title: Role of exchange and dipolar interactions in the radical pair model of the avian magnetic compass.
abstract: It is not yet understood how migratory birds sense the Earth's magnetic field as a source of compass information. One suggestion is that the magnetoreceptor involves a photochemical reaction whose product yields are sensitive to external magnetic fields. Specifically, a flavin-tryptophan radical pair is supposedly formed by photoinduced sequential electron transfer along a chain of three tryptophan residues in a cryptochrome flavoprotein immobilized in the retina. The electron Zeeman interaction with the Earth's magnetic field ( approximately 50 microT), modulated by anisotropic magnetic interactions within the radicals, causes the product yields to depend on the orientation of the receptor. According to well-established theory, the radicals would need to be separated by >3.5 nm in order that interradical spin-spin interactions are weak enough to permit a approximately 50 microT field to have a significant effect. Using quantum mechanical simulations, it is shown here that substantial changes in product yields can nevertheless be expected at the much smaller separation of 2.0 +/- 0.2 nm where the effects of exchange and dipolar interactions partially cancel. The terminal flavin-tryptophan radical pair in cryptochrome has a separation of approximately 1.9 nm and is thus ideally placed to act as a magnetoreceptor for the compass mechanism.
authors.affiliation: Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, United Kingdom.
authors.forename: Olga
authors.initials: O
authors.lastname: Efimova
authors.forename: P J
authors.initials: PJ
authors.lastname: Hore
mesh_terms: D025041:Animal Migration; D000818:Animals; D001717:Birds; D003198:Computer Simulation; D056931:Cryptochromes; D004579:Electron Transport / Q000502:physiology / Q000528:radiation effects; D005420:Flavoproteins / Q000737:chemistry / Q000378:metabolism; D008280:Magnetics; D009949:Orientation / Q000502:physiology / Q000528:radiation effects*; D010777:Photochemistry; D010786:Photoreceptor Cells / Q000378:metabolism / Q000528:radiation effects*; D011789:Quantum Theory; D012160:Retina / Q000378:metabolism / Q000528:radiation effects*; D014364:Tryptophan / Q000737:chemistry / Q000378:metabolism
chemical_list: D056931:Cryptochromes; D005420:Flavoproteins; D014364:Tryptophan