Coherent Vibrational Wave Packet Motion in ErCry4a Proteins Monitors the Redox State of the Flavin Chromophore

Cryptochromes are blue-light-sensitive flavoproteins that play central roles in biological function. In European robin (Erithacus rubecula) ErCry4a proteins, optical excitation of their flavin chromophore forms a long-lived radical pair through a sequence of electron-transfer steps across a tetradic chain of tryptophan residues, making them primary candidates for magnetoreception in night-migratory songbirds. Recent quantum chemical calculations indicate that nonadiabatic couplings play a central role in the energy- and charge-transfer processes initiated by optical excitation. Here, we study these dynamics in ErCry4a using ultrafast transient absorption spectroscopy with 10 fs time resolution in the 450 nm spectral range. We uncover a rapid, sub-50 fs red shift in stimulated emission, quenched within 360 fs by electron transfer from a nearby tryptophan moiety. While high-frequency excited-state vibrations are rapidly damped, coherent motion involving several low-frequency vibrations persists during both the initial energy relaxation and the subsequent electron transfer. This is evidenced by probing the coherent vibrational motion of the formed FAD•- radical anion and is independently validated by blocking the electron transfer through site-selective tryptophan mutation. Our results provide insight into the role of nonadiabatic couplings for the initial steps of cryptochrome photoactivation and suggest a general strategy for redox-state-specific monitoring of charge transfer dynamics by probing coherent vibrational motion.