Detection of remote proton-nitrogen correlations by 1H-detected 14N overtone solid-state NMR at fast MAS

Detecting proton and nitrogen correlations in solid-state nuclear magnetic resonance (NMR) is important for the structural determination of biological and chemical systems. Recent advances in proton detection-based approaches under fast magic-angle spinning have facilitated the detection of H-1-N-14 correlations by solid-state NMR. However, observing remote H-1-N-14 correlations by these approaches is still a challenge, especially for N-14 sites having large quadrupolar couplings. To address this issue, we introduce the H-1-N-14 overtone continuous wave rotational-echo saturation-pulse double-resonance (H-1-N-14 OT CW-RESPDOR) sequence. Unlike regular 2D correlation experiments where the indirect dimension is recorded in the time domain, the H-1-N-14 OT CW-RESPDOR experiment is directly observed in the frequency domain. A set of H-1-N-14 OT CW-RESPDOR filtered H-1 spectra is recorded at varying N-14 OT frequencies. Thanks to the selective nature of the N-14 OT pulse, the filtered H-1 spectra appear only if the N-14 OT frequency hits the positions of the N-14 OT central band or one of the spinning sidebands. This set of filtered H-1 spectra represents a 2D H-1-N-14 OT correlation map. We have also investigated the optimizable parameters for CW-RESPDOR and figured out that these parameters are not strictly needed for our working magnetic field of 14.1 T. Hence, the experiment is easy to set up and requires almost no optimization. We have demonstrated the experimental feasibility of H-1-N-14 OT CW-RESPDOR on monoclinic l-histidine and l-alanyl l-alanine. The remote H-1-N-14 correlations have been efficiently detected, no matter how large the N-14 quadrupolar interaction is, and agree with the crystal structures. In addition, based on the remote H-1-N-14 correlations from the non-protonated N-14 site of l-histidine, we can unambiguously distinguish the orthorhombic and monoclinic forms.