  A popular hypothesis ascribes magnetoreception to a magnetosensitive
recombination reaction of a pair of radicals in the protein cryptochrome. Many
theoretical studies of this model have ignored inter-radical interactions,
particularly the electron-electron dipolar coupling (EED), which have a
detrimental effect on the magnetosensitivity. Here, we set out to elucidate if
a radical pair allowed to undergo internal motion can yield enhanced
magneto-sensitivity. Our model considers the effects of diffusive motion of one
radical partner along a one-dimensional reaction coordinate. Such dynamics
could in principle be realized either via actual diffusion of a mobile radical
through a protein channel, or via bound radical pairs subjected to protein
structural rearrangements and fluctuations. We demonstrate that the suppressive
effect of the EED interactions can be alleviated in these scenarios as a result
of the quantum Zeno effect and intermittent reduction of the EED coupling
during the radical's diffusive excursions. Our results highlight the importance
of the dynamic environment entwined with the radical pair and ensuing
magnetosensitivity under strong EED coupling, where it had not previously been
anticipated, and demonstrate that a triplet-born radical pair can develop
superior sensitivity over a singlet-born one.
