Long-range 1H-15N heteronuclear shift correlation at natural abundance

Despite the inherently low sensitivity of N-15 NMR because of its low gyromagnetic ratio (gamma(N)) and its relatively low natural abundance (0.37%), this important nuclide still has useful potential as a structural probe even at natural abundance. Inverse-detected NMR methods coupled with major advances in NMR probe designs have made it possible to acquire long-range H-1-N-15 heteronuclear shift correlation data on samples as small as a micromole overnight. Chemical shift, referencing schemes for N-15 and the range of N-15 shifts are discussed, followed by a discussion of the currently available pulse sequences, pulse calibration, parametrization and processing of long-range H-1-N-15 data, and the implications of probe selection. These topics are followed by a review of the applications contained in the literature that have utilized H-1-N-15 heteronuclear shift correlation experiments at natural abundance, with emphasis placed on the observed long-range coupling pathways.