title: Origin of light-induced spin-correlated radical pairs in cryptochrome.
abstract: 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 laevis has been examined as a representative system. The terminal radical-pair state FAD(•)···W324(•) of X. laevis 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 triplet 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.
authors.affiliation: Institute of Physical Chemistry, Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany. Stefan.Weber@physchem.uni-freiburg.de
authors.forename: Stefan
authors.initials: S
authors.lastname: Weber
authors.forename: Till
authors.initials: T
authors.lastname: Biskup
authors.forename: Asako
authors.initials: A
authors.lastname: Okafuji
authors.forename: Anthony R
authors.initials: AR
authors.lastname: Marino
authors.forename: Thomas
authors.initials: T
authors.lastname: Berthold
authors.forename: Gerhard
authors.initials: G
authors.lastname: Link
authors.forename: Kenichi
authors.initials: K
authors.lastname: Hitomi
authors.forename: Elizabeth D
authors.initials: ED
authors.lastname: Getzoff
authors.forename: Erik
authors.initials: E
authors.lastname: Schleicher
authors.forename: James R
authors.initials: JR
authors.lastname: Norris
mesh_terms: D000818:Animals; D056931:Cryptochromes / Q000737:chemistry* / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport / Q000528:radiation effects; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D008027:Light; D008958:Models, Molecular; D011487:Protein Conformation; D029867:Xenopus Proteins / Q000737:chemistry / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D056931:Cryptochromes; D005609:Free Radicals; D029867:Xenopus Proteins; D005182:Flavin-Adenine Dinucleotide