What is the fundamental mechanism by which radical pairs enable magnetoreception in migratory birds, including the role of electron spin and hyperfine interactions?

Radical pairs, which are two molecules each possessing an unpaired electron, are believed to be the basis for magnetoreception in migratory birds. The mechanism relies on the fact that the spins of the unpaired electrons in the radical pair can be either correlated (in a singlet state) or uncorrelated (in a triplet state). The interconversion between the singlet and triplet states is influenced by the Earth's magnetic field. The key lies in hyperfine interactions, which are the interactions between the electron spins and the nuclear spins of nearby atoms, such as hydrogen or nitrogen. These hyperfine interactions create local magnetic fields that can affect the electron spin dynamics. The external magnetic field, such as the Earth's magnetic field, interacts with the electron spins and modulates the rate of singlet-triplet interconversion. Specifically, the Earth's magnetic field alters the energy difference between the singlet and triplet states, influencing the probability of the radical pair being in either state. The ratio of singlet to triplet state radical pairs influences the chemical reactions that occur. For example, the singlet state might lead to the formation of one product, while the triplet state leads to another. The relative amounts of these products then depend on the strength and direction of the magnetic field. Birds can then sense the ratio of these products via a biochemical pathway, effectively acting as a compass. The enzyme cryptochrome, found in the bird's retina, is a candidate molecule for forming these light-sensitive radical pairs. In summary, the mechanism involves light-induced formation of radical pairs in cryptochrome, whose spin dynamics are influenced by both hyperfine interactions and the external magnetic field, leading to changes in the relative yields of chemical products that are then transduced into a neural signal providing directional information.