Origin of Light-Induced Spin-Correlated Radical Pairs in Cryptochrome

Blue-light excitation of cryptochromes and homologues uniformly triggers electron transfer (ET) from the protein surface to the flavin adenine dinucleotide (FAD) cofactor A cascade of three conserved tryptophan residues has been considered to be critically involved in this photoreaction If the FAD is initially in its fully oxidized (diamagnetic) redox state light-induced ET via the tryptophan triad generates a series of short-lived spin-correlated radical pairs comprising an FAD radical and a tryptophan radical Coupled doublet-pair species of this type have been proposed as the basis, for example, of a biological magnetic compass in migratory birds, and were found critical for some cryptochrome functions in vivo In this contribution, a cryptochrome-like protein (CRYD) derived from Xenopus lams has been examined as a representative system The terminal radical-pair state FAD W324 of X lams CRYD has been characterized in detail by time-resolved electron-paramagnetic resonance (TREPR) at X-band microwave frequency (9 68 GHz) and magnetic fields around 345 mT and at Q-band (34 08 GHz) at around 1215 mT Different precursor states, singlet versus triplet, of radical-pair formation have been considered in spectral simulations of the experimental electron-spin polarized TREPR signals Conclusively we present evidence for a singlet-state precursor of FAD W324 radical-pair generation because at both magnetic fields, where radical pairs were studied by TREPR, net-zero electron-spin polarization has been detected Neither a spin-polarized triplet precursor nor a tnplet at thermal equilibrium can explain such an electron-spin polarization It turns out that a two-microwave-frequency TREPR approach is essential to draw conclusions on the nature of the precursor electronic states in light-induced spin-correlated radical pair formations