  Creatures as varied as mammals, fish, insects, reptiles, and migratory birds
have an intriguing `sixth' sense that allows them to distinguish north from
south by using the Earth's intrinsic magnetic field. Yet despite decades of
study, the physical basis of this magnetic sense remains elusive. A likely
mechanism is furnished by magnetically sensitive radical pair reactions
occurring in the retina, the light-sensitive part of the eyes. A photoreceptor,
cryptochrome, has been suggested to endow birds with magnetoreceptive abilities
as the protein has been shown to exhibit the biophysical properties required
for an animal magnetoreceptor to operate properly. Here, we propose a concrete
light-driven reaction cycle in cryptochrome that lets a magnetic field
influence the signaling state of the photoreceptor. The reaction cycle ties
together transient absorption and electron-spin-resonance observations with
known facts on avian magnetoreception. Our analysis establishes the feasibility
of cryptochrome to act as a geomagnetic compass in birds, gives insight into
structural and dynamic design features required for optimal detection of the
geomagnetic field direction, and suggests further theoretical and experimental
studies.
