The primary step
in the mechanism by which migratory
birds sense
the Earth’s magnetic field is thought to be the light-induced
formation of long-lived magnetically sensitive radical pairs within
cryptochrome flavoproteins located in the birds’ retinas. Blue-light
absorption by the non-covalently bound flavin chromophore triggers
sequential electron transfers along a chain of four tryptophan residues
toward the photoexcited flavin. The recently demonstrated ability
to express cryptochrome 4a from the night-migratory European robin
(Erithacus rubecula),ErCry4a, and to replace each of the tryptophan residues by a redox-inactive
phenylalanine offers the prospect of exploring the roles of the four
tryptophans. Here, we use ultrafast transient absorption spectroscopy
to compare wild typeErCry4a and four mutants having
a phenylalanine at different positions in the chain. We find that
each of the three tryptophan residues closest to the flavin adds a
distinct relaxation component (time constants: 0.5, 30, and 150 ps)
in the transient absorption data. The dynamics of the mutant containing
a phenylalanine at the fourth position, furthest from the flavin,
are very similar to those of wild typeErCry4a, except
for a reduced concentration of long-lived radical pairs. The experimental
results are evaluated and discussed in the framework of real-time
quantum mechanical/molecular mechanical electron transfer simulations
based on the density functional-based tight binding approach. This
comparison between simulation results and experimental measurements
provides a detailed microscopic insight into the sequential electron
transfers along the tryptophan chain. Our results offer a route to
the study of spin transport and dynamical spin correlations in flavoprotein
radical pairs.