Determination of Cα chemical shift tensor orientation in peptides by dipolar-modulated chemical shift recoupling NMR spectroscopy

We present a new method for determining the orientation of chemical shift tensors in polycrystalline solids with site resolution and demonstrate its application to the determination of the Cot chemical shift tensor orientation in a model peptide with beta-sheet torsion angles. The tensor orientation is obtained under magic angle spinning by modulating a recoupled chemical shift anisotropy (CSA) pattern with various dipolar couplings. These dipolar-modulated chemical shift patterns constitute the indirect dimension of a 2D spectrum and are resolved according to the isotropic chemical shifts of different sites in the direct dimension. These dipolar-modulated CSA spectra are equivalent to the projection of a 2D static separated-local-field spectrum onto its chemical shift dimension, except that its dipolar dimension is multiplied with a modulation function. Both C-13-H-1 and C-13-N-15 dipolar couplings can modulate the CSA spectra of the Calpha site in an amino acid and yield the relative orientations of the chemical shift principal axes to the C-H and C-N bonds. We demonstrate the C-H and C-N modulated CSA experiments on methylmalonic acid and N-tBoc-glycine, respectively. The MAS results agree well with the results of the 2D separated-local-field spectra, thus confirming the validity of this MAS dipolar-modulation approach. Using this technique, we measured the Val Calpha tensor orientation in N-acetylvaline, which has beta-sheet torsion angles. The sigma(11) axis is oriented at 158degrees (or 22degrees) from the C-H bond, while the sigma(22) axis is tilted by 144degrees (or 36degrees) from the C-N bond. Both the orientations and the magnitude of this chemical shift tensor are in excellent agreement with quantum chemical calculations.