Paramagnetic biomolecules bridge the gap between chemical reactivity through redox processes and quantum mechanics through their inherent electron spins. In this way, they provide a rich set of fundamental systems to elucidate complex electron spin dynamics and investigate their potential effects on native biological functions. Particularly, paramagnetic molecules, via the Zeeman effect, are able to act as molecular quantum bits (qubit) in an external magnetic field by generating a two-level system that operates quantum mechanically. The specificity and synthetic control of molecular spin qubits make them attractive targets for quantum sensing applications. Conventional quantum sensing modalities are often solid-state sensors lacking broad tunability, and magnetic resonance techniques rely on abundant nuclear spins for imaging. Conversely, molecular quantum sensors using electron spins offer chemical tunability of both spin coherences via electronic structure and precise targeting for local chemical microenvironments. Two main classes of quantum sensors are described in this work: organic radical-labeled micelles and paramagnetic metalloproteins. The former can be chemically tuned to target specific areas of interest and are active across broad temperature ranges. These micelles were utilized as a model system towards cell membranes and demonstrated sensitivity of electron spin relaxation to salt concentrations within the physiological range. The latter are found ubiquitously across the kingdoms of life and offer native sensing targets. Three electron transfer metalloproteins that display paramagnetic states were studied: putidaredoxin, cytochrome c, and plastocyanin. With putidaredoxin and cytochrome c, decoherence dynamics observed in conventional solid-state qubits were first observed in molecular systems and utilized for sensing single point mutations, protein unfolding, and solvent contributions. Subsequently, plastocyanin provided the first metalloprotein system to demonstrate electron spin coherences under ambient conditions via all-optical time-resolved Faraday rotation spectroscopy. Through its function as electron donor to Photosystem I, plastocyanin enabled investigation of the role spin coherences play in canonical biological processes, such as photosynthesis. Across these systems, the advantages of native biomolecular qubits as quantum sensors were demonstrated alongside the capacity to gain fundamental biophysical insight.
Christian Alexander Totoiu (Thu,) studied this question.