Magnetic sensing by cryptochrome may work better with four tryptophans than three

Purified Cry4a from migratory European robins shows that the third radical pair state is more magnetically sensitive than the fourth.

• The magnetic compass sensor in migratory songbirds may involve photo-induced radical pairs in cryptochrome flavoproteins.
• Four radical pair states are formed in Cry4a through sequential electron transfer along four tryptophan residues.
• The third radical pair state is suggested to play a significant role in magnetic sensing due to its greater magnetic sensitivity.
• The fourth radical pair may be more effective in starting magnetic signaling, potentially influenced by nearby tyrosine.
• This dynamic equilibrium between the two radical pair states could allow for optimized sensing and signaling functions.

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