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Optimal Conditions for a Cryptochrome Based Magnetic Compass


European robins can detect the direction of the Earth’s magnetic field, forming an internal compass which aids them in migration and other navigational tasks. The physical basis of this magnetic sense arises from a light-initiated chemical reaction which proceeds through a magnetically sensitive radical-pair intermediate. The pathway of this reaction is sensitive to Earth strength fields, with the products encoding directional information. The photoreceptor cryptochrome has been suggested as a magnetic receptor, owing to its ability to form radical pairs in response to light absorption. However, numerous challenges remain in establishing cryptochrome as the molecular basis of magnetoreception.

Thermal noise causes the positions of the radicals to fluctuate, affecting the magnetic environment and therefore the reaction pathway. Using model calculations, we show that a magnetic compass based on cryptochrome is robust to thermal fluctuations, and that its performance improves, becoming more robust to other sources of noise.

Motivated by this result we investigate the limits of its performance by determining the optimal radical pairs. Using a genetic based algorithm we show: (1) the simplest radical-pair system possible exhibits a high degree of sensitivity and (2) the radical pair formed on cryptochrome is nearly optimal.

Since the magnetic compass is light-dependent, it has been challenging to understand how night-migration is possible, given the low-light conditions. Furthermore, behavioral experiments have demonstrated that if the light intensity is too high or low (i.e. exhibits a functional window), the magnetic compass no longer works. Using a suggested photocycle for cryptochrome we show: (1) a magnetic compass based on cryptochrome performs better under low-light conditions and (2) the functional window can be understood as a consequence of its photocycle.

Another challenge facing cryptochrome is establishing which reaction step is magnetically sensitive. Two reactions have been proposed – the forward light-dependent step and the ‘dark’ backward reaction. Combining cryptochrome’s photocycle with a phosphorylation binding model, we present calculations in alternating light and magnetic conditions which support the dark reaction as being magnetically sensitive.

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