Determination of the ¹⁵N and ¹³C Chemical Shift Tensors of L-[¹³C]Alanyl-L-[¹⁵N]alanine from the Dipole-Coupled Powder Patterns

The¹³C and¹⁵N chemical shift tensors of L-[l-¹³C]alanyl-L-[¹⁵N]alanine have been determined from the dipole-coupled powder patterns and verified with the decoupled spectra. The principal values of the¹³C tensor are σ₁₁= -115.5, σ₂₂= -42.3, and 0–₃₃= 33.5 ppm, and the polar angles relating the¹³C-¹⁵N bond to the chemical shift tensor are β = 90° and a = -39.5°. Although the¹³C powder pattern widths of Ala Ala and AcGlyAlaNH₂differ by only 2%, the anisotropy and asymmetry parameters differ by 10 and 25%, respectively. The principal values for the¹⁵N chemical shift tensor are σ₁₁= 65.3,σ₂₂= 78.1, and σ₃₃= 215.5 ppm, and the orientation of the¹³C-¹⁵N bond to the σ₃₃axis is 106°. AlaAla and AcGlyAlaNH₂show a striking difference in asymmetry parameters (0.06 vs. 0.16) and anisotropy (144 vs. 165).