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^[1].

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 donors 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 donors shown with halos). (In an alternative view, see that FAD and the three tryptophans from the protein's inside to its outside). At this stage, where FAD is in its active signalling state, 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 (shown as a dotted line) is aligned with the line extending between the earth's 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 or the other is affected by a local magnetic field, which is in this case primarily determined by the nuclear spins of several (naming, as in fig. 5 of Schulten et al., 2007), the current spin state of the entangled electrons, and the external magnetic field (emanating from earth). Only when the line between FAD and trp324 is parallel to the line connecting the north and south poles, is the earth's (external) magnetic field biasing the electrons spins to the same direction (parallel;triplet) spinning. Otherwise, the nuclear spins are the main determinants, and the the spins are approximately equivalently likely to be in the same or opposite directions.

Because many cryptochrome proteins are involved 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 - "the transition rate" - is perhaps measured by the brain as the time until 50% of the cells do not have active FAD molecules. By moving its head about in different directions, a bird can find position at which the signalling last longest. That places the bird along the world's north-south pole axis.

(But how a bird know whether it is facing due north or south is a question which cannot be figured out using this protein compass, the research emphasize in their study).