Orientational restraints resulting from a wide range of anisotropic nuclear spin interactions such as chemical shifts, heteronuclear dipolar interactions, and quadrupolar interactions, have been widely used for determining the structures of peptides and proteins from aligned samples. The aligned samples have a unique orientation with respect to the magnetic field axis, such that the information from the orientation dependent nuclear spin interactions within a peptide plane allows for the characterization of the peptide plane orientation with respect to the alignment axis. By obtaining the orientations of all the peptide planes with respect to the same alignment axis, a model can be generated for the three-dimensional backbone conformation.

Proton (¹H) chemical shifts from solid state nuclear magnetic resonance (NMR) have been underutilized as an orientational restraint for backbone conformations in peptides and proteins. This is largely due to discrepancies in the model for the chemical shift tensors. A two dimension heteronuclear correlation (HETCOR) sequence was developed to obtain accurate chemical shift tensors to form a useable model of the orientations of the principal axes of the amide ¹H chemical shift. Further improvements to HETCOR were made to form the two dimensional pulse sequences, WIM-HETCOR and WIM-HETCOR Echo. Both WIM-HETCOR and WIM-HETCOR Echo pulse sequences incorporate a windowless isotropic mixing (WIM) sequence for cross polarization. WIM-HETCOR Echo was used to obtain the amide ¹H CSA tensor element magnitudes for a representative dipeptide, alanyl-¹⁵N-leucine (AL). The orientation of the amide ¹H CSA tensor was obtained by comparing simulated chemical shift correlation spectra with the experimental WIM-HETCOR Echo spectra. The variability and application of the ¹H CSA tensor was explored using membrane peptides: gramicidin A and piscidin 1.