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, theoretical biophysicists Drs. Schulten and Solov'yov describe a nanomechanism within the birds' retina tissue, inside the rod cells, inside cryptochrome proteins known to process blue light for entraining circadian cycles, but now perhaps also deserving to be known as the seat of these organism's ability to sense magnetic fields.

They hypothesize the birds perceive the effect of Earth's magnetic field by measuring the reaction dynamics of a process involving a pair of entangled electrons. When a bird first encounters blue light, the electrons separate in the many cryptochrome proteins, such that one free radical is found on a tryptophan amino acid, and the second free radical - originating from the same tryptophan - is found on a nearby FAD factor. When the blue light stimulation stops, the lone electron on FAD returns - in an irreversable reaction - to the tryptophan where it originated. The backtransfer, or return, of the lone electron to tryptophan, is partially a function of the angle the line between the two electrons makes relative to Earth's poles. The "transition time" from when the first cryptochrome returns to its unstimulated electron configuration until when all the cryptochrome protein's have returned, is one example of measurement of the reaction dynamics involving the back-transfer of the electron, and which is likely used by the bird to perceive its position relative to the earth's magnetic field.

More specifically, when the entangled electrons exist in the opposite-direction, or antiparallel spin state - as opposed to the alternative same-direction, parallel state - then the electron from FAD can return to tryptophan. The earth's magnetic field biases which spin state the electrons are found. Since, regardless of the orientation of the Earth's magnetic field, there will be some "extreme" cryptochrome protein that immediately return to the unstimulated state, therefore, by the orientation to Earth's magnetic field affecting the average rate (averaged over all the cryptochromes) at which the electron of FAD returns to tryptophan, it affects the transition time for the whole population to return to the unstimulated electron configuration, which the bird brain can use to determine its navigational trajectory.

Drs. Schulten and Solov'yov also introduce the involvement of a superoxide radical in order to increase the difference in time to about a millisecond for the back reaction electron transfer in different magnetic fields. By stabilizing the electron on FAD in the triplet state for as long as a millisecond when in the corresponding magnetic field, the difference in transition times (from one crytochrome until all the cryptochrome proteins return to the unstimulated state) reaches the magnitude consistent with prevalent timescales for signaling systems. A further condition of this model is that one rotational axis of the cryptochrome be restricted, which they say can easily be accomplished by tethering the cryptochrome to the cell membrane.

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).