An efficient NMR method for the characterisation of 14N sites through indirect 13C detection

Abstract

Nitrogen is one of the most abundant elements and plays a key role in the chemistry of biological systems. Despite its widespread distribution, the study of the naturally occurring isotope of nitrogen, ¹⁴N (99.6%), has been relatively limited as it is a spin-1 nucleus that typically exhibits a large quadrupolar interaction. Accordingly, most studies of nitrogen sites in biomolecules have been performed on samples enriched with ¹⁵N, limiting the application of NMR to samples which can be isotopically enriched. This precludes the analysis of naturally occurring samples and results in the loss of the wealth of structural and dynamic information that the quadrupolar interaction can provide. Recently, several experimental approaches have been developed to characterize ¹⁴N sites through their interaction with neighboring ‘spy’ nuclei. Here we describe a novel version of these experiments whereby coherence between the ¹⁴N site and the spy nucleus is mediated by the application of a moderate rf field to the ¹⁴N. The resulting ¹³C/¹⁴N spectra show good sensitivity on natural abundance and labeled materials; whilst the ¹⁴N lineshapes permit the quantitative analysis of the quadrupolar interaction.