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 (4700 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.