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

Abstract

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.

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