Decrypting Cryptochrome: Revealing the Molecular Identity of the Photoactivation Reaction

Migrating birds fly thousands of miles or more, often
without visual
cues and in treacherous winds, yet keep direction. They employ for
this purpose, apparently as a powerful navigational tool, the photoreceptor
protein cryptochrome to sense the geomagnetic field. The unique biological
function of cryptochrome supposedly arises from a photoactivation
reaction involving radical pair formation through electron transfer.
Radical pairs, indeed, can act as a magnetic compass; however, the
cryptochrome photoreaction pathway is not fully resolved yet. To reveal
this pathway and underlying photochemical mechanisms, we carried out
a combination of quantum chemical calculations and molecular dynamics
simulations on plant (Arabidopsis thaliana) cryptochrome. The results demonstrate that after photoexcitation
a radical pair forms, becomes stabilized through proton transfer,
and decays back to the protein’s resting state on time scales
allowing the protein, in principle, to act as a radical pair-based
magnetic sensor. We briefly relate our findings on A. thaliana cryptochrome to photoreaction pathways
in animal cryptochromes.