Alternative radical pairs for cryptochrome-based magnetoreception

There is growing evidence that the remarkable ability of animals, in particular birds, to sense the direction of the Earth's magnetic field relies on magnetically sensitive photochemical reactions of the protein cryptochrome. It is generally assumed that the magnetic field acts on the radical pair [FAD(center dot-) TrpH(center dot+)] formed by the transfer of an electron from a group of three tryptophan residues to the photo-excited flavin adenine dinucleotide cofactor within the protein. Here, we examine the suitability of an [FAD(center dot-) Z(center dot)] radical pair as a compass magnetoreceptor, where Z(center dot) is a radical in which the electron spin has no hyperfine interactions with magnetic nuclei, such as hydrogen and nitrogen. Quantum spin dynamics simulations of the reactivity of [FAD(center dot-) Z(center dot)] show that it is two orders of magnitude more sensitive to the direction of the geomagnetic field than is [FAD(center dot-) TrpH(center dot+)] under the same conditions (50 mu T magnetic field, 1 mu s radical lifetime). The favourable magnetic properties of [FAD(center dot-) Z(center dot)] arise from the asymmetric distribution of hyperfine interactions among the two radicals and the near-optimal magnetic properties of the flavin radical. We close by discussing the identity of Z(center dot) and possible routes for its formation as part of a spin-correlated radical pair with an FAD radical in cryptochrome.