New tools in bio-NMR: indirect detection of nitrogen-14 in solids.

This paper presents an overview of recently developed methods for the indirect detection of 14N nuclei (spin l = 1) in spinning solids by nuclear magnetic resonance spectroscopy. These methods exploit the transfer of coherence from a neighboring 'spy' nucleus with spin S = 1/2, such as 13C or 1H, to single- or double-quantum transitions of 14N nuclei. The two-dimensional correlation methods presented here are closely related to the well-known heteronuclear single- and multiple-quantum correlation (HSQC and HMQC, respectively) experiments, already widely used for the investigation of molecules in liquids. Nitrogen-14 NMR spectra exhibit powder patterns characterized by second- and third-order quadrupolar couplings which can provide important information about structure and dynamics of molecules in powder samples.