Dynamic Nuclear Polarization-Enabled Quantum Sensing for Investigating Peptide Configurations

Abstract

Introducing persistent free radicals into biochemical systems, a mechanism referred to as Site-Directed Spin Labeling (SDSL) is a biophysical research feat established by Wayne Hubbell. Integrating double electron–electron resonance (DEER) within SDSL enabled intermolecular distance measurements in the long-range (nanometers) that were otherwise unfeasible using nuclear magnetic resonance (NMR) techniques. DEER is commonly used in the structural analysis of peptides and polymers to probe the distance between electron spin labels (\(r_{ee}\)), typically within the range of 2.0–8.0 nm. However, this technique is generally employed at low-to-intermediate magnetic fields, such as X-Band (0.35 T), Q-Band (1.0 T), or W-Band (3.4 T) mainly due to the microwave power limitation at high fields. This may restrict both the structural resolution and the ability to probe a shorter distance. We propose a novel approach for distance measurement in SDSL peptides or polymers. This approach utilizes dynamic nuclear polarization (DNP) to investigate the distances between unpaired electron spins. DNP is a nuclear spin hyperpolarization technique that has revolutionized solid-state NMR by enhancing its sensitivity. It works by transferring high electron spin polarization to coupled nuclear spins under microwave irradiation. The efficiency of cross-effect DNP transfer is determined by the magnitude of the dipole–dipole coupling between two electron spins. By exploiting the distance dependence between two electron spins, we can sense the configurations of SDSL peptides. One significant advantage of DNP-enabled quantum sensing is that the method can be applied at higher magnetic fields and under magic-angle spinning conditions using existing instrumentation. The approach also enables observation of shorter distances. Overall, our proposed method opens up new possibilities for structural analysis and distance measurements in the study of peptides and polymers, combining with ingenious SDSL technique introduced by the esteemed scientist, Professor Wayne Hubbell.