Optically detected and radio wave-controlled spin chemistry in flavoproteins

Optically addressable spin systems, such as nitrogen-vacancy centers in diamond, have been widely studied for quantum sensing applications. In this work, we demonstrate that certain flavoproteins, specifically cryptochrome and iLOV, which generate spin correlated radical pairs upon optical excitation, also exhibit optically detected magnetic resonance (ODMR). Remarkably, the iLOV protein, commonly used in cellular imaging, displays ODMR contrast approaching 50%. We present initial applications including widefield magnetic field sensing and spatial modulation of photoluminescence using radiofrequency pulses and magnetic field gradients. Our results establish radical pairs in proteins as a novel platform for optically addressable spin systems, offering the key advantages of molecular designability and genetic encodability. Moreover, due to the spin-selective nature of radical pair chemistry, the results lay the groundwork for future radiofrequency-based manipulation of biological systems.

💡 Research Summary

In this work the authors demonstrate that flavoproteins—specifically the cryptochrome from Chlamydomonas reinhardtii (Cra Cry) and the fluorescent reporter iLOV—behave as optically addressable spin systems. Upon blue‑light excitation (447 nm) these proteins generate spin‑correlated radical pairs (SCRPs) consisting of the flavin cofactor (FMN) and a nearby amino‑acid donor. The radical pair is initially created in a defined singlet or triplet spin state; hyperfine‑driven singlet–triplet interconversion is modulated by external magnetic fields, giving rise to the well‑known magnetic‑field effect (MFE) on photoluminescence (PL). By applying radio‑frequency (RF) fields the authors induce electron‑spin transitions between the triplet sub‑levels (T0↔T±) and read out the resulting change in PL, thereby achieving optically detected magnetic resonance (ODMR) in a biological molecule.

The experimental scheme keeps the RF frequency fixed (typically 1470 MHz) while sweeping the static magnetic field. For Cra Cry a clear PL increase appears around 525 G, corresponding to a g≈2 electron spin resonance. The resonance shifts linearly with magnetic field, confirming its electronic origin. iLOV, after optimization of excitation power and sample conditions, exhibits an ODMR contrast approaching 50 %—far larger than that of Cra Cry—and a comparable g≈2 resonance. Linewidth analysis (≈70 MHz) shows little dependence on 15N isotopic labeling, indicating that hyperfine coupling to nitrogen nuclei is not the dominant broadening mechanism. Cra Cry displays a broader line (~100 MHz), a puzzling observation that may reflect a heterogeneous ensemble of radical‑pair configurations.

Time‑resolved measurements reveal that the ODMR signal builds up over milliseconds to seconds under continuous illumination and RF irradiation. This slow kinetics points to the accumulation of long‑lived reaction intermediates (e.g., redox or deprotonated flavin states) whose equilibrium populations are shifted by RF‑driven spin mixing. Pulsed ODMR experiments, where optical excitation and RF manipulation are temporally separated, confirm that spin control can be exerted within the lifetime of the flavin triplet state (µs), i.e., during the formation of the SCRP itself.

Beyond proof‑of‑concept, the authors exploit iLOV for magnetic‑field imaging. By placing a small permanent magnet near the sample they generate a magnetic‑field gradient across the microscope field of view. Pixel‑wise fitting of the ODMR spectra yields a spatial map of the local magnetic field, validated against a solid‑state spin reference (boron‑nitride nanotubes). This demonstrates the feasibility of a genetically encodable, protein‑based magnetic sensor that can operate at ambient temperature and does not suffer from the distance limitations of diamond‑NV microscopy.

Finally, the combination of a magnetic‑field gradient with frequency‑selective RF excitation enables spatial control of PL. Only regions where the local field satisfies the resonance condition for a chosen RF frequency show enhanced PL, effectively “slicing” the sample in magnetic‑field space. This approach could be used to suppress background fluorescence, improve signal‑to‑noise, or implement a new form of super‑resolution imaging where spatial resolution is encoded in the magnetic‑field gradient rather than optical diffraction.

In summary, the paper establishes that radical‑pair chemistry in flavoproteins can be harnessed for ODMR, providing a platform that merges quantum‑spin sensing with the inherent tunability of proteins (through rational design or directed evolution). The work opens avenues for quantum technologies, bio‑imaging, and RF‑controlled biochemistry, including the prospect of RF‑driven regulation of gene expression or signaling pathways in living cells.