Non-equilibrium vibrational dynamics govern ultrafast electron transfer in avian cryptochrome

Photoactivated intermolecular electron transfer (ET) in cryptochromes proceeds along chains of aromatic residues and creates a spatially separated pair of radical electrons. Ultrafast time-dependent spectroscopy can provide experimental insight into this process and theoretical estimates of charge transfer rates are commonly obtained via Marcus theory. Here, we present a new perspective on the ET in European robin cryptochrome 4a (ErCry4a) that synthesizes insights from real-time ET calculations, ultrafast spectroscopic measurements and analytical derivations. The simulations exemplify that molecular vibrations play an essential role in enabling the ET dynamics, which was further rationalized through analytical derivations. Ultrafast pump-probe spectroscopy provided experimental access to the first 1.5 ns of the ET cascade, where multiple radical pair recombination rates arise due to the dynamic equilibrium along the ET chain. We show that the motions of the protein environment and the ET dynamics are inseparably coupled, violating the timescale separation required for Marcus theory. The presented results highlight that non-equilibrium coupling between electronic and nuclear motion dominates ET kinetics in ErCry4a during the first nanosecond after photo-excitation. The findings exemplify the limits of Marcus theory and refine the interpretation of ultrafast spectroscopic signatures in cryptochromes.