Quantum transduction from electron spin state to a signaling state in a wild-type LOV photoreceptor

Whether coherent spin dynamics can control biological regulation remains an open question. Here we show that the electron spin state in the wild-type Avena sativa phototropin 1 Light-Oxygen-Voltage domain 2 (AsLOV2) modulates FMN-cysteine photoadduct formation, the covalent bond that defines its signaling state. Although AsLOV2 was thought magnetically inactive, we ob- serve a magnetic field effect (MFE) on its fluorescence, emerging above ~100 mT and strengthening to 1300 mT. The MFE establishes a spin-correlated radical pair on the photoadduct pathway, one rendered inaccessible to direct spec- troscopy by the same strong coupling that sets the high-field onset. It arises from g-factor asymmetry within the closely spaced FMN-cysteine radical pair overcoming its dipolar coupling to drive coherent singlet-triplet interconversion. We furthermore observe an MFE in UV-vis absorption that, unlike fluorescence, shows the field modulates productive bond formation itself. LOV domains are ubiquitous biological regulators, suggesting that quantum transduction relying on radical pairs as molecular qubits may be a general property among natural and engineered photoreceptors.