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 ¹H-¹⁴N correlations by solid-state NMR. However, observing remote ¹H-¹⁴N correlations by these approaches is still a challenge, especially for ¹⁴N sites having large quadrupolar couplings. To address this issue, we introduce the ¹H-¹⁴N overtone continuous wave rotational-echo saturation-pulse double-resonance (¹H-¹⁴N OT CW-RESPDOR) sequence. Unlike regular 2D correlation experiments where the indirect dimension is recorded in the time domain, the ¹H-¹⁴N OT CW-RESPDOR experiment is directly observed in the frequency domain. A set of ¹H-¹⁴N OT CW-RESPDOR filtered ¹H spectra is recorded at varying ¹⁴N OT frequencies. Thanks to the selective nature of the ¹⁴N OT pulse, the filtered ¹H spectra appear only if the ¹⁴N OT frequency hits the positions of the ¹⁴N OT central band or one of the spinning sidebands. This set of filtered ¹H spectra represents a 2D ¹H-¹⁴N 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 ¹H-¹⁴N OT CW-RESPDOR on monoclinic L-histidine and L-alanyl L-alanine. The remote ¹H-¹⁴N correlations have been efficiently detected, no matter how large the ¹⁴N quadrupolar interaction is, and agree with the crystal structures. In addition, based on the remote ¹H-¹⁴N correlations from the non-protonated ¹⁴N site of L-histidine, we can unambiguously distinguish the orthorhombic and monoclinic forms.