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 (the atoms involved in resonance are shown with halos). FAD is protonated by a (the proximate ones shown with halos), and the electron hole is filled through a series of electron transfers - a chain reaction involving three tryptophan amino acids (the nitrogen donators shown in halo) which form an extending from FAD in the interior to the surface of the protein. At this stage, FAD is in its active signalling state, and also the extra electron on FAD and lone electron on the final tryptophan amino acid (324) (location of the electron shown with halos). The pair is entangled, such that they spin in opposite or same directions, but only when they spin in the opposite directions, can the extra electron on FAD tunnel back to the hole left in tryptophan 324.

Researchers Klaus Schulten at University Illinois at Urbana Champaign and Ilya Solov'yov, now at the University of Southern Denmark, connect this system to the fascinating ability of many birds, and other flying species, to migrate while sensing the earth's magnetic field. Through simulations, they show that where the bird's cryptochrome compass's "FAD-trp324 needle" is aligned with the line extending between the poles, the entangled electrons will 'on average' spend more time in the same spinning state (also known as triplet; or parallel), and therefore by delaying the electrons return to trp324, FAD will 'on average' be in its signalling mode for longer.

Mechanistically, the propensity of the electrons to spin in one direction of the other is affected by a local magnetic field, which is in this case primarily determined by the nuclear spins of several key nitrogen and hydrogen atoms,

Because many cryptochrome proteins are invovled in registering blue light photons - millions of proteins per cell, and many cells across the retina, a change in the average time spent in the signalling state is perhaps measured by the brain as the time until 50% of the cells do not have active FAD molecules. By

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).