions, which occur with equal rate constants (kf) for singlet and triplet pairs, which is

assumed to be, or to lead to, the biochemical signaling state of the protein. (Adapted from Kattnig and Hore (2017).)

enhanced by interactions with its fuctuating environment, a property apparently found in many areas

of “quantum biology” (Kattnig and Hore, 2017).

In the current model, the spin-selective reaction channel is charge recombination (rate constant kb)

within the singlet state of the RP which regenerates the ground state of the protein. Te RP state also

undergoes proton transfer reactions, which occur with equal rate constants (kf) for singlet and triplet

pairs, to produce a secondary, long-lived radical pair state, S, which is assumed to be, or to lead to, the

biochemical signaling state of the protein. Te interaction of the electron spins with the MF can induce a

signifcant change in the yield of S if kb

−1 (the characteristic time of singlet recombination) is comparable

to or shorter than k⁻

f

1 (the time required for the formation of S), small compared to the electron spin

relaxation time (∼1 μs or possibly longer) and longer than the coherent singlet-triplet interconversion

time. Tese conditions mean that the radicals must not be too far apart, otherwise charge recombina­

tion will be too slow. Figure 5.3 shows the classical RPM.

Recent comprehensive calculations for the FAD^•−/W^•+ RP revealed that the key quantity, the difer­

ences in the yield of signaling state at diferent feld directions, might be perplexingly small (Kattnig

et al., 2016b). According to Hore et al., the fact that the compass performance in animals surpasses these

predictions (e.g., with respect to the acuity of the sensor, its function under very low light intensities, or

its sensitivity to weak radio frequency MFs) suggests the presence of a powerful, yet unknown, ampli­

fcation process and remarkable resilience to decoherence (Hore and Mouritsen, 2016; Kattnig et al.,

2016a). Te same group recently proposed an extended reaction scheme (Figure 5.4), that is predicted to

greatly (by a factor of 10 and more) enhance the compass sensitivity via the so-called “chemical Zeno

efect” (Letuta and Berdinskii, 2015), the efect of spin-dependent recombination on the singlet–triplet

RP evolution rate and frequency. Te chemical Zeno efect changes the S–T recombination rate and,

thus, has an efect on the yield of recombination products, quantum beat frequency, chemically induced

nuclear polarization, and magnetic isotope efect (Letuta and Berdinskii, 2015). Te model of Hore and

co-workers relies on a spin-selective reaction of one of the two radicals of the primary pair and a spin-

bearing, external scavenger. Tis scavenger is initially uncorrelated with respect to the RP, a situation

that resembles f-pairs, but eventually acquires correlation as a result of its spin selective reactivity, i.e.

the chemical Zeno efect. It has been shown that this additional reaction induces singlet−triplet conver­

sion in the original RP and serves as a spin-selective reaction channel. As a consequence, and contrary

to previous theories, spin-selective recombination of the primary RP (see Figure 5.3) is no longer essen­

tial, and the radicals could thus be farther apart than is necessary for efcient charge recombination.