Probing Bioinorganic Electron Spin Decoherence Mechanisms\nwith an Fe<sub>2</sub>S<sub>2</sub> Metalloprotein

Recent efforts have\nsought to develop paramagnetic molecular quantum\nbits (qubits) as a means to store and manipulate quantum information.\nEmerging structure–property relationships have shed light on\nelectron spin decoherence mechanisms. While insights within molecular\nquantum information science have derived from synthetic systems, biomolecular\nplatforms would allow for the study of decoherence phenomena in more\ncomplex chemical environments and further leverage molecular biology\nand protein engineering approaches. Here we have employed the exchange-coupled <i>S</i><sub>T</sub> = 1/2 Fe<sub>2</sub>S<sub>2</sub> active site\nof putidaredoxin, an electron transfer metalloprotein, as a platform\nfor fundamental mechanistic studies of electron spin decoherence toward\nspin-based biological quantum sensing. At low temperatures, decoherence\nrates were anisotropic, reflecting a hyperfine-dominated decoherence\nmechanism, standing in contrast to the anisotropy of molecular systems\nobserved previously. This mechanism provided a pathway for probing\nspatial effects on decoherence, such as protein vs solvent contributions.\nFurthermore, we demonstrated spatial sensitivity to single point mutations\nvia site-directed mutagenesis and temporal sensitivity for monitoring\nsolvent isotope exchange. Thus, this study demonstrates a step toward\nthe design and construction of biomolecular quantum sensors.