Probing amide bond nitrogens in solids using 14N NMR spectroscopy

A novel two-dimensional nuclear magnetic resonance (NMR) experiment is proposed for indirect observation of N-14 nuclei in various types of nitrogen-containing solids. In a method somewhat similar to the heteronuclear single-quantum correlation (HSQC) experiment widely used for protein structure determination in solutions, this technique correlates spin S = 1/2 nuclei, e.g., H-1, C-13, with the N-14 spin l = 1 nucleus in solids. The present experiment, however, transfers coherence from neighboring H-1 or C-13 nuclei to N-14 via a combination of J-couplings and residual dipolar splittings (RDS). Projections of the two-dimensional NMR spectra onto the N-14 dimension yield powder patterns that reflect the N-14 quadrupolar interaction, which can be used to study molecular structure and dynamics. Indirect detection of amide nitrogen-14 via H-1 and C-13 is shown experimentally on a model compound of N-acetyl-glycine.