N-14 COUPLED DIPOLAR-CHEMICAL SHIFT C-13 NMR-SPECTRA OF THE AMIDE FRAGMENT OF PEPTIDES IN THE SOLID-STATE

Carbon-13 NMR spectra of solid samples of acetanilide-C-13(CO) have been examined under conditions of cross-polarization (CP) with and without magic-angle spinning (MAS). The C-13 NMR line shapes are perturbed by direct dipolar coupling to the adjacent quadrupolar N-14 nucleus. The C-13 NMR spectra have been fully analyzed employing first-order perturbation theory for N-14. Residual C-13, N-14 splittings observed in the C-13 CP/MAS NMR spectra of the carbonyl and C(alpha) region of acetanilide indicate an upper limit of the N-14 nuclear quadrupolar coupling constant, chi, of -3.2 MHz. The largest component of the electric field gradient (EFG) tensor at N-14 is oriented perpendicular to the amide plane, while the smallest component lies in the amide plane about 20-degrees off the N-H bond axis toward C(alpha). This orientation of the EFG tensor at nitrogen is in excellent agreement with that calculated using MO theory. Carbon-13 NMR spectra of static samples of acetanilide-C-13(CO) exhibit typical N-14 dipolar coupled chemical shift powder line shapes similar to those observed for the amide fragment in peptides. Asymmetric N-14, C-13 dipolar splittings are apparent in the delta11 region of the powder pattern, but symmetrical splittings are observed at delta33, while the splittings at delta22 are not resolved. Analysis of the static C-13 NMR spectrum yields the magnitudes and orientations of the principal components of the carbonyl carbon chemical shift tensor. The most shielded component, delta33, is perpendicular to the amide plane, while the least shielded component is oriented 30-degrees off the C-N bond vector toward C(alpha). In contrast to the MAS spectra, the static spectrum is found to be insensitive to the magnitude and orientation of the N-14 EFG tensor, contrary to previous reports. The origin of the asymmetric N-14, C-13 dipolar splittings observed in the C-13 NMR spectra of static samples of acetanilide and the amide fragment of peptides is discussed in detail.