ИСТИНА

Cryptochrome photosensory proteins have been discussed as putative light-activated magnetoreceptors of migratory birds and other animals [1]. Formation and decay of a radical pair between the flavin cofactor and conserved tryptophane (Trp) triad is central in the cryptochrome sensory mechanism. According to the radical pair hypothesis, the yield of singlet-triplet interconversion of the radical pair depends on the cryptochrome orientation in a weak magnetic field. To realize such a mechanism allowing sufficient directional sensitivity, the radical pair should demonstrate a microsecond lifetime [2]. We investigated spin chemistry of Drosophila cryptochrome (dCry) using quantum-chemistry methods. To compute singlet and triplet energies we employed accurate and highly efficient extended multiconfiguration quasidegenerate perturbation theory to the second order (XMCQDPT2) realized in the Firefly software [3]. We performed computations of rather large molecular models consisting of the chromophore electron acceptor, Trp electron donors and proximal protein residues, reaching beyond the electrostatic classical model typically employed in the excited-state calculations of photosensory proteins. Moreover, we directly obtained electron-transfer energies (reaction energies and reorganization energies) as well as electronic-state couplings determining electron-transfer rates in the semi classical limit. Our results demonstrated that distance separation in the dCry radical pair is an isoenergetic reaction controlled by the substantial Trp-Trp electronic coupling. Recombination of the distance-separated radical pair on the microsecond timescale is also controlled by electronic coupling of the flavin with the Trps. Spin decoherence due to formation of the triplet flavin should have unfavorable energy. Overall, our ab initio analysis indicates that magnetosensitivity of the cryptochrome light receptor is warranted by the protein structure. References [1] P.J. Hore and H. Mouritsen, Ann. Rev. Biophys., 45, 299-344 (2016) [3] H. G. Hiscock et al, Proc. Natl. Acad. Sci. USA 113, 4634-4639 (2016) [2] A. Granovsky, J. Chem. Phys. 134, 214113 (2011)