The magnetic field-dependent fluorescence of MagLOV2 in live bacterial cells is consistent with the radical pair mechanism

MagLOV2 is an engineered flavoprotein designed to have large changes in fluorescence intensity in response to weak magnetic fields. Here, we characterize the magnitude of these fluorescence changes, known as the “magnetic field effect,” as a function of the strength of an externally applied magnetic field in E. coli colonies expressing MagLOV2. We observe that the magnetic field effect is positive at low magnetic fields, reaches a maximum positive value near 1 mT, and then decreases, reversing sign at approximately 2 mT. Furthermore, the effect starts to plateau above approximately 70 mT, with a decreased sensitivity of fluorescence changes to magnetic fields above this range. The non-monotonic behavior, as well as the diminished responsiveness to higher magnetic fields, are consistent with the changes in fluorescence being driven by electron spin-dependent chemical processes governed by the radical pair mechanism.