DIRECT OBSERVATION OF ASYMMETRIC H-1/N-14 TRIPLETS AND APPLICATIONS OF ASYMMETRIC DIPOLE-DIPOLE SPLITTINGS TO STRUCTURE DETERMINATION BY SOLID-STATE NMR-SPECTROSCOPY

High-resolution solid-state NMR spectra of H-1 nuclei directly bonded to N-14 nuclei can be extracted from two-dimensional H-1/N-14 heteronuclear correlation spectra of single crystal samples. The H-1 resonances are asymmetric triplets, since the heteronuclear dipole-dipole interaction between a spin one-half H-1 and a spin one N-14 is influenced by the large N-14 quadrupole coupling constant and can be observed in both N-14 fundamental (Delta m = 1) and N-14 overtone (Delta m = 2) spectra. N-14 fundamental spectra have two resonances for each magnetically distinct N-14 nucleus, each of which is a doublet in the heteronuclear correlation spectra; the asymmetric H-1 triplets can be reconstructed from the doublets of both resonances. N-14 overtone spectra are simpler, since they have a single asymmetric H-1 triplet associated with a single N-14 resonance for each magnetically distinct N-14 nucleus. H-1 chemical shift frequencies, the splittings from H-1/N-14 heteronuclear dipole-dipole couplings, N-14 quadrupole splittings, and N-14 second-order shifts can all be measured with both fundamental and overtone H-1/N-14 heteronuclear correlation spectra. In addition, the H-1 and N-14 resonance frequencies for the bonded hydrogen and nitrogen nuclei are correlated in these spectra. The spectral parameters from asymmetric H-1/N-14 triplets are demonstrated to offer substantial advantages over those of the corresponding symmetric N-14/H-1 doublets for determining structures of peptides by solid-state NMR spectroscopy, even though both sets of parameters arise from the same dipole-dipole interactions.