Measurement of ¹⁴N quadrupole couplings in biomolecular solids using indirect-detection ¹⁴N solid-state NMR with DNP

The quadrupolar interaction experienced by the spin-1 ¹⁴N nucleus is known to be extremely sensitive to local structure and dynamics. Furthermore, the ¹⁴N isotope is 99.6% naturally abundant, making it an attractive target for characterisation of nitrogen-rich biological molecules by solid-state NMR. In this study, dynamic nuclear polarization (DNP) is used in conjunction with indirect ¹⁴N detected solid-state NMR experiments to simultaneously characterise the quadrupolar interaction at multiple ¹⁴N sites in the backbone of the microcrystalline protein, GB3. Considerable variation in the quadrupolar interaction (>700 kHz) is observed throughout the protein backbone. The distribution in quadrupolar interactions observed reports on the variation in local backbone conformation and subtle differences in hydrogen-bonding; demonstrating a new route to the structural and dynamic analysis of biomolecules.