Quantum transduction from electron spin state to signaling in wild-type AsLOV2

Coherent electron-spin dynamics has been shown to modulate flavoprotein fluorescence in vivo, but not the chemistry by which a protein signals. Here we show that a magnetic field modulates formation of the covalent FMN-cysteine photoadduct that constitutes the signaling state of wild-type Avena sativa LOV2 (AsLOV2), via a spin-selective radical-pair reaction. AsLOV2 was thought magnetically inactive, but a fluorescence magnetic field effect (MFE) emerges near 100 mT and reaches 7% by 1300 mT. Tracking the photoadduct by UV-vis absorption, we find that 650 mT raises the signaling state population and shortens the product formation time constant by 6.7 +/- 0.6%. The short FMN-cysteine separation produces a large dipolar coupling that blocks hyperfine-driven triplet-singlet interconversion at low field; the unusually large flavin-cysteinyl radical g-factor difference overcomes it at high field. The same proximity enables covalent bond formation of radical pairs in a singlet state. This arrangement is canonical in LOV domains, so magnetically enhanced signaling likely extends across natural and engineered photoreceptors