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.

Molecular Tour:

The cryptochrome protein obsorbs a single phton of blue light of 2.7 eV which excites an . FAD is protonated by a , and the electron hole is filled through a series of electron transfers - a chain reaction invovling three tryptophan amino acids which form a line extending from FAD in the interior to the surface of the protein.

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).