Role of exchange and dipolar interactions in the radical pair model of the avian magnetic compass

¹ University of Oxford
² Oxford University

^* To whom correspondence should be addressed. E-mail: peter.hore{at}chem.ox.ac.uk.

Submitted on August 8, 2007
Revised on August 29, 2007
Accepted on 12 September 2007

	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 immobilised in the retina. The electron Zeeman interaction with the Earth's magnetic field (~50 µT), 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 more than 3.5 nm in order that inter-radical spin-spin interactions are weak enough to permit a ~50 µT 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 ~1.9 nm and is thus ideally placed to act as a magnetoreceptor for the compass mechanism.

Key Words: cryptochrome, electron transfer, flavoprotein, magnetic field effect, magnetoreception, photoreceptor