Quantum Spin Resonance in Engineered Magneto-Sensitive Fluorescent Proteins Enables Multi-Modal Sensing in Living Cells

Abstract

Quantum mechanical phenomena have been identified as fundamentally significant to an increasing number of biological processes. Simultaneously, quantum sensing is emerging as a cutting-edge technology for precision biosensing. However, biological based candidates for quantum-sensors have thus far been limited to in vitro systems, are prone to light induced degradation, and require sophisticated experimental setups making high-throughput studies prohibitively complex. We recently created a new class of magneto-sensitive fluorescent proteins (MFPs) [1], which we now show overcome these challenges and represent the first biological quantum-based sensor that functions at physiological conditions and in living cells. Through directed evolution, we demonstrate the possibility of engineering these proteins to alter properties of their response to magnetic fields and radio frequencies. These effects are explained in terms of the spin correlated radical pair (SCRP) mechanism, involving the protein backbone and a bound flavin cofactor. Using this engineered system we demonstrate the first observation of a fluorescent protein exhibiting Optically Detected Magnetic Resonance (ODMR) in living bacterial cells at room temperature, at sufficiently high signal-to-noise to be detected in a single cell, paving the way for development of a new class of in vivo biosensors. Magnetic resonance measurements using fluorescent proteins enable unprecedented technologies, for instance 3D spatial localisation of the fluorescence using gradient fields (i.e. Magnetic Resonance Imaging but using an endogenous probe). We further demonstrate the use of multiple variants of MFPs for multiplexing or lock-in amplification of fluorescence signals, opening a new approach to combining or extracting multiple signals from a biological measurement. Taken together, our results represent a new intersection of imaging and perhaps actuation modalities for engineered biological systems, based on and designed around understanding the quantum mechanical properties of MFPs.

Footnotes

• Title revised to be more accurate. Section 2 updated for clarification. Discussion updated to include spatial resolution calculation. Abstract updated to mention this application.

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