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Accurate distance measurements between proton and nitrogen can provide detailed information on the structures and dynamics of various molecules. The combination of broadband phase-modulated (PM) pulse and rotational-echo saturation-pulse double-resonance (RESPDOR) sequence at fast magic-angle spinning (MAS) has enabled the measurement of multiple  1 H- 14 N distances with high accuracy. However, complications may arise when applying this sequence to systems with multiple inequivalent  14 N nuclei, especially a single  1 H sitting close to multiple  14 N atoms. Due to its broadband characteristics, the PM pulse saturates all  14 N atoms; hence, the single  1 H simultaneously experiences the RESPDOR effect from multiple  1 H- 14 N couplings. Consequently, no reliable H-N distances are obtained. To overcome the problem, selective  14 N saturation is desired, but it is difficult because  14 N is an integer quadrupolar nucleus. Alternatively,  14 N overtone (OT) NMR spectroscopy can be employed owing to its narrow bandwidth for selectivity. Moreover, owing to the sole presence of two energy levels ( m  = ± 1), the  14 N OT spin dynamics behaves similarly to that of spin-1/2. This allows the interchangeability between RESPDOR and rotational-echo double-resonance (REDOR) since their principles are the same except the degree of  14 N OT population transfer; saturation for the former whereas inversion for the latter. As the ideal saturation/inversion is impractical due to the slow and orientation-dependent effective nutation of  14 N OT, the working condition is usually an intermediate between REDOR and RESPDOR. The degree of  14 N OT population transfer can be determined from the results of protons with short distances to  14 N and then can be used to obtain long-distance determination of other protons to the same  14 N site. Herein, we combine the  14 N OT and REDOR/RESPDOR to explore the feasibility of selective  1 H- 14 N distance measurements. Experimental demonstrations on simple biological compounds of L-tyrosine.HCl, N-acetyl-L-alanine, and L-alanyl-L-alanine were performed at 14.1 T and MAS frequency of 62.5 kHz. The former two consist of a single  14 N site, whereas the latter consists of two  14 N sites. The experimental optimizations and reliable fittings by the universal curves are described. The extracted  1 H- 14 N distances by OT-REDOR are in good agreement with those determined by PM-RESPDOR and diffraction techniques.

Selective 1H-14N Distance Measurements by 14N Overtone Solid-State NMR Spectroscopy at Fast MAS