The quantum environment in cryptochrome enhances light absorption of FAD

Cryptochromes are blue-light photoreceptors that regulate circadian rhythms and, in some species, contribute to magnetic-field sensing through excitation of the flavin chromophore FAD. However, theoretical descriptions of cryptochrome photoexcitation usually treat only the chromophore quantum mechanically, neglecting quantum effects of the surrounding pro-tein because of the computational cost of electronic-structure calculations. Here we present a computational framework that extends the quantum-mechanical region from the chromophore alone to approximately 1,200 atoms, enabling explicit treatment of the surrounding amino acids and cofactors. The approach combines relaxed-accuracy real-time time-dependent Hartree-Fock simulations with a post-processing correction that compensates for systematic errors. The framework resolves the spatial origin, wavelength and oscillator strength of individual absorption processes throughout the quantum region. Our calculations reveal that the quantum-mechanical protein environment substantially enhances the light absorption of FAD, demonstrating that neighboring residues actively modulate the photophysical response of cryptochrome rather than merely providing a structural scaffold.