Extraordinary Proteins. Extreme lifestyles sometimes require sensing the earth's magnetic field. Trytophan and aspartic acid residues may be key to an organism's ability to pick up where it is relative to the earth's poles.

Birds, turtles, butterflies and other animals migrate with the help of the compasses built into their bodies. Little is known about the mechanistic nature of these compasses, and to fill the gap in knowledge, two theoretical biophysicists describe a systems based on cryptochrome, a flavoprotein found in bird rod cells which is known to process blue light for entraining circadian cycles, but should now perhaps also be known as the seat of these organism's ability to sense magnetic fields.

The fundamental mechanisms involves a pair of entangled free radicals within the flavoprotein. The angle between the Earth's magnetic field of the entangled pair then biases the pair to be in either the singlet or triplet state (spinning parallel of antiparallel), which would then result in either of two respective reaction products - which then somehow is perceived by the brain. With this system in place, a bird seeking to measure its place within Earth's magnetic field, would move its head about, and keep track of the relative amounts of the two possible reaction products, since they depended on the spin state of the radical pair. In short, earth's magnetic field would effect a perceivable change in the reaction products, or a reaction that depended on the earth's magnetic field because of its affect of the spin states of the rate determining spin state of an entangled pair of electrons.

Further conditions of this model are that one rotational axis of the cryptochrome be restricted, which they say can easily be accomplished by tethering the cryptochrome to the cell membrane. And Drs. Schulten and Solov'yov also introduce the involvement of a superoxide radical in order to increase the difference in time for the back reaction electron transfer to about a millisecond, in order that it consistent with prevalent timescales for signaling systems.

Molecular Tour:

Klaus Schulten of the UIUC and Illia Solov'yov, now at the University of Southern Denmark, hypothesize that the FAD factor and just several residues of a crytochrome protein is all it takes to register the magnetic field of the earth. The they describe involves the . When light in the blue range hits the FAD factor it becomes excited, with the excitement diffused over its (the atoms involved in resonance are shown with halos). Then, one of the donates a hydrogen proton from its hydroxyl group (the proximate ones shown with halos). The FAD factor then receives an electron from the neighboring tryptophan, from the tryptophan's nitrogen atom (shown in halo). The proton and electron that FAD received are attached to one of the nitrogen atoms on its ring (shown with a halo). Next, this tryptophan received an electron from its , and then the second tryptophan received an electron from its neighbor, a third tryptophan. Finally, the third tryptophan loses a proton to a neighboring element. At this stage, the magnetic core contains an entangled pair of free radicals. The FAD factor contains a (shown with a halo), as does the third tryptophan residue on its donating nitrogen atom(shown with a halo).