| Summary: | <p>The remarkable ability of migratory birds to use the geomagnetic field for their navigation has been fascinating scientists for more than 50 years. Extensive studies, from experiments to theories, suggest that avian magnetoreception may arise from photo-induced radical-pair reactions in the birds’ eyes. Although the energy for the interaction of a single molecule with a magnetic field as weak as ∼50 μT is more than a million times smaller than the thermal energy, such weak fields can still significantly influence spin-selective reactions via the modulation of radical-pair spin dynamics.</p>
<p>A navigational cue requires the radical-pair magnetoreceptor to be sensitive to the direction of an external magnetic field. In Chapter 3, I explore the anisotropic weak magnetic field effect on the eigenstructure and the spin dynamics of a model radical pair with respect to the field direction. Then, the feasibility of the “reference-probe” model for radical-pair magnetoreception is studied.</p>
<p>Birds’ disorientation in a static magnetic field caused by extremely weak radiofrequency fields is considered to originate in magnetic effects on the sensory molecules, though it is not yet understood exactly in quantitative detail. A more convincing test - reorientation rather than disorientation in a radiofrequency field in the absence of a static magnetic field - is proposed in Chapter 4. Elaborate theoretical study of the resonance effect induced by a radiofrequency field is followed by spin dynamics simulations for flavin-based radical pairs to evaluate the possibility of a “radiofrequency compass”.</p>
<p>The “B1/2” value, a parameter that characterises the magnetic field-dependence of the reactions, was found to be larger than expected on the basis of the hyperfine interactions and to have significant time dependence. This behaviour is considered as a strong indication of spin relaxation. In Chapter 5, I present several possible candidate spin relaxation mechanisms and find the specific one, singlet-triplet dephasing (STD), that could account for the time-dependent magnetic field effect. In cryptochromes, the only candidate magnetoreceptor, STD probably originates in the electron hopping between tryptophans. Furthermore, simulation shows that STD, even quicker than chemical reactions, does not seem to destroy the directional sensitivity of the FAD-Trp radical pair to weak magnetic fields.</p>
<p>Chiral molecules could act as spin filters that preferentially transmit electrons with spins polarized parallel or antiparallel to their direction of motion. This chiral-induced spin selectivity (CISS), as a spin polarization effect that could be observable by electron paramagnetic resonance (EPR) experiments, is included in the radical pair mechanism in Chapter 6. CISS effects offer the possibility of evolving a more sensitive or precise compass. The associated lack of field- inversion symmetry potentially explains the observation of a polar magnetic compass response.</p>
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