The Amide ¹⁵N Chemical Shift Tensors of Four Peptides Determined from ¹³C Dipole-Coupled Chemical Shift Powder Patterns

The¹⁵N chemical shift tensors of a homologous series of peptides of the form N-acetyl[l-¹³C]glycyl[¹⁵N]-X-amide (X = glycine, alanine, and tyrosine) and the unprotected dipeptide [l-¹³C]glycyl[¹⁵N]glycine hydrochloride have been determined from¹³C dipole-coupled¹⁵N powder patterns. It was found that the shift tensor principal values differ greatly while their molecular orientation does not. The common shift tensor orientation places σ₂₂perpendicular to the peptide plane, and σ₃₃at a 99° angle with respect to the C-N bond. The orientations of σ₁₁ and σ₂₂were previously unknown for¹⁵N chemical shift tensors of amides. Comparison of magic angle spinning (MAS) spectra with solution spectra shows significantly different solid and solution isotropic chemical shifts for several of the peptides studied, demonstrating that at least part of the variation in principal values is due to lattice effects. This conclusion is borne out by the MAS spectrum of N-acetyl[l-¹³C]glycl-[¹⁵N]phenylalaninamide, which shows at least three peaks corresponding to different lattice environments.