Mutual orientation of three magnetic tensors in a polycrystalline dipeptide by dipole-modulated ¹⁵N chemical shift spectroscopy

This study demonstrates the mutual orientation of three tensor interactions in a single NMR experiment. The orientation of the ¹⁵N chemical shift tensor relative to the molecular frame has thus been determined in polycrystalline L-[1-¹³C] alanyl-L-[¹⁵N] alanine. The ¹³C-¹⁵N and ¹⁵N-¹H dipole interactions are determined using the ¹H dipole-modulated, ¹³C dipole-coupled ¹⁵N spectrum obtained as a transform of the data in t₂. From simulations of the experimental spectra, two sets of polar angles have been determined relating the ¹³C- ¹⁵N and ¹⁵N-¹H dipoles to the ¹⁵N chemical shift tensor. The values determined are β_CN = 106°, α_CN = 5° and β_NH = -19°, α_NH = 12°. The experiment verifies, without reference to single crystal data, that σ₃₃ lies in the peptide plane and σ₂₂ is nearly perpendicular to the plane.